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

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

Fig. 13.3, p.198

Fig. 13.3, p.198

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

Genetic Code: RNA Triplets

From DNA to Polypeptide

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

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

A peptide bondforms between thesecond and thirdamino acids (here,valine and leucine).

A peptide bondforms between thefirst two amino acids(here, methionineand valine).

An initiator tRNAcarries the amino acidmethionine, so the firstamino acid of the newpolypeptide chain will be methionine. A second tRNA binds the second codon of the mRNA (here, thatcodon is GUG, so thetRNA that binds carriesthe amino acid valine).

Elongation

The first tRNAis released and theribosome moves tothe next codon in themRNA. A third tRNAbinds to the thirdcodon of the mRNA(here, that codon isUUA, so the tRNAcarries the aminoacid leucine).

A peptide bondforms between thethird and fourthamino acids(here, leucineand glycine)

The second RNAis released and theribosome moves to the next codon. A fourth tRNA binds the fourth mRNA codon (here, that codon is GGG, sothe tRNA carries the amino acid glycine).

Steps d and e are repeated over and over until the ribosome encounters a STOP codon in the mRNA. The mRNA transcript and the new polypeptide chain are released from the ribosome. The two ribosomal subunits separate from each other. Translation isnow complete. Either the chain will join the pool of proteins in the cytoplasm or it will enter rough ER of the endomembrane system (Section 4.8).

Termination

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

Some Causes of Mutations

Natural and synthetic chemicals• Cigarette smoke

Environmental agents

Fig. 13.11, p.206

mRNA

Final protein

cytoplasmicpools ofamino acids,ribosomalsubunits,and tRNAs

Convergenceof RNAs

Transcription Assembly of RNA on unwound regions of DNA molecule

At an intactribosome,synthesis ofa polypeptidechain at thebinding sitesfor mRNAand tRNAs

Translation

mRNAprocessing

maturetRNA

ribosomalsubunits

mature mRNAtranscripts

proteins

tRNArRNA