Chapter 13

From DNA to Protein

Proteins

• All proteins consist of polypeptide chains

– A linear sequence of amino acids

• Each chain corresponds to the nucleotide base

sequence of a gene

The Path From Genes to Proteins

1. Transcription

– Enzymes use the base sequence of a gene as a

template to make a strand of RNA

2. Translation

– Information in the RNA strand is decoded

(translated) into a sequence of amino acids

DNA RNA Proteintranscription translation

Prokaryotes and Eukaryotes

• In prokaryotic cells (no nucleus)

– Transcription and translation occur in cytoplasm

• In eukaryotic cells

– Genes are transcribed in the nucleus

– Resulting mRNA is translated in the cytoplasm

Key Concepts:

INTRODUCTION

• Life depends on enzymes and other proteins

• All proteins consist of polypeptide chains

• Chains are sequences of amino acids that correspond to sequences of nucleotide bases in DNA called genes

• The path leading from genes to proteins has two steps: transcription and translation

Transcription: DNA to RNA

• Two DNA strands unwind in a specific region

• RNA polymerase assembles a strand of RNA

– Covalently bonds RNA nucleotides (adenine,

guanine, cytosine, uracil) according to the

nucleotide sequence of the exposed gene

Three Types of RNA

• Messenger RNA (mRNA)

– Carries protein-building codes from DNA to ribosomes

• Ribosomal RNA (rRNA)

– Forms ribosomes (where polypeptide chains are

assembled)

• Transfer RNA (tRNA)

– Delivers amino acids to ribosomes

RNA and DNA Compared

RNA Base Pairing

phosphate

group

base

(uracil)

sugar (ribose)

Fig. 13.3, p.198

gene region

RNA polymerase, the enzyme

that catalyzes transcription

DNA template

unwinding

newly forming

RNA transcript

DNA template

winding up

Fig. 13.3, p.198

RNA Modification: Alternative Splicing

• Before mRNA leaves the nucleus:

– Introns are removed

– Some exons are removed along with introns;

remaining exons are spliced together in different

combinations

– Poly-A tail is added to 3’ end of new mRNA

The Poly-A Tail

• The longer its poly-A tail, the more time an

mRNA transcript (and its protein-building

message) will remain intact in the cytoplasm

unit of transcription in DNA strand

mature mRNA transcript

exon exon exonintronintron

transcription into pre-mRNA

cap poly-A tail

snipped out snipped out

5' 3'

Key Concepts:

TRANSCRIPTION

• During transcription, the two strands of the DNA double helix are unwound in a gene region

• Exposed bases of one strand become the template for assembling a single strand of RNA (a transcript)

• Messenger RNA is the only type of RNA that carries DNA’s protein-building instructions

The Genetic Code

• Messenger RNA (mRNA) carries DNA’s

protein-building information to ribosomes for

translation

• mRNA’s genetic message is written in codons

– Sets of three nucleotides along mRNA strand

Codons

• Codons specify different amino acids

– A few codon signals stop translation

• Sixty-four codons constitute a highly

conserved genetic code

From DNA to Polypeptide

Variation in Genetic Code

• Variant codons occur among prokaryotes,

prokaryote-derived organelles (such as

mitochondria), and some ancient lineages of

single-celled eukaryotes

Key Concepts:

CODE WORDS IN THE TRANSCRIPTS

• The nucleotide sequence in RNA is read three bases at a time

• Sixty-four base triplets that correspond to specific amino acids represent the genetic code, which has been highly conserved over time

tRNA and rRNA

Function in Translation

• Transfer RNA (tRNA)

– Anticodon binds to mRNA codon

– Also binds amino acid specified by codon

• Different tRNAs carry different amino acids

– tRNAs deliver free amino acids to ribosomes

during protein synthesis

tRNA

rRNA

• Ribosomal RNA (rRNA) and proteins make up

the two subunits of ribosomes

SUMMARY: Protein Synthesis

Three Stages of Translation

• mRNA-transcript information directs synthesis

of a polypeptide chain during translation

• Translation proceeds in three stages

– Initiation

– Elongation

– Termination

Initiation

• One initiator tRNA, two ribosomal subunits,

and one mRNA come together as an initiation

complex

Initiation

Elongation

• tRNAs deliver amino acids to the ribosome in

the order specified by mRNA codons

• Ribosomal rRNA catalyzes the formation of a

peptide bond between amino acids

Elongation

Elongation

Elongation

Termination

• Translation ends when RNA polymerase

encounters a STOP codon in mRNA

– New polypeptide chain and mRNA are released

– Ribosome subunits separate from each other

Termination

Key Concepts:

TRANSLATION

• During translation, amino acids become bonded together into a polypeptide chain in a sequence specified by base triplets in messenger RNA

• Transfer RNAs deliver amino acids one at a time to ribosomes

• Ribosomal RNA catalyzes the formation of peptide bonds between the amino acids

Mutated Genes and

Their Protein Products

• Mutations are permanent, small-scale

changes in the base sequence of a gene

• Common mutations include insertions,

deletions, and base-pair substitutions

Common Gene Mutations

Transposable Elements

• Another type of insertion mutation is caused

by the activity of transposable elements

• Segments of DNA that can insert themselves

anywhere in a chromosome

Some Causes of Mutations

• Natural and synthetic chemicals

– Cigarette smoke

• Environmental agents

– Ionizing radiation

– Nonionizing radiation

Key Concepts:

MUTATIONS IN THE CODE WORDS

• Mutations in genes may result in changes in

protein structure, protein function, or both

• The changes may lead to variation in traits

among individuals