Chapter 12: Protein SynthesisWhat is DNA?

– The DNA of the gene is transcribed into RNA• Which is translated into protein

• The flow of genetic information from DNA to RNA to Protein is called the CENTRAL DOGMA

DNA

Transcription

RNA

Protein

Translation

Chapter 13 Protein Synthesis - Overview

Central Dogma - FLOW IS FROM DNA TO RNA TO PROTEIN

Chapter 12 Protein Synthesis (Overview)

FLOW IS FROM DNA TO RNA TO PROTEIN

• Genes on DNA are expressed through proteins, which provide the molecular basis for inherited traits

• A particular gene, is a linear sequence of many nucleotides

– Specifies a polypeptide (long protein made of amino acids)

Chapter 13 Protein Synthesis (Overview)

Genes - discrete units of hereditary information comprised of a nucleotide sequence found in a DNA molecule.

Chapter 12 Protein Synthesis (Overview)

12.1 RNAThree Types of RNA

• Messenger RNA (mRNA) – carries copies of genes (DNA) to the rest of the cell.

• Ribosomal RNA (rRNA) – make up the ribosomes.

• Transfer RNA (tRNA) – transfers the amino acids to the ribosomes as specified by the mRNA

12-1 Messenger (mRNA)

1. Three main differences between mRNA and DNA

• Ribose instead of deoxyribose• mRNA is single stranded• mRNA has uracil in place of thymine (U

instead of T)

12.1 TRANSCRIPTION: The process of making mRNA from DNA

– Why do you need this process?• Location of DNA?

Nucleus

• Location of Ribosome? Cytoplasm

– mRNA takes code from DNA in the nucleus to the cytoplasm

T A C T T C A A A A T C

A T G A A G T T T T A G

A U G A A G U U U U A G

Transcription

Translation

RNA

DNA

Met Lys PhePolypeptide

Startcondon

Stopcondon

Strand to be transcribed

Transcription produces genetic messages in the form of mRNA12.1 In the nucleus, the DNA helix unzips

• And RNA polymerase lines up nucleotides along one strand of the DNA, following the base pairing rules

– As the single-stranded messenger RNA (mRNA) peels away from the gene

• The DNA strands rejoinRNApolymerase

RNA nucleotides

Direction of transcription

Template Strand of DNA

Newly made RNA

TC

A T C C A A TT

GG

CC

AATTGGAT

G

U

C A U C C AA

U

12.1 Eukaryotic mRNA is processed before leaving the nucleus

– Noncoding segments called introns are spliced out leaving only the coding exons

• A 5’ cap and a poly A tail are added to the ends of mRNA

• Cap and tail protect mRNA

Exon Intron Exon Intron Exon

DNA

Cap TranscriptionAddition of cap and tail

RNAtranscript with capand tail

Introns removedTail

Exons spliced together

mRNA

Coding sequence Nucleus

Cytoplasm

5’ 3’

Direction of transcription

T

C

AT C C A A T

T

GG

C

CA

ATTGGAT

G

U

C A U C C AA

U

12-3 Protein Synthesis - Translation

• Translation is defined as going from mRNA to protein– tRNA which have amino acids attached are

going to the ribosome in the cytoplasm.• What are amino acids? monomers of proteins

• Does the order of amino acids matter? Yes, they must be in order for the protein to fold correctly.

Amino acid attachment site

– How does the correct tRNA (with amino acid attached) bind to the mRNA? The tRNA contains an anticodon which matches up with the mRNA sequence (codon).

Amino acid attachment site

Hydrogen bond

RNA polynucleotide chain

AnticodonAnticodon

Amino acid attachment site

Transfer RNA (tRNA) molecules serve as interpreters during translation

Ribosomes build polypeptides (proteins)– A ribosome consists of two subunits

• Each made up of proteins and a kind of RNA called ribosomal RNA

• Translation at RibosometRNAmolecules

mRNA Small subunit

Growingpolypeptide

Largesubunit

12.3 Translation

– The subunits of a ribosome• Hold the tRNA and mRNA close together during

translation

12.3 Translation

Met Met

Initiator tRNA

1 2mRNA Small ribosomal

subunit

Startcodon

Large ribosomalsubunit

A siteU A CAU C

A U G A U G

P site

Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation

– Once initiation is complete amino acids are added one by one to the first amino acid

– The mRNA moves a codon at a time• A tRNA with a complementary anticodon pairs with

each codon, adding its amino acid to the peptide chain

Animation

1. Take the DNA and transcribe it into mRNAExample: TAC ATA CTA GCG ACT

mRNA:

2. Take the mRNA sequence and decode it using the codon chart.

AUG UAUGAU CGC UGA

AUG = METUAU = TYRGAU = ASPCGC = ARG

Figure out the exact sequence of amino acids needed