Ch2 chromosome structure (In brief)

SBB 1054 Genetics

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Prepared by Pratheep Sandrasaigaran

Lecturer at Manipal International University

IMPORTANT DATES..!

1. Test 1- Week 6 (20th – 24th MAC 2017)

2. Test 2- Week 11 ( 24th – 28th APR 2017)

3. Assignment 2- 3rd May 2017 (Week 12)

4. Study Break- Week 15 (22nd – 26th MAY 2017)

5. Exam- Week 16 & 17 (29th MAY – 9th JUN 2017)


Figure taken from Internet

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Lecturer at Manipal International University

2.0 Chromosome structure

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Figure taken from Internet

• Define the basis of “Central Dogma”

• Chromosome Structure and DNA packaging

• Understanding genetics Code

• Compare the nuclear DNA with

- Mitochondrial DNA

- Chloroplast DNA

By the end of this chapter you should be able to:

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2.1 The Central Dogma


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The “Central Dogma”

• Central dogma of:

- Molecular Biology

- Genetics

• Primarily to direct the production of proteins.

• Each DNA molecule can carry thousands of genes which in return plan for building a particular protein, or part of a particular protein.


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The “Central Dogma”

• The type of protein produced decides everything about a cell and eventually the human body as a whole:

- The color of the hair and skin

- Propensity to certain diseases

- Unique ability

- Fat, thin, tall, short, bold, hairy….


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Diagram adopted from: Human Genetics: Concepts and Applications, 9th Edition, Lewis


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a. Transcription

• Transcription occurs in three stages: - Initiation - Elongation - Termination.


a. Transcription

• Initiation is the control point that determines which genes are transcribed.


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a. Transcription

• RNA nucleotides are added during elongation


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a. Transcription

• A terminator sequence in the gene signals the end of transcription.


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a. Transcription


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• Many identical copies of RNA are transcribed simultaneously.

• Usually 100 or more DNA bases lie between RNA polymerases.



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• The double helix become detached from each other and exposes the genes (sequences of bases)

• Just one of the strands serves as a template to produce the RNA strand.

• RNA polymerase binds to the DNA and moves along it, attaching free nucleotides making strand of mRNA.

• What is the sequence of mRNA that you expect?

a. Transcription

Diagram adopted from The Facts On File Illustrated Guide to the Human Body: Cells and Genetics


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• The mRNA consists of a single chain of nucleotides with that the base uracil (U) occurs instead of thymine (T).

• mRNA carries the genetic code, in the form of base triplets, or codons.

• What happen to the other strand of DNA?

• They are not transcribed

a. Transcription

i. Transcription- mRNA Processing


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i. Transcription- Alternative mRNA Splicing


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ii. Ribosomal RNA


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iii. Transfer RNA


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Types of RNA


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TEST YOUR KNOWLEDGE 1


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1. DNA is the genetic code.

2. Transcription is needed to only synthesis mRNA.

3. Both strand of DNA involve in the transciption process.

4. Guanine and Thymine are the purine component of the nitrogenous bases that are found in DNA.

5. Instead of Adenine, Uracil is replaced in the RNA molecule.

6. RNA is a single strand molecule and information is read in triplet code of codon.

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRxqFQoTCLD_qsHP0MgCFZcHjgodLkkC4w&url=http%3A%2F%2Fwww.notaio.org%2Fconcorso_notarile2.htm&psig=AFQjCNF2sitVXDq8A_X_k11kNST3Sp4PyQ&ust=1445415730506564

b. Translation


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• Initiation • Elongation • Termination

Diagram adopted from internet


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Diagram adopted from internet

Termination



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2.2 Chromosome Structure and DNA packaging


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Introduction

• A gene provides each cell in the body with instructions for making (synthesizing) a particular protein (or part of a protein).

• Each protein, either directly or indirectly, determines a particular measurable characteristic or cell function.

• Proteins are formed either as enzymes or structural proteins.


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Where and when genes are active?

• Except for sex cells, all human cells contain a set of approximately 25,000 genes.

• Which genes manufacture proteins depends on where in the body a cell is situated.

• Where is the gene is located in the body and how does such vast information can be coordinated?


