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From DNA to Proteins
Chapter 13
Byssus: Marvelous Mussel Adhesive
• Mussels live in the surf zone and are subjected to continual pounding by waves
• Mussel binds itself to rocks with threads coated with the protein bysuss
Same two steps produce ALL proteins:
1) DNA is transcribed to form RNA– Occurs in the nucleus– RNA moves into cytoplasm
2) RNA is translated to form polypeptide chains, which fold to form proteins
Steps from DNA to Proteins
Three Classes of RNAs
• Messenger RNA
– Carries protein-building instruction
• Ribosomal RNA
– Major component of ribosomes
• Transfer RNA
– Delivers amino acids to ribosomes
A Nucleotide Subunit of RNA
phosphate group
sugar (ribose)
uracil (base)
Base Pairing During Transcription
• A new RNA strand can be put together
on a DNA region according to base-
pairing rules
• As in DNA, C pairs with G
• Uracil (U) pairs with adenine (A)
Transcription & DNA Replication
• Like DNA replication– Nucleotides added in 5’ to 3’ direction
• Unlike DNA replication– Only small stretch is template
– RNA polymerase catalyzes nucleotide addition
– Product is a single strand of RNA
Promoter
• A base sequence in the DNA that signals the start of a gene
• For transcription to occur, RNA polymerase must first bind to a promoter
Gene Transcription
transcribed DNA winds up again
DNA to be transcribed unwinds
mRNAtranscript
RNA polymerase
Adding Nucleotides
growing RNA transcript5’
3’5’
3’
direction of transcription
Transcript Modificationunit of transcription in a DNA strand
exon intron
mature mRNA transcript
poly-A tail
5’
5’ 3’
3’
snipped out
snipped out
exon exonintron
cap
transcription into pre-mRNA
3’ 5’
Genetic Code
• Set of 64 base triplets
• Codons– Nucleotide bases read in
blocks of three
• 61 specify amino acids
• 3 stop translation
Code Is Redundant
• Twenty kinds of amino acids are
specified by 61 codons
• Most amino acids can be specified by
more than one codon
• Six codons specify leucine
– UUA, UUG, CUU, CUC, CUA, CUG
tRNA Structure
codon in mRNA
anticodon in tRNA
amino acid OH
tRNA molecule’s attachment site for amino acid
Ribosomestunnel
small ribosomal subunit large ribosomal subunit intact ribosome
Three Stages of Translation
Initiation
Elongation
Termination
Initiation
• Initiator tRNA binds to small ribosomal subunit
• Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon
• Large ribosomal subunit joins complex
Binding Sites on Large Subunit
binding site for mRNA
P (first binding site for tRNA)
A (second binding site for tRNA)
Elongation
• mRNA passes through ribosomal subunits
• tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA
• Peptide bonds form between the amino acids and the polypeptide chain grows
Elongation
Termination
• A stop codon in the mRNA moves onto the ribosomal binding site
• No tRNA has a corresponding anticodon
• Proteins called release factors bind to the ribosome
• mRNA and polypeptide are released
Polysome
• A cluster of many ribosomes translating one mRNA transcript
• Transcript threads through the multiple ribosomes like the thread of bead necklace
• Allows rapid synthesis of proteins
What Happens to the New Polypeptides?
• Some just enter the cytoplasm
• Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified
Overview
Transcription
Translation
mRNA rRNA tRNA
Mature mRNA transcripts
ribosomal subunits
mature tRNA
Gene Mutations
Base-Pair Substitutions
Insertions
Deletions
Effect of Base-Pair Substitution
original base triplet in a DNA strand
As DNA is replicated, proofreadingenzymes detect the mistake andmake a substitution for it:
a base substitution within the triplet (red)
One DNA molecule carries the original, unmutated sequence
The other DNAmolecule carries a gene mutation
POSSIBLE OUTCOMES:
OR
Frameshift Mutations
• Insertion
– Extra base added into gene region
• Deletion
– Base removed from gene region
• Both shift the reading frame
• Result in many wrong amino acids
Frameshift Mutation
ARGININE GLYCINE TYROSINE TRYPTOPHAN ASPARAGINE
ARGININE GLYCINE LEUCINE GLUTAMATELEUCINE
mRNA
PARENTAL DNA
amino acid sequence
altered mRNA
BASE INSERTION
altered amino acid sequence
Transposons
• DNA segments that move spontaneously about the genome
• When they insert into a gene region, they usually inactivate that gene
Mutation Rates
• Each gene has a characteristic mutation rate
• Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation
• Only mutations that arise in germ cells can be passed on to next generation