Theme: Molecular basis of heredity. Realization of hereditary information Lecturer: ass. prof....
-
Upload
dustin-adams -
Category
Documents
-
view
215 -
download
0
Transcript of Theme: Molecular basis of heredity. Realization of hereditary information Lecturer: ass. prof....
Theme: Molecular basis of heredity.
Realization of hereditary information
Lecturer: ass. prof. Tetyana Bihunyak
The questions of the lecture:
1. Molecular biology as science 2. The chemistry of nucleic acids2.1. Deoxyribonucleic acid (DNA) 2.2. Ribonucleic acid (RNA) 3. DNA Replication 4. Genetic code5. Gene Expression6. Functions of Proteins in the organism
Molecular biology is the study of biology at a molecular level.
Molecular biology concerns itself with understanding the interactions between the various systems of a cell, including the interactions between DNA, RNA and protein biosynthesis and learning how these interactions are regulated.
Nucleus contains genetic materials encoded in DNA of chromosomes
Only nucleus directs protein synthesis in the cytoplasm via ribosomal RNA (rRNA), messenger RNA (mRNA) and transport RNA (tRNA), which are synthesized in the nucleus
Nucleic acids• The nucleic acids are polymers of
smaller units called nucleotides.
• There are 2 types of nucleic acids:
DNA (deoxyribonucleic acid)
RNA (ribonucleic acid).
Structure of nucleotide
1) five-carbon sugar (deoxyribose C5H10O4 in DNA and ribose C5H10O5 in RNA);
2) a phosphate group (PO4);
3) one of five types nitrogen-containing compounds called nitrogenous bases.
Nucleic acids
The nitrogenous bases are:
• Purines, which are larger – Adenine (A), Guanine (G);
• Pyramidines, which are smaller – Thymine (T), Cytosine (C), Uracil (U).
DNA Basics
• DNA is a long, double-stranded, linear molecule composed of multiple nucleotide sequences.
• DNA contains Adenine, Guanine, Cytosine, Thymine.
DNA Basics
The DNA double helix consists of two complementary DNA strands held together by hydrogen bonds between the base pairs A-T and G-C
A-T base pair
G-C base pair
Chargaff’s rule: The content of A equals the content of T, and the content of G equals the content of C in double-stranded DNA from any species
Hydrogen bonding of the bases
Chargaff's rules said that A = T and G = C. The model shows that A is hydrogen bonded to T and G is hydrogen bonded to C. This so-called complementary base pairing means that a purine is always bonded to a pyrimidine. Only in this way will the molecule have the width (2 nm) dictated by its X-ray diffraction pattern, since 2 pyrimidines together are too narrow and 2 purines together are too wide.
DNA Basics
In the formation of a nucleic acid chain the phosphate group of the nucleotide binds to the hydroxyl group of another, forming what is called a phosphodiester bond, which is very strong.
Watson JamesCrick Francis
The double helix of DNA was discovered in 1953 by Crick F. and Watson J. Nobel prize in 1962
Structure of DNA
Watson and Crick model shows that DNA is a double helix with sugar-phosphate backbones on the outside and paired bases on the inside. This arrangement first the mathematical measurements provided by the X-ray diffraction data for the spacing between the base pairs (0.34 nm) and for a complete turn of the double helix (3.4 nm)
The main biological DNA functions:
• DNA stores hereditary information about primary protein structure.
• The order of the bases specifies the order of amino acids in polypeptides.
• DNA-replication – maintaining genetic information.
DNA Replication
• The Watson and Crick model suggests that DNA can be replicated by means of complementary base pairing.
• During replication, each old DNA strand of the parent molecule serves as a template for a new strand in a daughter molecule.
• A template is most often a mold used to produce a shape complementary to itself
The human cell cycle
G1
S
G2M
G0
Growth and preparation forcell division
Rapid growth and preparation forDNA synthesis
Quiescent cells
phase
phase
phase
phase
Mitosis
S phaseis the synthetic
phase, resulting in duplication of the chromosomes: one
replicated chromosome
consisting of two chromatids
Steps of replication:
1. Unwinding
• The old strands that make up the parent DNA molecule are unwound and "unzipped" (i.e., the weak hydrogen bonds between the paired bases are broken)
• There is a special enzyme called helicase that unwinds the molecule
Helicase
DNA replication
• DNA helicase (enzyme) unwinds the DNA. The junction between the unwound part and the open part is called a replication fork.