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Basic structure of a chromosome

• Genes are organized into structures called chromosomes, which serve as vehicles for transmitting genetic information.

• In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin.

• Two types of chromatin can be seen with electron microscopy.

- Heterochromatin

- Euchromatin


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a. Heterochromatin

• Is an electron dense and distributed around the periphery of the nucleus and in discrete masses within the nucleus.

• The DNA is in close association with nucleoproteins, and it is not active in RNA synthesis.

• Serves as structural purpose during the chromosomal stages.


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b. Euchromatin

• Is an electron lucent and represents DNA that is actually or potentially active in RNA synthesis.

• Example, the protein coding gene region


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Basic structure of a chromosome

• Chromosomes are usually diffuse, threadlike structures, not easily distinguishable from each other within the nucleus.

• Just before and during cell division the chromosomes condense (become shorter and fatter), so that their different shapes become visible under a microscope.

• The chromosome also copies itself, making two identical chromatids that meet at a narrow point called the centromere.


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2 type of Chromosome • Sex • Autosomal

a. Sex chromosomes

• These chromosomes determine gender.

• Human cells contain two sex chromosomes.

• If you’re female, you have two X chromosomes, and if you’re male, you have an X and a Y chromosome.


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b. Autosomal chromosomes

• Autosomal simply refers to non-sex chromosomes.

• So, sticking with the human example, do the math, and you can see that humans have 44 autosomal chromosomes.

• In humans, chromosomes come in pairs.

• How many pairs of chromosome that you will find in your cells?


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The 46 Human chromosomes


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Diagram adopted from Genetics for Dummies, Tara. R. R

Packing DNA into Small Spaces

• Human cells contain 6 billion base pairs of DNA, which would measure 1.8 meters stretched end to end.

• In the nucleus of a normal human cell, there are 46 chromosomes each containing 48–240 million bases of DNA

• Smallest human chromosome, the DNA would stretch 14,000 times the length of the nucleus.

• Virtually all of the genomic DNA is distributed among the 23 chromosomes that reside in the cellular nucleus


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Packing DNA into Small Spaces

• However, the average nucleus has a diameter of approximately 5 μm.

• Tightly packaging is required to fit DNA into such small spaces.

• How it is possible?

• Complexed with histone protein and tightly packed within the nucleus of the cell


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Histone protein

• Positively charged proteins of five major types: H1, H2A, H2B, H3, and H4.

• The positive charges attract the negative charges on the phosphates of DNA and holds the DNA in contact with the histones


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Diagram adopted from Genetics for Genetics- Conceptual Approach, Benjamin A. Pierce


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Chromatosome consists of a nucleosome plus the H1 histone

Diagram adopted from Genetics for Genetics- Conceptual Approach, Benjamin A. Pierce


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Diagram adopted from Concepts of Genetics, Klug.W.S., 10th ED

Chromatin fibers Metaphase chromosomes

Metaphase chromosomes



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1. Heterochromatin are the dense region of chromosome that involve in the RNA transcriptions.

2. Human autosomal chromosome accounts 46 all together.

3. Chromosome has both DNA and Protein.

4. H1, H2, H3 and H4 are the protein parts of histone protein.



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Chromatid

Centromere

Telomeres

Heterochromatin Euchromatin

Diagram adopted from Internet


Centromeres

• Each metaphase chromosome is composed of two identical sister chromatids.

• Chromatids are connected at a central region called the centromere

• Centromeres consist of hundreds of kilobases of repetitive DNA and are responsible for the movement of chromosomes at cell division.

• Each centromere divides the chromosome into short (p) and long (q) arms.


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Anatomy of a chromosome showing the three shapes


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Diagram adopted from Crash course: Cell Biology and Genetics, 4th ED


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p arm

Centromere

Diagram adopted from Internet

q arm

A

B

C

Diagram adopted from Concepts of Genetics, Klug.W.S., 10th ED

Centromere locations and designations of chromosomes


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Telomeres

• The ends of chromosomes are protected by DNA structures called telomeres.

• Telomeres are tandem repeats of the hexameric sequence ‘TTAGGG’ and loops back on itself to form the T-loop.


