Central Dogma; Big Idea 3 , Essential Knowledge 3.A.1 b-c and 3.B.1

DNA RNA

- Double stranded molecule

- Contains thymine

- Contains deoxyribose sugar

- Found only in nucleus

- Single stranded molecule

- Contains uracil

- Contains ribose sugar

- Found in nucleus and cytoplasm

- Made of nucleotides

- Contain adenine, guanine, and cytosine

Does this diagram represent DNA or RNA? …how can you tell?

Central Dogma- DNA to protein DNA serves as a genetic

code for the synthesis (creation) of proteins

We eat food, and that food is reassembled to make US (you are made of proteins, which are made of amino acids)

DNA codes for RNA, which guides the synthesis of proteins (basically in order to read and express genes, it goes from DNA to RNA to protein)

Types of RNA

The 3 main types of RNA are: Messenger RNA (mRNA)

– these travel from the nucleus to the ribosome to direct the synthesis of a specific protein

Ribosomal RNA (rRNA) – help form ribosomes in the cytoplasm (remember, ribosomes help with protein synthesis); reads and decodes RNA

Transfer RNA (tRNA) – transport amino acids (building blocks of protein) to the ribsomes

Central Dogma - Protein synthesis Protein synthesis is the assembly of

amino acids (by RNA) into proteins Involves two steps:

1. Transcription – copying DNA code into mRNA; occurs in nucleus

2. Translation – reading the mRNA code and assembling amino acids into a polypeptide chain (protein); occurs in cytoplasm

Transcription overview

Performed in nucleus by mRNA mRNA “reads” single DNA strand and

forms the complementary copy (replacing thyamine with uracil)

Chinese characters transcribed to English alphabet:

Summary of Transcription

The three stages of transcription: Initiation (RNA polymerase attaches to the DNA) Elongation (mRNA is created) Termination (DNA zips back together because of

the hydrogen bonds holding the base pairs together)

Vocab

The DNA sequence where RNA polymerase attaches is called the promoter; in prokaryotes, the sequence signaling the end of transcription is called the terminator

The stretch of DNA that is transcribed is called a transcription unit

Transcription

First step of the central dogma involves the creating of mRNA from DNA

Occurs in nucleus 1. Initiation – The DNA is

unzipped in the nucleus (by the enzyme helicase) RNA polymerase

(another enzyme) binds to a specific section where an mRNA will be synthesiszed.

Transcription factors mediate the binding of RNA polymerase and initiation of transcription

Transcription cont.

2. Elongation - Uracil is used instead of thymines when the bases pair up and mRNA is made The mRNA then gets a 5’ cap

on one end, a poly A tail on the other 3’ end (which is just a bunch of adenines in a row)

We get rid of introns, which are areas that don’t code for a gene (exons are areas that do code for a gene) and then we splice together the mRNA (using something called slicesomes)

3. Termination - The mRNA takes the copied code into the cytoplasm for protein synthesis

Practice

Please create a strand of mRNA from this template strand of DNA (write this in your notes):

TTAACGCATGCATAC

Translation

Translation occurs in ribosomes (in cytoplasm)

All three types of RNA work together during translation to produce proteins

Transcribed Chinese words translated to English words:

Translation

Once the mRNA is made, it moves out into the cytoplasm and attaches to a ribosome (can be located on the E.R.)

When it connects to the ribosome, the code is read and makes a protein through a process called translation

tRNA act as interpreters of the mRNA

Molecules of tRNA are not identical: Each carries a specific amino

acid on one end Each has an anticodon on the

other end; the anticodon base-pairs with a complementary codon on mRNA

Steps of Translation 1. mRNA leaves the nucleus 2. mRNA attaches to a ribosome

(between the 2 subunits, which are made of protein and rRNA)

3. tRNA molecules bring amino acids (building blocks of protein) to the ribosome

4. Every 3 letters in the mRNA code for a single amino acid – 3 bases form a “codon” The tRNA has a 3 letter message

that matches the codon on the mRNA, called the ANTICODON

5. Amino acids get linked together in a “polypeptide chain”, which form a protein

6. The chain folds into a 3-D protein (looks kind of like a 3 leaf clover)

*DNA – mRNA – ribosome - amino acids are brought by tRNA – polypeptide chain – protein

Targeting Polypeptides to Specific Locations

Two types of ribosomes are in the cell: free ribsomes (in the cytosol) bound ribosomes (attached to

the ER)

Free ribosomes mostly synthesize proteins that function in the cytosol

Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell

Ribosomes are identical and can switch from free to bound

Ribosomes cont.

