D2.2 Gene Expression

Genotype?

All its gentic information

Phenotype

All aspects of structures and functions - its characteristics

What is gene expression

Process of turning genotype into phenotype

Function of the promoter

Is an example of a non-coding DNA with a function:

It is a sequence located near a gene

It is the binding site of RNA polymerase which catalyses covalent bond formation between nucleotides during RNA synthesis

Regulation of transcription by proteins

e.g. TATA box

• A general transcription factor binds to it, indicating to RNA polymerase where transcription should start.

Selectivity is based on DNA-binding proteins

Double helix has two grooves where proteins can bind to specific sequences - ‘major groove’ ‘minor group’

DNA-binding proteins that regulate gene transcription are called transcription factors

Transcription factors can act as activator or repressors which turn genes ‘on’ or ‘off’ by binding to regulatory sequences called enhancers and silencers

Enhancers are sequences to which activators bind and increase rate of transcription

Silencers are sequences to which repressors bind and decrease rate of transcription.

Degradation of mRNA

Mechanism to regulate gene expression after transcription occurs

After transcription, a long sequence of adenine nucleotides is added to the 3’ end of the mRNA.

This is a ‘poly a tail’

It increases stability but is shortened over time.

Shorter it is less likely the mRNA translated and the more likely it is to be degraded by a nuclease

Epigenesis

Epigenesis is the concept that multicellular organisms develop from undifferentiated cells

Differentiation is achieved by the activation of some genes and the silencing of others

• Chemical modifications to DNA and to associated proteins determine which genes are activated or deactivated.

• These modifications are known as epigenetic tags.

• Epigenetic modification does not influence an organism’s genotype, just the phenotype.

• DNA base sequences are not altered.