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Telomeres

• Telomeres have several functions in preserving chromosome stability:

• Preventing abnormal end-to-end fusion of chromosomes

• Protecting the ends of chromosomes from degradation

• Ensuring complete DNA replication

• Having a role in chromosome pairing during meiosis.


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Chromosome structure- Summary

Diagram adopted from Genetics for Dummies, Tara. R. R



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Karyotype Analysis


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1. Look at the following Karyotype; is this organism male or female?

2. How many chromosomes are found in the somatic cells of this organism?

3. How many chromosomes are found in sperm cells from this organism?

4. How many pairs of chromosomes does this organism’s karyotype contain?

5. How many chromosomes would be found in this organism’s skin cells?

6. How many chromosomes would be found in this organism’s egg cells?

Figure adopted from Internet

Define these terms • Gene

- A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses)

• Locus

- A specific place along the length of a chromosome where a given gene is located

• Gamete

- A haploid reproductive cell, such as an egg or sperm. Gametes unite during sexual reproduction to produce a diploid zygote.


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Define these terms

• Male gamete

- Sperm

• Female gamete

- Eggs

• Asexual reproduction

- The generation of offspring from a single parent that occurs without the fusion of gametes (by budding, division of a single cell, or division of the entire organism into two or more parts).

- In most cases, the offspring are genetically identical to the parent.


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1. What is a somatic cell? Give examples of two human somatic cell types

- A somatic cell is any cell in a multicellular organism except a sperm or egg or their precursors. Examples may vary but could include bone cells, skin cells, blood cells, etc.

2. How does a somatic cell compare to a gamete in terms of chromosome number?

- Unlike somatic cells, gametes contain a single set of chromosomes.

- Such cells are called haploid cells, and each has a haploid number of chromosomes (n).

- For humans, the haploid number is 23.


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3. Can you distinguish allele, gene and locus?

- Homologous chromosomes can have different alleles on them.

- Alleles are variants of the same gene that occur on the same place on a chromosome. (Through a mutation, they are different).

- A locus refers to the location on the chromosome where the gene is found.


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• Human genome encodes app. 100,000 proteins.

• Coding portions of a gene app. 2000

• Coding sequences are interrupted by from 1 to 50 noncoding sequences


2.3 Understanding genetics Code


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- Identify the chemical components of DNA. - Assemble the double helix: The structure of

DNA. - Genetic code



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- Identify the chemical components of DNA.

• Nucleic acids are produced from nucleotide polymerization.

• During synthesis a series of nucleic acid condensation reactions occur between phosphate and sugar groups.

• A-T-C-G (DNA) A-U-C-G (RNA)


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Nucleic Acid



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Nucleic Acid

• Pentose sugar • Nitrogenous

base • Phosphates

• Pentose sugar • Nitrogenous

base

Nucleotides

Nucleosides

Phosphates Nucleosides + Nucleotides =


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Nucleic Acid

N-glycosidic bond

Phosphodiester bonds


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DNA vs RNA



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DNA vs RNA- Sugar

Sugar: • RNA – ribose (OH) • DNA – deoxyribose (H)


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DNA vs RNA- Sugar

A B

• The nucleotide bases in nucleic acids contain nitrogen derived from either purines or pyrimidines.

• Purines (Double ring)

• Adenine

• Guanine

• Pyrimidines (Single ring)

• Cytosine

• Thymine*

• Uracil*


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DNA vs RNA

PURINES PYRIMIDINES

• Discovered by Watson and Crick in 1953

• Base composition analysis of hydrolyzed samples of DNA

• X-ray diffraction studies of DNA.

• Adenine and thymine pair via two hydrogen bonds between opposing strands.

• Guanine and cytosine pair via three hydrogen bonds.

• Base pairing results in two complementary polynucleotides, which run antiparallel to each other.


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DNA is double helix


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DNA is double helix


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The Road to the Double Helix

• Messenger RNA

• Carries genetic information from the nucleus into the cytoplasm.

• In eukaryotes, it is derived by splicing the initial RNA transcript (heteronuclear RNA that holds introns)


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RNA is Single strand- mRNA


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RNA is Single strand- mRNA


A A A A A A A A

• Transfer RNA

• Linear molecule with an average of 76 nucleotides

• Exhibits extensive intramolecular base pairing, giving it a ‘clover-leaf’-shaped secondary structure

• Carries specific amino acids to the site of protein synthesis.