• DNA polymerase adds the complementary nucleotides and binds the sugars and phosphates. DNA polymerase travels from the 3' to the 5' end.
Steps of replication:
2. Complementary base pairing
New complementary nucleotides,
always present in the nucleus,
are positioned by the process
of complementary base pairing.
DNA replication• DNA polymerase adds
complementary nucleotides on the other side of the ladder. Traveling in the opposite direction.
• One side is the leading strand - it follows the helicase as it unwinds.
• The other side is the lagging strand - its moving away from the helicase
DNA replication• Problem: it reaches the
replication fork, but the helicase is moving in the opposite direction. It stops, and another polymerase binds farther down the chain.
• This process creates several fragments, called Okazaki Fragments, that are bound together by DNA ligase.
Steps of replication:
3. Joining• The complementary nucleotides
become joined together to form new strands.
• Each daughter DNA molecule contains an old strand and a new strand.
• Steps 2 and 3 are carried out by the enzyme DNA polymerase.
DNA replication• During replication, there are many points along the
DNA that are synthesized at the same time (multiple replication forks).
• It would take forever to go from one end to the other, it is more efficient to open up several points at one time.
A model for DNA replication
DNA replication is termed semiconservative replication because one of the old strands is conserved, or present, in each daughter double helix. Semiconservative replication was experimentally confirmed by Matthew Meselson and Franklin Stahl in 1958.
Accuracy of Replication• The mismatched nucleotide causes a
pause in replication, and during this time, the mismatched nucleotide is excised from the daughter strand.
• The errors that slip through nucleotide
selection and proofreading cause a gene mutation to occur.
• Actually it is of benefit for mutations to occur occasionally because variation is the raw material for the evolutionary process.
Rate of Gene Mutations
• Per cell cycle, gene mutations don't occur very often
• There are several mechanisms that protect against the occurrence of mutations.
• The bases are on the interior of the DNA molecule, and the supercoiling of the molecule in eukaryotes also lends stability.
• During replication, DNA polymerases proofread the new strand against the old strand and detect any mismatched pairs, which are then replaced with the correct nucleotides. In the end, there is usually only one mistake for every one billion nucleotide pairs replicated.
Excision repair
ATGCUGCATTGATAGTACGGCGTAACTATC
thymine dimer
AT AGTACGGCGTAACTATC
ATGCCGCATTGATAGTACGGCGTAACTATC
ATGCCGCATTGATAGTACGGCGTAACTATC
excinuclease
DNA polymerase
DNA ligase
(~30 nucleotides)
ATGCUGCATTGATACGGCGTAACT
ATGC GCATTGATACGGCGTAACT
AT GCATTGATACGGCGTAACT
deamination
ATGCCGCATTGATACGGCGTAACT
ATGCCGCATTGATACGGCGTAACT
uracil DNA glycosylase
repair nucleases
DNA polymerase
DNA ligase
Base excision repair Nucleotide excision repair
DNA repair activity
Life
sp
an
1
10
100 human
elephant
cow
hamsterratmouseshrew
Correlation between DNA repair activity in fibroblast cells from various mammalian species and the life span of the organism
Point Mutations
• Point mutations involve a change in a single nucleotide and therefore a change in a specific codon.
• When one base is substituted for another, the results can be variable. For example, if UAC is changed to UAU, there is no noticeable effect, because both of these codons code for tyrosine. This is called a silent mutation.
• If UAC is changed to UAG, however, the result could very well be a drastic one because UAG is a stop codon. If this substitution occurs early in the gene, the resulting protein may be too short and may be unable to function. Such an effect is called a nonsense mutation. Finally, if UAC is changed to CAC, then histidine is incorporated into the protein instead of tyrosine. This is a missense mutation.
• A change in one amino acid may not have an effect if the change occurs in a noncritical area or if the 2 amino acids have the same chemical properties. In this instance, the polarities of tyrosine and histidine differ; this substitution most likely will have a deleterious effect on the functioning of the protein. Recall that the occurrence of valine instead of glutamate in the beta (B) chain of hemoglobin results in a sickle-cell disease.
