7.2

3 parts of RNA

3 parts of RNA

-promoter

-coding sequence

-terminator

Promoter

-The non-coding sequence responsible for the initiation of transcription

-binding site for RNA polymerase

Coding Sequence

-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

Terminator

-RNA polymerase will continue to transcribe the DNA until it reaches a terminator sequence

Antisense Strand

-A gene (DNA) consists of two polynucleotide strands, but only one is transcribed into RNA

-Antisense strand is transcribed

-Aka template strand

-Directed 5’ to 3’ direction

-Contains anti-codons

Sense Strand

-not transcribed

-Directed 3’ to 5’ direction

-Contains codons

Transcription

-Nucleoside triphosphates (NTPs) line up opposite their complementary base partner

-RNA polymerase covalently binds the NTPs together

-this releases 2 additional phosphates

-transcription occurs in 5’ to 3’ direction

3 steps of transcription

-initiation

-elognation

-termination

Initiation

RNA polymerase binds to the promoter and the DNA unwinds

Elognation

RNA polymerase moves along, synthesising RNA in a 5’ to 3’ direction

Termination

-When RNA polymerase reaches the terminator RNA strand detaches and the DNA rewinds

Post-transcriptional events to form mature mRNA

-capping

-polyadenylation

-splicing

Capping

-Addition of a methyl group to the 5’ end of the transcribed RNA

-provides protection against degradation by exonucleases

-allows the transcript to be recognised

Polyadenylation

-the addition of a long chain of adenine nucleotides to the 3’-end of the transcript

-improves the stability of the RNA transcript

Splicing

-removes introns

-exons end up fusing together

Alternative splicing

-removal of exons can results in the formation of different polypeptides from the same gene sequence

Regulatory Proteins

-activator

-repressor

bind to control elements

Activator proteins

They bind to enhancer sites and increase the rate of transcription

Repressor proteins

They bind to silencer sequences and decrease the rate of transcription

Control elements types

-close to the promoter→ proximal elements

-distant to the promoter→ distal elements

most genes have multiple control elements and gene expression is tightly controlled

Typically binds to proximal control elements

transcription factors

Typically binds to distal control elements

regulatory proteins

Histone tails charge

They have a positive charge and hence associate tightly with the negatively charged DNA

Acetylation of Nucleosomes

-neutralises the charge, making DNA less tightly coiled and increases transcription

Methylation of Nucleosomes

-maintains the positive charge, making the DNA more coiled and reduces transcription

Heterochromatin

-inaccessible for transcription

-methylation

Euchromatin

-accessible for transcription

-acetylation

What is epigenetics

The study of changes in phenotype as a result of variations in gene expression levels

Methylation patterns

-have a genetic basis but are influenced by the environment

-control transcription and hence the expression of disease