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RNA is Single strand- tRNA


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RNA is Single strand- tRNA

• Terminal CCA group can accept a specific amino acid.

• anticodon arm, recognizes the corresponding mRNA codon.

• Specific base pairing within the five arms helps to maintain the secondary structure.


• Ribosomal RNA (ribosomal component)

• In a eukaryotic cell each ribosome consists of two unequal subunits, made up of proteins and RNA, called the S (small) and L (large) subunits held together by magnesium ions.

• The RNA molecules undergo extensive intramolecular base pairing, which determines the ribosomal structure.


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RNA is Single strand- rRNA



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1. What are nucleic acids? • DNA and RNA, the nucleic acids, are the molecules responsible for the

hereditary information that commands the protein synthesis in living beings.

2. What are the units nucleic acids constituted? What are the chemical entities that compose that unit?

• Nucleic acids are formed by sequences of nucleotides

• Nucleotides are constituted by one molecule of sugar (deoxyribose in DNA and ribose in RNA) bound to one molecule of phosphate and to one nitrogen-containing base

3. Which two groups can the nitrogen-containing bases that form DNA and RNA be classified? • The nitrogen-containing bases that form DNA and RNA are classified as

pyrimidine and purine bases


4. What is the criterion used in purine-pyrimidine classification?

• Cytosine, thymine and uracil, have only one nitrogenized carbon ring. The others, adenine and guanine, have two nitrogenized associated carbon rings

5. Which type of chemical bond maintains the pairing of each chain in the DNA molecule?

• To form the DNA molecule, purine bases bind to pyrimidine bases by intermolecular bonds called hydrogen bonds. Hydrogen bonds occur when there is hydrogen near one of these electronegative elements: fluorine, oxygen or nitrogen

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6. What is the completing sequence of nitrogen-containing bases for a AGCCGTTAAC fragment of a DNA chain?

7. What are the three main types of RNA?

• Messenger RNA, or mRNA, transfer RNA, or tRNA, and ribosomal RNA, or rRNA, are the three main types of RNA.

• TCGGCAATTG

8. What is meant by heterogeneous RNA?

• The newly formed RNA molecule, a precursor of mRNA, is called heterogeneous RNA (hnRNA). The heterogenous RNA bears portions called introns and portions called exons. The hnRNA is processed in many chemical steps, introns are removed and mRNA is created formed only of exons, the biologically active nucleotide sequences

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9. Concerning their biological function what is the difference between DNA and RNA?

10. Is there any situation in which DNA is made based on a RNA template?

• The process in which DNA is synthesized having as template a RNA chain is called reverse transcription. In cells infected by retroviruses (RNA viruses, like the AIDS or SARS viruses) reverse transcription occurs and DNA is made from information contained in the viral RNA. Viral RNA within the host cell produces DNA with the help of an enzyme called reverse transcriptase.

• DNA is the source of information for RNA production (transcription) and thus for protein synthesis. DNA is still the basis of heredity due to its replication capability

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- Assemble the double helix: The structure of DNA.

• Between 1949 and 1953, Erwin Chargaff and his colleagues used chromatographic methods to separate the four bases in DNA samples from various organisms.


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Evidence 1: Base-Composition Studies

• The amount of adenine residues is proportional to the amount of thymine residues in DNA.

• The amount of guanine residues is proportional to the amount of cytosine residues.

• The sum of the purines (A + G) equals the sum of the pyrimidines (C + T).

• The percentage of (G + C) does not necessarily equal the percentage of (A + T).


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What can be derived from Erwin Chargaff?

• When fibers of a DNA molecule are subjected to X-ray bombardment, the X rays scatter (diffract) in a pattern that depends on the molecule’s atomic

• William Astbury (1938) detected a periodicity 3.4 angstroms (3.4-Å)

• Bases were stacked like coins on top of one another.


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Evidence 2: X-Ray Diffraction Analysis


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Evidence 2: X-Ray Diffraction Analysis

• Between 1950 and 1953, Rosalind Franklin suggested that the structure of DNA was some sort of helix.