Defects in DNA repair or replicationAll are associated with a high frequency of chromosome
and gene (base pair) mutations; most are also associated with a predisposition to cancer, particularly leukemias
• Xeroderma pigmentosum• caused by mutations in genes involved in nucleotide excision repair• associated with a >1000-fold increase of sunlight-induced skin cancer and with other types of cancer such as melanoma
• Ataxia telangiectasia• caused by gene that detects DNA damage• increased risk of X-ray• associated with increased breast cancer in carriers
• Fanconi anemia• caused by a gene involved in DNA repair• increased risk of X-ray and sensitivity to sunlight
• Bloom syndrome• caused by mutations in a DNA helicase gene• increased risk of X-ray• sensitivity to sunlight
• Cockayne syndrome• caused by a defect in transcription-linked DNA repair• sensitivity to sunlight
• Werner’s syndrome• caused by mutations in a DNA helicase gene• premature aging
DNA and RNA differ
RNA is single-stranded (but it can fold back upon itself to form secondary structure, e.g. tRNA)
In RNA, the sugar molecule is ribose rather than deoxyribose
In RNA, the fourth base is uracil rather than thymine.
The major bases found in DNA and RNA
DNA RNA
Adenine Adenine Cytosine Cytosine Guanine Guanine Thymine Uracil (U)
uracil-adenine base pairthymine-adenine base pair
RNA Basics• Messenger RNA carries the genetic code to the
cytoplasm to direct protein synthesis.• 1. This single-stranded molecule (hundreds to
thousands of nucleotides).• 2. mRNA contains codons that are complementary
to the DNA codons from which it was transcribed
• Ribosomal RNA associates with many different proteins (including enzymes) to form ribosomes.
• 1. rRNA associates with mRNA and tRNA during protein synthesis.
• 2. rRNA synthesis takes place in the nucleolus and is catalyzed by RNA polymerase.
Transfer RNA - the adapter
Transfer RNA is folded into a cloverleaf shape and contains about 80 nucleotides.
• 1. Each tRNA combines with a specific amino acid that has been activated by an enzyme.
• 2. One end of the tRNA molecule possesses an anticodon, a triplet of nucleotides that recognizes the complementary codon in mRNA.
Transcription: DNA-Directed RNA Synthesis
• Transcription has three phases:InitiationElongationTermination
• RNA is transcribed from a DNA template after the bases of DNA are exposed by unwinding of the double helix.
• In a given region of DNA, only one of the two strands can act as a template for transcription.
Transcription: DNA-Directed RNA Synthesis - Elongation
• Nucleotides are added by complementary base pairing with the template strand
• The substrates, ribonucleoside triphosphates, are hydrolyzed as added, releasing energy for RNA synthesis.
All organisms use the same genetic codeEach set of three nucleotides codes for an
amino acid = “The Genetic Code”
Genetic code
The Genetic Code is universal
• All organisms use the same genetic code
• Each set of three nucleotides codes for an amino acid = “The Genetic Code”
AUG = Met
The genetic code
– consists of 64 triplet codons (A, G, C, U) 43 = 64– all codons are used in protein synthesis
• 20 amino acids• 3 termination (stop) codons: UAA, UAG, UGA• AUG (methionine) is the start codon (also used
internally)– multiple codons for a single amino acid =
degeneracy– Genetic code is unambiguous. Each triplet codon has
only one meaning– 5 amino acids are specified by the first two
nucleotides only– 3 additional amino acids (Arg, Leu, and Ser) are
specified by six different codons
Gene Expression• The process by which a gene produces a product, usually a
protein, is called gene expression.• DNA not only serves as a template for its own replication, it is also a
template for RNA formation. • Gene Expression in prokaryotes:• transcription, translation. • Gene Expression in eukaryotic cells:• transcription, processing, translation.
Transcription
• It is the first step required for gene expression.
• During transcription, a mRNA molecule is formed that has a sequence of bases complementary to a portion of one DNA strand;
• A, T, G, or С is present in the DNA template,
• U, A, C, or G is incorporated into the mRNA molecule
The process by which a mRNA copy is made of a portion of DNA
Transcription
• Transcription begins at a region of DNA called a promoter.
• A promoter is a special sequence of DNA bases where RNA polymerase attaches and the transcribing process begins. A promoter is at the start end of the gene to be transcribed.
• Elongation of the mRNA molecule occurs as long as transcription proceeds. Finally, RNA polymerase comes to a terminator sequence at the other end of the gene being transcribed.
• The terminator causes RNA polymerase to stop transcribing the DNA and to release the mRNA molecule, now called a RNA transcript
Transcription: DNA-Directed RNA Synthesis - Termination
• Termination: Special DNA sequences and protein helpers terminate transcription.
• The transcript is released from the DNA.
