From DNA to Protein

Chapter 13

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

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

13.1 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

Fig. 13.3, p.198

Fig. 13.3, p.198

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

Fig. 13.3, p.198

direction of transcription

growing RNA transcript

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

Post-Translational RNA Modification

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

13.2 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

Genetic Code: RNA Triplets

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

13.3 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

13.4 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

13.5 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

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

Fig. 13.11, p.206

mRNA

Final protein

cytoplasmic pools of amino acids, ribosomal subunits, and tRNAs

Convergence of RNAs

Transcription Assembly of RNA on unwound regions of

DNA molecule

At an intact ribosome, synthesis of a polypeptide chain at the binding sites for mRNA and tRNAs

Translation

mRNA processing

mature tRNA

ribosomal subunits

mature mRNA transcripts

proteins

tRNA rRNA SUMMARY:

Protein

Synthesis