Gr 12 Genes, Gene Expression and Gene Regulation

What is Gene Expression?

Simple Gene Expression

In a multicellular organism, usually every cell contains the complete genome. All genes are present in every nucleus.

However, in any given cell, only some of the genes are actually producing mRNA. Many of the genes are inactive and never do anything.

Other genes only produce mRNA at certain points in the life of a cell or in response to certain conditions.

The production of mRNA is known as gene expression.

Genes that do not produce mRNA and do not contribute to the phenotype are not being expressed.

The non-coding sequence responsible for the initiation of transcription
The core promoter is typically located immediately upstream of the gene’s coding sequence
The promoter functions as a binding site for RNA polymerase (the enzyme responsible for transcription)
The binding of RNA polymerase to the promoter is mediated and controlled by an array of transcription factors in eukaryotes
These transcription factors bind to either proximal control elements (near the promoter) or distal control elements (at a distance)

After RNA polymerase has bound to the promoter, it causes the DNA strands to unwind and separate
The region of DNA that is transcribed by RNA polymerase is called the coding sequence

RNA polymerase will continue to transcribe the DNA until it reaches a terminator sequence
The mechanism for transcriptional termination differs between prokaryotes and eukaryotes

The packaging of DNA with histone proteins results in a greatly compacted structure, allowing for more efficient storage

Organisation of eukaryotic chromosomes can be summarised as follows:

DNA is complexed with eight histone proteins (an octamer) to form a complex called a nucleosome
Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of chromatosomes
These then coil to form a solenoid structure (~6 chromatosomes per turn) which is condensed to form a 30 nm fibre
These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin
Chromatin will then supercoil during cell division to form chromosomes that are visible (when stained) under microscope

Transcriptional activity is regulated by two groups of proteins that mediate binding of RNA polymerase to the promoter

Transcription factors form a complex with RNA polymerase at the promoter

RNA polymerase cannot initiate transcription without these factors and hence their levels regulate gene expression

Regulatory proteins bind to DNA sequences outside of the promoter and interact with the transcription factors

Activator proteins bind to enhancer sites and increase the rate of transcription (by mediating complex formation)
Repressor proteins bind to silencer sequences and decrease the rate of transcription (by preventing complex formation)

The presence of certain transcription factors or regulatory proteins may be tissue-specific