• This Primary Transcript is called the “pre-mRNA”
• The pre-mRNA is processed to generate the mature mRNA
RNA Processing • introns are noncoding portions of the original mRNA tape;
they do not contain information for the sequencing of amino acids in a protein
• exons: portions of the mRNA transcript that code for amino acids
• special molecular splicing complexes cut out sections of introns. Then the remaining portions of the mRNA (exons) tape are spliced together
Translation
During translation, the sequence of codons in mRNA directs the sequence of amino acids in a protein.
Two other types of RNA are needed for protein synthesis.
• rRNA is contained in the ribosomes, where the codons of mRNA are read
• tRNA carries amino acids to the ribosomes so that protein synthesis сan occur.
Codon-anticodon interactions• codon-anticodon base-pairing is antiparallel• the third position in the codon is frequently degenerate• one tRNA can interact with more than one codon (therefore 50 tRNAs)• wobble rules
C with G A with U G with C U with A
5’ 3’
A U G
U A C
3’ 5’ tRNAmet
mRNA
5’ 3’
C U A G
G A U
3’ 5’ tRNAleu
mRNA
wobble base
• one tRNAleu can read two of the leucine codons
The Central Dogma
The Flow of Information: DNA RNA protein
DNA Replication
Transcription Translation
Regulation of Gene expression in prokaryotes
lac operon in E. Coli•Function - to produce enzymes which break down lactose (milk sugar). When lactose (inducer)is present, they turn on and produce enzymes (5) •Two components - repressor genes (1) and functional strutural genes (4)•Promoter (P) - aids in RNA polymerase binding •Operator (O) - "on/off" switch (3) - binding site for the repressor protein •Repressor (lacI) gene - produces repressor protein w/ two binding sites, one for the operator and one for lactose. The repressor protein is under allosteric control - when not bound to lactose, the repressor protein can bind to the operator
Regulation of Gene expression in prokaryotes
lac operon in E. Coli
Operation if lactose is present:
When lactose is present, an isomer of lactose, allolactose, will also be present in small amounts. Allolactose binds to the allosteric site and changes the conformation of the repressor protein so that it is no longer capable of binding to the operator
Operation if lactose is not present:
the repressor gene produces repressor, which binds to the operator. This blocks the action of RNA polymerase, thereby preventing transcription
Proteins are polymers of amino acids
• There are 20 different amino acids in cells that differ only by R groups
• All amino acids contain 2 important functional groups: an amino, - NH2, group and a carboxyl (acid) –СООН, group, both of which ionize at normal body pH
•All enzymes are proteins.• Storing amino acids as nutrients and as building blocks for the growing organism.•Transport function (proteins transport fatty acids, bilirubin, ions, hormones, some drugs etc.).•Proteins are essential elements in contractile and motile systems (actin, myosin).• Protective or defensive function (fibrinogen, antibodies).• Some hormones are proteins (insulin, somatotropin).•Structural function (collagen, elastin).
Functions of Proteins in the organism
Types of proteins• Enzymes -Quicken chemical reactions (surcease
brocks sugar to glucose and fructose)• Hormones - chemical messengers (growth hormone)• Transport –move other molecules (hemoglobin)• Contractive –movement (myosin and actin -allow
muscles to contract)• Protective - healing, defense against invader
(fibrinogen: stops bleeding antibodies: kill bacterial invaders)
• Structural –mechanical support (keratin: hair collagen: cartilage)
• Storage - stores nutrients (ovalbumin: egg white: used as nutrient for embryos)
• Toxins - defense, predations (bacterial diphteria toxin)
• Communication – cell signaling (glicoprotein: receptors on cell surface)
TEST QUESTIONS
1. Which one of the following nucleotides is not present in DNA?A. Thymine. B. Adenine. C. Uracil. D. Cytosine. E. Guanine.2. DNA is duplicated in the cell cycle during the:A. G1 phase. B. S-phase. C. G2-phase. D. Prophase. E. Metaphase.3. If one strand of DNA has the base sequence ATCGTA, what will the complementary strand of
mRNA have?A. TAGCAT B. UAGCAUC. CAGTCTD. ATCGTAE. All of these.4. Which of the following statements concerning transcription is false:A. A gene is transcribed into DNA on RNA molecule by RNA polymerase.B. Both exons and introns are transcribed.C. DNA molecule unwinds at the end of transcriptionD. The codon always represents a single amino acid.E. The chain terminator on the DNA molecule stops the transcription process.