From DNA to Proteins

RNA PROTEINStranscription translation

in-text, p. 201

DNA

Proteins are coded for by Genes- long stretches of DNA that code for a particular amino acid sequence

Occurs in the nucleus Occurs in the cytoplasm

Same two steps produce ALL proteins:Example: insulin from pancreas

1) DNA is transcribed to form RNA

– Occurs in the nucleus– mRNA 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 (What enzyme?)

• As in DNA: C pairs with G

• Uracil (U) pairs with adenine (A)

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

The

Terminator

Gene Transcription

mRNAtranscript

Where is the promoter?

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’

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

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

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