• X-ray diffraction photograph by Rosalind Franklin using the purified DNA fibers.

Diagram adopted from William S and Klug. Concept of Genetics, 10th ED

../Ch3- The Genetics Code/Rosalind Franklin - NHD Documentary.mp4


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The Watson–Crick Model


• Two long polynucleotide chains are coiled around a central axis, forming a right-handed double helix.

• The two chains are antiparallel; that is, their C-5’-to-C-3’orientations run in opposite directions.

• The bases of both chains are flat structures lying perpendicular to the axis; they are “stacked” on one another, 3.4 Å (0.34 nm) apart, on the inside of the double helix.



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• The nitrogenous bases of opposite chains are paired as the result of the formation of hydrogen bonds

• In DNA, only A -T and G-C pairs occur.

• Each complete turn of the helix is 34 Å (3.4 nm) long; thus, each turn of the helix is the length of a series of 10 base pairs.



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• A larger major groove alternating with a smaller minor groove winds along the length of the molecule.

• The double helix has a diameter of 20 Å (2.0 nm).



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DNA and Inheritance

• The structure of DNA is a linear sequence of deoxyribonucleotides.

• There are regions within the DNA that contain protein-coding genes.

• How is the information within DNA decoded for translation of proteins?

• Central dogma – flow of information from?




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A

B

C

D

E


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1. Who were James Watson, Francis Crick and Maurice Wilkins

• Watson (North American), Crick (British) and Wilkins (New Zealander) were the discoverers of the molecular structure of DNA, the double helix made of two polynucleotide chains paired by their nitrogen-containing bases. They won the Nobel prize in Medicine in 1962 for the discovery.

2. What can you conclude from the table below?




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- Genetic code


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Deciphering the Code


• Marshall Nirenberg and Heinrich Matthaei at the National Institutes of Health used a precise and logical series of experiments to “crack the code”.

• They were among the first to characterize specific coding sequences.

• Made possible by advancements that:

• Allowed protein synthesis in vitro

• Synthesizing RNA strands in vitro

Diagram adopted from Human Genetics concepts and Application 9th ed


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Deciphering the Code


https://profiles.nlm.nih.gov/ps/access/JJBCBS.pdf

https://profiles.nlm.nih.gov/ps/access/JJBCBR.pdf

8 Characteristics of the Genetic Code

1. Written in linear form of ribonucleotide bases (mRNA).

2. Each word consists of 3 ribonucleotide letters which (triplet code- codon) specifies one amino acid.

3. The code is non-overlapping

4. The code is degenerate, given amino acid can be specified by more than one triplet codon.


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The Genetic code

Diagram adopted from Internet source

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5. The code is commaless; Once translation of mRNA begins, the codons are read one after the other with no breaks between them (until a stop signal is reached).

6. The code contains 1 start and 3 stop codons

7. The code is (nearly) universal. With only minor exceptions, a single coding dictionary is used by almost all viruses, prokaryotes, archaea, and eukaryotes.

8. The code is unambiguous- each triplet specifies only a single amino acid.


8 Characteristics of the Genetic Code

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The Genetic Code Uses Ribonucleotide

Bases as “Letters”


• Even though genetic information is stored in DNA, the code that is translated into proteins resides in RNA.

• How only four nucleotides could specify 20 the amino acids?

Diagram adopted from William S and Klug. Concept of Genetics, 10th ED 100



• List down the characteristics of DNA that you know?

• List down the characteristics of RNA that you know?

• Explain how information is passed from DNA to the genetic code?

• Explain how Gene is translated?


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2. Evidence for the Triplet Code

• How Many RNA Bases Specify One amino acid code; 20 amino acids code?

• If a codon consisted of only one mRNA base?

• Two base, for example, provides only 16 unique code words (42). Not enough..!

• A triplet code yields 64 words (43) and therefore is sufficient for the 20 amino acids.

• A four-letter code (44), which would specify 256 words.


Theory

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• Experimental work of Francis Crick, Leslie Barnett, Brenner and R. J. Watts-Tobin provided the solid evidence for a triplet code.

• The insertion of a single nucleotide shifts all subsequent triplets out of the reading frame (frameshift mutations).

