Genes to phenotypes (transcription/translation)

Central dogma of molecular biology

flow of information from DNA to RNA to protein

Transcription

DNA used as a template to make a complimentary RNA transcript in the nucleus

Only the template strand is transcribed

Central dogma of molecular biology

flow of information from DNA to RNA to protein

Transcription

DNA used as a template to make a complimentary RNA transcript in the nucleus

Only the template strand is transcribed

Transcription what actually happens

Ribonucleotides are added to the 3' end of the RNA transcript via the polymerisation reaction

Bases complimentary to those in the DNA are added (U not T with A)

Which enzyme carries out the polymerisation reaction and what happens within the enzyme?

RNA polymerase

The two DNA strands separate to form a transcription bubble

allows RNA-DNA duplex to form, add nucleotides, release the transcript and restore original DNA double helix

What is a promoter?

region of a few hundred bases where the RNA polymerase and associated proteins bind

(TATA box)

Where does transcription stop?

at a terminator region

How is promoter recognition facilitated in bacteria and eukaryotes?

Bacteria - sigma fator protein

Eukaryotes - 6 general transcription factor proteins

Which enzyme to general transcription factors recruit to the transcription site?

RNA polymerase II (Pol II)

What do proteins on the enhancer sequence recruit?

A mediator complex

How many base pairs long is the transcription bubble and RNA-DNA duplex?

Transcription bubble 14 base pairs

RNA-DNA duplex 8 base pairs

Primary transcript

The initial mRNA transcript that is transcribed from a protein coding gene. Also called pre-mRNA.

Prokaryotic primary transcript

messenger RNA

RNA processing step 1

1. 5' caps - made of 7-methylguanosine - helps ribosome recognise the mRNA

RNA processing step 2

Addition of 250 consecutive adenines to the 3' end of mRNA (polyA tail) - transcription termination and export of mRNA to the cytoplasm

RNA processing step 3

RNA splicing

Introns are removed (noncoding) - catalysed by RNA and proteins called a spliceosome

Why introns?

primary transcripts from same gene can be spliced in different ways to yield different protein products

Ribosomal RNA

makes up the bulk of ribosomes (translation)

Transfer RNA

carries individual amino acids for use in translation

Small nuclear RNA

an essential component of the spliceosome

How many amino acids are there?

20

What is a codon?

a sequence of three nucleotides that together form a unit of genetic code in a DNA or RNA molecule.

How many codons are there?

64

Where does translation occur?

Within ribosomes

Structure of a ribosome

Large subunit and a small subunit

Three binding sites = exit site, peptidyl site, Aminoacyl site

Translation step 1

Initiation - initiaion complex moves along mRNA until it encounters first AUG triplet - requires initiation factors which bring up tRNA charged with methionine - forms first peptide bond - then shifted one codon to the right freeing up the A site for the next tRNA

Translation step 2

the tRNA in the E sit is ejected - a covalent bond forms between the amino acid Arg bonded to the tRNA in the A site and the amino acid Val - the subunit then moves down one codon etc...

Translation step 3

Termination - stop codons (UAA, UAG, UGA) - protein release factor binds to site A and the bond connected to the polypeptide of the tRNA breaks - creates carboxyl terminus and completes the chain

what protein does the MC1R gene make?

Melanocortin 1 receptor which plays a role in normal pigmentation

Epigenetics

the study of environmental influences on gene expression that occur without a DNA change

Chromatin remodelling

exposes different stretches of DNA to the nuclear environment

Chemical modification

Prevents gene expression by adding a chemical group (usually methyl) to DNA

Chromosomal level gene regulation

random inactivation in different cells

Transcriptional regulation

regulatory transcription factors can either promote or repress transcription by binding with enhancer sequences or silencers