GENE EXPRESSION

What is a gene?• Mendel

– Unit of inheritance conferring a phenotype

• Modern definition– Unit of DNA directing

the synthesis of a polypeptide or functional RNA molecule

GENE EXPRESSION

How is a gene expressed?

DNA RNA protein

• DNA RNA– Transcription

• RNA protein– Translation

TRANSCRIPTION

• Synthesis of RNA from a DNA template

• Occurs in the nucleus of a eukaryotic cell

• Similar to DNA synthesis

• Some differences

TRANSCRIPTION

• How is it different from DNA synthesis?• Start at promoter • Unidirectional

– Single strand transcribed

• Continuous (not discontinuous)• Catalyzed by RNA polymerase• Termination sequence• No mismatch repair or ligation• Thymine is replaced with Uracil

TRANSCRIPTION

TRANSCRIPTION

TRANSCRIPTION

TRANSCRIPTION

RNA PROCESSING

• RNA transcripts must be modified prior to transport from the nucleus to the cytoplasm

RNA PROCESSING

• Splicing– Removal of introns (junk)

• Capping– Backwards G-residue

at 5’ end

• Polyadenylation– Addition of

A-residues to 3’ end

TRANSLATION

• Synthesis of a protein from an mRNA template

• Occurs in the cytoplasm of eukaryotic cells

• Requires ribosomes, tRNAs, and various other factors

• Utilizes genetic code

TRANSLATION

Messenger RNA• mRNA• Temporary copy of gene• Information deciphered

using the genetic code• Functional unit is codon

– Three consecutive nucleotides

– “Triplet”

TRANSLATION

mRNA codons

• 3 consecutive nucleotides– 64 different codons

(43)• 61 encode specific

amino acids

• 61 > 20; some duplication

TRANSLATION

mRNA codons• 3 consecutive nucleotides

– One of these (AUG) is a start codon

– 3 are stop codons (UAG, UGA, UAA)

• Terminate translation• Recognized by tRNAs

TRANSLATION

Transfer RNA (tRNA)• Short segments of RNA

– ~80 nucleotides long• Two important regions

– Anticodon• Nucleotide triplet• Complementary to codon of

mRNA– Amino acid attachment site

• Amino acid corresponding to codon is covalently attached to the tRNA

TRANSLATION

Ribosomes

• Cellular organelles

• Workhorse

• Site of translation

• Facilitate interaction between mRNA and tRNA

• Facilitate polymerization of amino acids

TRANSLATION

TRANSLATION

TRANSLATION

MUTATION

Mutation

• Heritable– Change in DNA sequence

• Relatively rare

• Typically occurs during DNA replication or repair

• May affect gene expression

• Spontaneous or mutagen

MUTATION

Mutation• Different effects

– Harmful– Neutral– Beneficial

• Most are neutral or harmful• A minority are beneficial• Ultimate source of genetic

variation– Natural selection acts upon

variation

MUTATION

Single gene mutations• How do mutations

affect gene expression?

• A change in DNA can give rise to a change in RNA codons, which can encode different amino acids

MUTATION

Base substitutions mutations

• Affect only a single base

• “Point mutation”– Affects a single base pair

• One base replaced by another, does not change “downstream” triplet

MUTATION

Base Substitution mutations

• Can cause change in amino acids

• e.g., GAA GTA (glu val)

• Phenotype may be affected

MUTATION

Frameshift mutations

• Insertion or deletion of base(s)– e.g., GGA GAGA (gly glu)

• Alter reading frame– All downstream amino acids altered

• Protein function affected– Effect similar to nonsense mutation

Frameshift