• They exposed the chemicals that add or remove one, two, or three bases at the rII locus of bacteriophage T4 and examine their reproduction on E. coli K12 (viral gene is well known).


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• Alteration in DNA sequence happen due to disruption of the reading frame.

• However, continues experiment shows no huge variation in the protein production and the reproduction ability of T4 phage… WHY?

• The result was a protein with a stretch of the wrong amino acids, like a sentence with a few words in the middle that are misspelled



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https://profiles.nlm.nih.gov/ps/access/JJBCBT.pdf

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3. Evidence for a Non-Overlapping Code

• If code was overlapping:

• Amino acid sequences would be restricted- Sydney Brenner

• Translation would be too complex to be efficient- Francis Crick


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• Consider a hypothetical mRNA sequence:

AUGCCCAAG

• If the genetic code is triplet and a DNA sequence is “read” in a nonoverlapping manner, then this sequence has only three codons and specifies three amino acids.

• Do you agree?

• However, if the DNA sequence is overlapping, the sequence specifies seven codons.


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• Even though the genetic code is non-overlapping, it is possible to read any DNA or RNA sequence in three different reading frames, depending upon the “start” base.



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4 & 5. Evidence for a Commaless and Degenerate Code

• Crick hypothesized, on the basis of genetic evidence, that the code would be commaless.

• Only 20 of the 64 possible codons would specify an amino acid and that the remaining 44 would carry no coding assignment. What do you think?

• More than one codon specifies the same amino acid.

• Crick’s frameshift studies suggest that wild-type function is restored when there is, (+) with (-); (++) with (- -); and (+++) with (- - -); the original frame of reading is restored.


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6. The code contains 1 start and 3 stop codons

• Chemical analysis eventually showed that the genetic code includes directions for starting and stopping translation.

• The codon AUG signals “start,” and the codons UGA, UAA, and UAG signify “stop.”


AUG

UGA

UAA

UAG

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7. The code is (nearly) universal

• All life evolved from a common ancestor, hence all species use the same mRNA codons to specify the same amino acids.

• Do you think there is exceptions to the universality of the genetic code?

• Mitochondria

• Certain single-celled eukaryotes (ciliated protozoa)

• These deviations may be tolerated because they do not affect the major repositories of DNA.


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8. The code is unambiguous

• Which codons specify which amino acids?

• Marshall Nirenberg and Heinrich Matthaei experiments.



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Marshall Nirenberg and Heinrich Matthaei experiments

• Synthesized simple mRNA molecules were added to test tubes that contained all the chemicals and structures needed for translation, extracted from E. coli.

• The 1st synthetic mRNA they made had the sequence UUUUUU…. Result: Peptides of phenylalanine.

• Conclusion: The codon UUU specifies the amino acid phenylalanine


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• The 2nd experiments AAAAAA….. Result: Peptide of lysine

• The 3rd experiments CCCCCC…. Result: Peptide for Proline.

• GGGGGG…. was unstable, so this part of the experiment could not be done.

• Next to prove other codon-amino acid pairs hence researchers synthesized chains of alternating bases


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• Synthetic mRNA of sequence AUAUAU . . . introduced codons AUA and UAU alternating Isoleucines and Tyrosines, but which one is which?

• Another experiment with a more complex sequence answered the question.

• The mRNA UUUAUAUUUAUA, UUU codes for phenylalanine, AUA code for isoleucine.

• Hence if AUA codes for isoleucine, then UAU must code for tyrosine… right?


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8. The code is unambiguous

• Sixty of the possible 64 codons specify particular amino acids

• Three indicate “stop,” and one encodes both the amino acid methionine and “start.”

• This means that some amino acids are specified by more than one codon.

• For example, both UUU and UUC encode phenylalanine.


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The Genetic code

Diagram adopted from Internet source

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The Genetic code

• Almost all amino acids are specified by two, three, or four different codons.

• Three amino acids (arginine, serine, and leucine) are specified by six codons.

• Methionine and tryptophan are encoded by single codons.

• In many codons specifying the same amino acid (synonymous codons), the first two positions are the same but the third position differs (Proline); degenerate


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Wobble hypothesis

• Crick postulated the wobble hypothesis.

