GENE EXPRESSION

Gene Expression

Our phenotype is the result of the expression of proteins

Different alleles encode for slightly different proteins

Protein variation is the basis for normal phenotypic variation - blue or brown eyes

It is also the basis for abnormal phenotypes- cystic fibrosis

 

In the late 1930s, early 40s, Beadle and Tatum

- working with a mold – Neurospora

- observed that a mutation in a single gene caused the loss of a single enzyme, and that this resulted in a mutant phenotype

This established that genes produce phenotypes through the action of proteins

Awarded the Nobel Prize in Medicine in 1958

Cytoplasm

Nucleus

DNA

Central Dogma

RNA

Protein

Replication

Transcription

Translation

Gene Expression

Together transcription and translation are called gene expression.

The genetic information encoded in the DNA of an embryo includes all of the genes needed to develop and maintain the organism.

Different cell types express different subsets of genes.

Transcription

DNA is used as a template for creation of RNA using the enzyme RNA polymerase.

DNA

5’

3’

5’

3’

G T C A T T C G G

C A G T A A G C C

Transcription

RNA polymerase reads the nucleotides on the template strand from 3’ to 5’ and creates an RNA molecule that looks like the coding strand.

DNA template strand

DNA

5’

3’

5’

3’

G T C A T T C G G

C A G T A A G C C

DNA coding strand

Transcription

The new RNA molecule is formed by incorporating nucleotides that are complementary to the template strand.

DNA coding strand

DNA template strand

DNA

5’

3’

5’

3’

G T C A T T C G G

C A G T A A G C C

G

RNA

5’

GG U C A U U C3’

Two types of nucleic acids

RNA

Usually single-stranded

Has uracil as a base

Ribose as the sugar

Carries protein-encoding information

Can be catalytic

DNA

Usually double-stranded

Has thymine as a base

Deoxyribose as the sugar

Carries RNA-encoding information

Not catalytic

# of strands

kind of sugar

bases used

Types of RNA

Abbrev. Function

mRNA Messenger RNA - encodes protein

rRNA Ribosomal RNA - part of ribosome

- used to translate mRNA into protein

tRNA Transfer RNA - couples the region which binds the mRNAcodon and its amino acid

rRNA is part of ribosome, used to translate mRNA into protein

tRNA is a connection between anticodon and amino acid

Transcription

Occurs in three steps:

Initiation

Elongation

Termination

RNA processing

mRNA transcripts are modified before use as a template for translation:

- Addition of capping nucleotide at the 5’ end - Addition of polyA tail to 3’ end

Important for moving transcript out of nucleus And for regulating when translation occurs

Splicing - the removing internal sequences - introns are sequences removed - exons are sequences remaining

RNA processing

• The process of reading the RNA sequence of an mRNA and creating the amino acid sequence of a protein is called translation.

Transcription

Codon Codon Codon

Translation

DNA

T T C A G T C A G

DNAtemplatestrand

mRNA

A A G U C A G U C MessengerRNA

Protein Lysine Serine ValinePolypeptide(amino acidsequence)

Translation

The genetic code

There is a 3 to 1 correspondence between RNA nucleotides and amino acids.

The three nucleotides used to encode one amino acid is called a codon.

The genetic code refers to the codons that encode each amino acids.

What is the correspondence between the mRNA nucleotides and the amino acids of the protein?

Proteins are formed from 20 amino acids in humans.

Codons of one nucleotide:AGCU

Can only encode 4 amino acids

Codons of two nucleotides:AA GA CA UAAG GG CG UGAC GC CC UCAU GU CU UU

Can only encode16 amino acids

Codons of three nucleotides:

AAA AGA ACA AUA AAG AGG ACG AUGAAC AGC ACC AUC AAU AGU ACU AUUGAA GGA GCA GUA GAG GGG GCG GUGGAC GGC GCC GUC GAU GGU GCU GUUCAA CGA CCA CUA CAG CGG CCG CUGCAC CGC CCC CUC CAU CGU CCU CUUUAA UGA UCA UUA UAG UGG UCG UUGUAC UGC UCC UUC UAU UGU UCU UUU

Allows for 64 potential codons => sufficient!

A codon of three nucleotides determines choice of amino acid

The genetic code is non-overlapping

- All known organisms use the same genetic code. (Rare organisms use one codon for an additional amino acid.)

The genetic code is universal

The genetic code is degenerate

Some codons encode the same amino acid. e.g. GGU, GGC, GGA, and GGG all encode glycineDegeneracy is mostly at the third base of the codon.

Some codons have additional functions

AUG encodes methionine. Methionine can be used within a protein sequence and

is often the first amino acid cueing the beginning of translation.

UAA, UAG, and UGA do not encode an amino acid.These codons signal termination of the protein.