Protein Synthesis

Transcription and Translation

The Central Dogma

The information encoded with the DNA nucleotide sequence of a double helix is transferred to a mRNA molecule.

The mRNA molecule travels out of the nucleus and attaches to a ribosome

Using the RNA nucleotide sequence and the genetic code, the ribosome assembles a protein

The Central Dogma (brief)

DNA is copied to mRNA

mRNA is used as blueprint to make protein

DNA Protein: in 3 easy steps!

1. Transcription

2. RNA modification

3. Translation

Genes and DNA A gene is a specific sequence of

DNA nucleotides

For each specific protein used by a cell, there is a specific DNA sequence (gene) located on a chromosome

1 gene 1 polypeptide

RNA Structure

RNA (ribonucleic acid) are nucleotides very similar to DNA

Nitrogenous bases include Cytosine, Guanine, Adenine, and Uracil (instead of Thymine)

Form three basic structures mRNA – messenger RNA rRNA – ribosomal RNA tRNA – transfer RNA

RNA vs DNA RNA has an oxygen on

the 2’ carbon of the ribose sugar

RNA vs DNA

RNA is single stranded, DNA is double stranded

GCAT vs CUGA

Types of RNA molecules

mRNA (messenger): Relays DNA sequence information to ribosome

rRNA (ribosomal): Combines with proteins to form ribosomes

tRNA (transfer): Acts as bridge between nucleotide sequence and

growing polypeptide chain

Transcription

The process by which the nucleotide base sequence of a DNA molecule is copied into a mRNA molecule

3 steps: Initiation Elongation Termination

Proteins required: RNA polymerase Transcription factors

RNA Polymerase

Creates a mRNA molecule complimentary to template strand of DNA

Works in the 5’ 3’ direction

Requires transcription factors to begin its work

Initiation

Proteins called transcription factors bind to DNA region upstream from gene

Proteins bind to region called promoter

RNA polymerase attaches to double helix at beginning of gene

Elongation

RNA polymerase creates a mRNA molecule with bases complimentary to the template strand

Template strand = Anti-sense strand

Termination

RNA polymerase reaches end of gene and detaches from double helix

mRNA transcript is released

Animations

Transcription showing full complex Transcription – cool sounds

Sense or Anti-sense?

The sense strand of a gene has the same base sequence as the mRNA transcript

The anti-sense strand is used as the template

Transcript Modification

Before a mRNA transcript exits the nucleus it is modified in 3 three (tres) ways…

1. Addition of 5’ cap

2. Addition of poly-A tail

3. Removal of introns

5’ cap and poly-A tail

Protective cap is placed on 5’ end

A long repetitive sequence of adenine nucleotides are added to 3’ end, also for protection

mRNA splicing Not all of a transcribed DNA sequence will be

translated Genes are composed of introns and exons Introns are removed from mRNA transcripts by

splicosomes

Transcription Review

1. How is RNA polymerase similar to DNA polymerase III? How are they different?

2. Will the mRNA transcript have the same nucleotide sequence as the sense or anti-sense strand of DNA?

3. How are RNA and DNA different?

4. Name 3 things that happen during mRNA modification.

Translation

messenger RNA (mRNA) is decoded at a ribosome to produce a specific polypeptide according to the rules specified by the genetic code.

4 steps: Activation Initiation Elongation Termination

Requires: Ribosomes (rRNA + proteins), mRNA, tRNA, and amino

acids

Activation

Amino acid is joined with the correct tRNA

Reaction catalyzed by aminoacyl-tRNA-synthetase

Occurs continuously

tRNA - transfer

Specified amino acids are attached to tRNA

each anti-codon corresponds to the amino acid specified by the genetic code

Each tRNA has an anti-codon (3 nucleotides)

Anti-codon region base pairs with mRNA trascript

Initiation

Small ribosome subunit recognizes start sequence on mRNA and binds to it

Start codon, AUG, is recognized by tRNA carrying a Methionine amino acid

Large subunit completes the complex

Elongation

Ribosome moves down the mRNA in a 5’ 3’ direction

Every three mRNA nucleotides another amino acid is added to the growing polypeptide

3 steps: Codon recognition Peptide bond formation Translocation

Elongation: Codon Recognition

When the appropriate tRNA anticodon H-bonds to a mRNA codon at the ribosomal complex

Elongation: Peptide Bond Formation

A peptide bond is created between polypeptide chain and new amino acid

polypeptide is transferred to incoming tRNA

Elongation: Translocation

Ribosome shifts 3 nucleotides (reading frame) down mRNA transcript

tRNA unattached to polypeptide is released

Elongation

Translocation

Codon Recognition

Peptide Bond Formation

Termination

The end of the mRNA coding sequence is reached Stop codon is recognized by a release factor Ribosome complex dissociates, protein is released

The Genetic Code

Each codon corresponds to a specific amino acid

Degenerate 64 possible codons only 20 amino acids Several codons can code for the same

amino acid Ex. CCU, CCA, CCG, CCC = Proline

Universal The same genetic code is used by all living

organisms

The Genetic Code

Animations

Translation Translation – no sound,

basic

Summary

Genetic information is encoded in the sequence of the DNA double helix. To access this information, the DNA sequence must be copied, or "transcribed", by enzymes known as RNA polymerases. The resulting messenger RNA (mRNA) molecules carry the genetic information to the protein-synthesizing machinery, where it is used to define the amino-acid sequence, and therefore the structure and function, of proteins.