• He predicted that the initial two ribonucleotides of triplet codes are often more critical than the third member in attracting the correct tRNA.

• He hypothesized that hydrogen bonding at the third position of the codon-anticodon interaction would be less spatially constrained.

• In addition, it may also need not to adhere strictly to the base-pairing rules.


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Wobble hypothesis

• 61 different tRNAs could theoretically exist, one for each codon that specifies an amino acid, however only 49 different genes encode tRNAs.

• This is because the same type of tRNA can detect synonymous codons that differ only in whether the wobble (third) position is U or C.

• The same type of tRNA, for example, binds to both UUU and UUC codons, which specify the amino acid phenylalanine.

• Synonymous codons ending in A or G use different tRNAs


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• The genetic code is linear

• The genetic code is triplet code- codon

• The code is non-overlapping

• The code is degenerate

• The code is commaless

• The code contains 1 start and 3 stop codons

• The code is (nearly) universal.

• The code is unambiguous

Summary- The Genetic Code


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1. In the film Jurassic Park, which is about cloned dinosaurs, a cartoon character named Mr. DNA talks about the billions of genetic codes in DNA. Why is this statement incorrect?


2. Titin is a muscle protein named for its size. its gene has the largest known coding sequence of 80,781 DNA bases. How many amino acids long is it?

3. An extraterrestrial life form has a triplet genetic code with five different bases. How many different amino acids can this code specify, assuming no degeneracy?

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4. Explain the work of Francis Crick at al. on how did they prove a codon is triplet.


5. What is Marshall Nirenberg and Heinrich Matthaei experiments is about? Explain in details.

6. What is a Wobble hypothesis?

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2.4 Compare the nuclear DNA With

The Mitochondrial DNA


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Mitochondrial DNA and the genetic code

• Mitochondria contain their own unique DNA, which in humans consists of 16 kb of circular dsDNA

• This makes 37 essential genes


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Mitochondrial DNA and the genetic code

• Mitochondrial DNA code for:

• 22 mitochondrial (mt) tRNAs

• Two mt rRNAs

• 13 proteins synthesized by the mitochondrion’s machinery; subunits of the oxidative phosphorylation pathway.


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• Codon/anticodon pairings show more ‘wobble’ pairings than in the process originating in the nucleus due to unusual mt tRNA sequences

Nuclear DNA vs Mitochondrial DNA


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Mitochondrial inheritance

• Mitochondrial DNA (mtDNA) is only maternally inherited

• Sperm mtDNA (mid-piece) normally will be degraded after fertilization.

• Hence, affected males cannot transmit the disease to their offspring, single ancestral lineage

• Used to track back the evolutionary history of human



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Mitochondrial Eve.mp4

Mitochondrial Eve.mp4

Mitochondrial inherited disease

• Leber hereditary optic neuropathy- an inherited form of vision loss

• Mitochondrial encephalomyopathy, lactic acidosis and stroke-like syndrome (MELAS)

• Myoclonus with epilepsy and with ragged red fibres (MERRF).


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2.4 Compare the nuclear DNA WIth

The Chloroplast DNA


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Chloroplast DNA and the genetic code

• Chloroplast DNA (cpDNA) has between 100 and 225 kb in length.

• It is circular and double stranded.

• The size of cpDNA is much larger than that of mtDNA, hence account for a larger number of genes.

• Most cpDNA are non coding and duplications of same DNA sequences


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Chloroplast DNA and the genetic code

• Numerous gene products encoded by chloroplast DNA function during translation within the organelle.

• Genes specific to the photosynthetic function have also been identified.

• Mutations in these genes may inactivate photosynthesis.


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Eg. Ribulose-1-5-bisphosphate carboxylase

• Also known as Rubisco

• This enzyme has its small subunit encoded by a nuclear gene, whereas the large subunit is encoded by cpDNA.


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1. Codon/ anticodon pairing in mitochondria are very wobble.

2. mt DNA is important to make 37 essential genes.

3. Most of the cp DNA are the junk DNA.

4. Mutation in Rubisco gene will cease the photosynthesis in plant.

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Ch2 chromosome structure (In brief)

Science

Transcript of Ch2 chromosome structure (In brief)