Protein synthesis - D.1.2

transcription

3’ to 5’ is the template strand: the one used by the mRNA.

helicase opens the DNA. RNA polymerase catalyses the production of mRNA. The mRNA is essentially the same as the coding strand

What directions do polymerases read genetic material and how do they produce new ones ?

Read from 3’ to 5’

Produce new strand 5’ to 3’

What order is mRNA read in ribosomes ?

5’ to 3’

percentage of expressed genes

25% - in whole body !

genes expression variation:

• genes related to essential metabolic reactions: respiration. always

• at later stage: stem→ specialised

• matured cells → memory B cells

• when signalled to→ insulin production

Point mutation

Mutation in which one nucleotide is changed. Usually happens in dna replication but can also happen in transcription

Types of point mutation

• insertion mutation

• Deletion mutation

Both of these cause frameshift mutation, meaning that all the following codons are affected by the mutation

• substitution mutation: due to code degeneracy, it sometimes is a silent mutation

Élongation of a polypeptide

1. tRNA comes and bonds to A site

2. The large subunit of the ribosome catalyses a reaction between the amino acid held by the tRNA in the A site and the one in the P site ( requires energy )

3. tRNA in P site moves to E site and exits ribosome, and one in A site moves to P site ( requires energy ) which leaves site A available for next tRNA

proteome

All cells used by human body

Proteasome

Protein complex which degrades and recycles unwanted proteins: hydrolyses peptide bonds between amino acids

Stages of translation

• initiation: mRNA attaches to small subunit of ribosome

• Elongation: peptide bonds are for,Ed

• TerminAtion: polypeptide released during termination

The promoter DNA

Non coding region in DNA just before a gene, where proteins known as positive transcription factors can bind, which will allow the RNA polymerase to bind to the dna

Non coding sequences in DNA roles

• regulators of gene expression : promoters

• Introns: parts of dna that are copied in mRNA but removed in post transcriptional modification

• Telomeres: parts at end of chromosomes made of repetitions which protects coding DNA; with every cell division 0, end of the telomeres are lost

• Genes for tRNA and rRNA ; ?

Directionality of transcription

Direction of RNA polymerase when on DNA (5’ to 3’);

Post transcriptional modification of mRNA

• addition of 5’ cap ( so ribosome can work ) and a poly-A-tail ( which protects a molecule from degradation )

• Splicing : excising introns ( splisosomes ) : and ligating exons to form mature mRNA

Introns and exons

Introns In eukaryotic genomes, don’t contain coding information. Exons do; they code for polypeptides

Alternative splicing

A gene can be modified by combing different exons and removing others; this can results in a different range of proteins which function differently. Like this, from a single gene multiple proteins can be made

polysome

several ribosomes may move along the mRNA at the same time. This structure ( mRNA, ribosomes and protein chains ) is called a polysome

Alternative splicing example

Troponin P, a protein involved in cardiac contraction, is sliced one particular way in foetuses which gives it a greater sensitivity to Ça+2 and acidosis. A few weeks after the birth of the baby, the troponin T gene gets spliced differently and these features are lost