Genes and Protein Synthesis

Genes and Protein Synthesis

Chapter 7

One Gene-One Polypeptide Hypothesis• DNA contains all of

our hereditary information

• Genes are located in our DNA

• ~25,000 genes in our DNA (46 chromosomes)

• Each Gene codes for a specific polypeptide

Main Idea• Central Dogma– Francis Crick (1956)

Overall Process• Transcription – DNA to RNA– Takes place

inside nucleus• Translation – Assembly of

amino acids into polypeptide

– Takes place in cytoplasm

DNA molecule

Gene 1

Gene 2

Gene 3

DNA strand

TRANSCRIPTION

RNA

Polypeptide

TRANSLATIONCodon

Amino acid

Key Terms • RNA transcription– Initiation,

Elongation, Termination

• TATA box • Introns, Exons• mRNA, tRNA, rRNA• Translation • Ribosome• Codon• Amino Acids• Polypeptide

DNA RNA

Double stranded Single stranded

Adenine pairs with Thymine Adenine pairs with Uracil (A with U)

Guanine pairs with Cytosine Guanine pairs with Cytosine (C with G)

Deoxyribose sugar Ribose sugar

Remember!! When transcribing DNA to RNA, T from DNA pairs with A for RNA and A from DNA pairs with U for RNA

DNA to Protein • Protein is

made of amino acid sequences

• 20 amino acids

• How does DNA code for amino acid?

DNA molecule

Gene 1

Gene 2

Gene 3

DNA strand

TRANSCRIPTION

RNA

Polypeptide

TRANSLATIONCodon

Amino acid

Genetic Code• Codon

– Three letter code– 5’ to 3’ order– Start codon

(AUG/methionine)– 3 Stop codons

• AA are represented by more than one codon

• 61 codons that specify AA

• 4³ = 64 minus 3 stop codons = 61

Amino acids

Transcription • DNA to RNA• Occurs in nucleus • Three process– Initiation – Elongation – Termination

RNA polymerase

DNA of gene

PromoterDNA Terminator

DNAInitiation

Elongation

TerminationGrowingRNA

RNApolymerase

Completed RNA

Initiation• RNA polymerase binds to DNA• Binds at promoter region

– TATA box• RNA polymerase unwinds DNA• Transcription unit

– Part of gene that is transcribed• Transcription factors bind to

specific regions of promoter • Provide a substrate for RNA

polymerase to bind beginning transcription

• Forms transcription initiation complex

Elongation • RNA molecule is

built– RNA polymerase

• Primer not needed• 5’ to 3’ direction • Template strand is

copied– 3’ to 5’ DNA

• Coding strand– DNA strand that is

not copied• Produces mRNA

– Messenger RNA • DNA double helix

reforms

Termination • RNA polymerase recognizes a termination sequence

– AAAAAAA (polyadenylation)• Nuclear proteins bind to string of UUUUUU on RNA• mRNA molecule releases from template strand

Post-Transcriptional Modifications• Pre-mRNA

undergoes modifications before it leaves the nucleus

• Poly(A) tail– Poly-A polymerase– Protects from RNA

digesting enzymes in cytosol

• 5’ cap– 7 G’s– Initial attachment

site for mRNA’s to ribosomes

• Removal of introns

Splicing the pre-mRNA• DNA comprised of – Exons

• sequence of DNA or RNA that codes for a gene

– Introns • non-coding sequence

of DNA or RNA• Being researched for

code responsible for different splicing arrangements

• Spliceosome– Enzyme that removes

introns from mRNA

Splicing Process• Spliceosome contains a handful of small

ribonucleoproteins– snRNP’s (snurps)

• snRNP’s bind to specific regions on introns

Alternative Splicing• Increases number and variety of proteins

encoded by a single gene• ~25,000 genes produce ~100,000 proteins

Translation• Takes place in

cytoplasm• mRNA to protein • Ribosomes read

codons • tRNA assists

ribosome to assemble amino acids into polypeptide chain

tRNA• Contains – triplet anticodon – amino acid

attachment site • Are there 61

tRNA’s to read 61 codons?

tRNA: Wobble Hypothesis • First two nucleotides of

codon for a specific AA is always precise

• Flexibility with third nucleotide

• Aminoacylation– process of adding an AA

to a tRNA – Forming aminoacyl-

tRNA molecule – Catalyzed by 20

different aminoacyl-tRNA synthetase enzymes

Ribosomes• Translate mRNA chains into amino acids• Made up of two different sized parts – Ribosomal subunits (rRNA)

• Ribosomes bring together mRNA with aminoacyl-tRNAs

• Three sites– A site - aminoacyl– P site – peptidyl– E site - exit

1 Codon recognition

Amino acid

Anticodon

AsiteP site

Polypeptide

2 Peptide bond formation

3 Translocation

Newpeptidebond

mRNAmovement

mRNA

Stopcodon

Translation process • Three stages– Initiation – Elongation – Termination

Initiation• Ribosomal subunits associate with mRNA • Met-tRNA (methionine)

– Forms complex with ribosomal subunits• Complex binds to 5’cap and scans for start codon (AUG) (scanning)• Large ribosomal subunit binds to complete ribosome • Met-tRNA is in P-site

Reading frame is established to correctly read codons

Elongation

• Amino acids are added to grow a polypeptide chain

• A, P, and E sites operate

• 4 Steps

Termination• A site arrives at a stop codon on mRNA – UAA, UAG, UGA

• Protein release factor binds to A site releasing polypeptide chain

• Ribosomal subunits, tRNA release and detach from mRNA

ba

Red object = ?

What molecules are present in this photo?

POLYSOME

Review • What is a gene?• Where is it

located?• What is the main

function of a gene?• Do we need our

genes “on” all the time?

• How do we turn genes “on” or “off”?