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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.