genes and protein synthesis

GENES AND PROTEIN SYNTHESISChapter 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

Translation Assembly of

amino acids into polypeptide

Using RNA

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 Guanine pairs with Cytosine Guanine pairs with Cytosine

Deoxyribose sugar Ribose sugar

DNA TO PROTEIN Protein is made of amino acid

sequences 20 amino acids How does DNA code for amino acid?

GENETIC CODE Codon

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

AA are represented by more than one codon

61 codons that specify AA

AMINO ACIDS Abbreviated

Three letters

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

ELONGATION RNA molecule is built

RNA polymerase Primer not needed 5’ to 3’ 3’ to 5’ DNA is template

strand Coding strand

DNA strand that is not copied

Produces mRNA Messenger RNA

DNA double helix reforms

TERMINATION RNA polymerase recognizes a

termination sequence – AAAAAAA 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

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 mRNA to protein Ribosomes read

codons tRNA assists

ribosome to assemble amino acids into polypeptide chain

Takes place in cytoplasm

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) –

known as 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

PROKARYOTIC RNA TRANSCRIPTION/TRANSLATION

Throughout cell Single type of RNA

polymerase transcribes all types of genes

No introns mRNA ready to be

translated into protein mRNA is translated by

ribosomes in the cytosol as it is being transcribed