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