A ribosome has three binding sites for tRNA: The A site holds the

tRNA that carries the next amino acid to be added to the chain

The P site holds the tRNA that carries the growing polypeptide chain

The E site is the exit site, where discharged tRNAs leave the ribosome

Termination of Translation Termination occurs when a

stop codon (see the stop codons in the picture) in the mRNA reaches the A site of the ribosome

The A site accepts a protein called a release factor

The release factor causes the addition of a water molecule instead of an amino acid

This reaction releases the polypeptide, and the translation assembly then comes apart

The Genetic Code DNA is a three base

code (eg. ATC). Three bases form a

“codon”. DNA codons are

converted into mRNA codons and are then interpreted by the gentic code.

DNA->mRNA->Amino Acids->Protein

Genetic Code cont.

There are 20 amino acids, but only 4 different nucleotide bases

they can combine in so many different ways, that they can create over 10,000 different proteins

Practice

The genetic code is a set of rules (see the chart) used to specify which amino acid is used during protein synthesis

Here is a chart of the genetic code ->

DNA codon: TAC mRNA: Amino Acid

More Practice

DNA: TACGGGTCTGGCATT

mRNA:

Amino Acid Sequence:

Evolution of the Genetic Code

The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals

Genes can be transcribed and translated after being transplanted from one species to another

Concept 17.6: Comparing gene expression in prokaryotes and eukaryotes reveals key differences Prokaryotic cells lack a

nuclear envelope, allowing translation to begin while transcription progresses

In a eukaryotic cell: The nuclear envelope

separates transcription from translation

Extensive RNA processing occurs in the nucleus

Mutations – a permanent change in a cell’s DNA Point mutations involve a chemical

change in just one base pair and can be enough to cause a genetic disorder (a point mutation where one base is exchanged for another is called a substitution) The change of a single nucleotide in a

DNA template strand can lead to production of an abnormal protein

Base-pair substitution can cause missense or nonsense mutations Missense mutations still code for an

amino acid, but not necessarily the right amino acid

Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein

Missense mutations are more common

*Ex – can cause some cancers, attributes to Tay-Sachs symptoms, color blindness

Mutation cont. Frameshift mutations – change

the multiples of three from the point of insertion or deletion and change the “frame” of the amino acid sequence Insertions are additions of a

nucleotide to the DNA sequence

Deletion – the loss of a nucleotide

These mutations have a disastrous effect on the resulting protein more often than substitutions do

Ex – can be a cause of cystic fibrosis, colorectal cancer, Crohn’s disease (inflammatory bowel disease)

Insertions and Deletions

Insertions and deletions are additions or losses of nucleotide pairs in a gene

These mutations have a disastrous effect on the resulting protein more often than substitutions do

Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation

Mutations Can be caused spontaneously

or by mutagens (certain chemicals or radiation)

Can be found in somatic cells or gametes with different results Somatic cells will pass the

mutation to all its daughter cells – can be a cause of cancer in the body

Gametes don’t necessarily affect the function of the organism, but may drastically affect their offspring

Can be good, bad, or neutral (may not even know you have it until there is a change in the environment)

Mutagens Spontaneous mutations

can occur during DNA replication, recombination, or repair

Mutagens are physical or chemical agents that can cause mutations

Ex – ultraviolet light, radiation, benzene (an industrial solvent found in synthetic rubber and some dyes), virus’s, sodium azide (found in car air bags)