Transcription

Do all Eukaryotic genes contain introns?

Nope some are fine as they are

Do prokaryotes add a cap?

5' end of transcript has a triphosphate, rather than a methylated cap, but still no official processing

Describe the tail type of processing done to mRNA after transcription

Addition of 100-200 Adenosines to the 3' end, known as the poly-A tail. NOT encoded by the gene. First, ribonuclease cleaves the primary transcript to form a new 3' end (sequence AAUAAA is encoded 11 to 30 nucleotides upstream of the position where the tail is added). Then poly-A polymerase adds As onto this new 3' end. Thought to stabilize mRNA (prevent degradation, increase half-life) and aid efficiency of translation

If some genes need huge areas spliced out (consuming lots of energy), why do we do it in the first place?

Allow alternative splicing --> diversity in products from same gene --> related to function = fewer genes but increased products. More introns more complexity (highest in mammals) Genome size does not correlate with complexity E.g Antibody production No antigen found = splicing for hydrophobic production to anchor to membrane Antigen found = splicing for hydrophilic production to release antibodies

What is trans-splicing?

Allow for trans-splicing: a form of alternative splicing in which an exon from one transcript can be joined to an exon from a different transcript (from 2 genes) Not as common, combines exons

If a mutation occurs between two genes, what are the effects?

Frame-shifts in 1 gene, do not affect other genes since initiation of genes is independent of others (no downstream effect)

Describe the process of transcription in prokaryotes Anything special with enzyme?

Holoenzyme (with core RNA polymerase and sigma factor to bind it to promoter)

Describe the cap type of processing done to mRNA after transcription

Only in Eukaryotes 1) Addition of methylated cap at the 5' end. A special capping enzyme adds a guanidine triphosphate in reverse orientation to the 5' end after polymerization of the transcript's first few nucleotides. This G is NOT encoded by the gene! Methyl transferases then add methyl groups to the backward G and to one or more of the succeeding nucleotides in the RNA. Critical for efficient translation of mRNA. First nucleotide has 3 phosphase since usually two are usually taken off for energy. 5'->5' connection Also some methylation occurs at the 5' end of the nucleotides

Subtle difference between primary transcript and mature mRNA between pro and eu?

Pro: Primary trans = mature mRNA Eu: Primary trans does not equal mature mRNA (since undergoes splicing and processing)

Relate WYSIWYG to prokaryotes and eukaryotes

Prokaryotes are WYSIWYG since the mRNA is it, no modification and related directly to the protein's that will be made Eukaryotes aren't since modifications such as alternative splicing take place.

How is transcription terminated in prokaryotes?

RNA is ejected out when hydrogen bonds reform between DNA strands 1) Terminator region base pairs with itself into a hairpin, when RNA pol goes across this, it destabalizes and disattaches. 2) Rho attaches to mRNA, making it into a hairpin, destabalizing and disatachign - Rho independent - no protein involved - Rho dependent - protein involved

Players in the transcription process?

RNA polymerase - the enzyme that catalyzes transcription. It doesn't need a primer (unique to it). It uses whatever strand but goes 5'->3' so can orientate based on that. Promoters - DNA sequences near the beginnings of genes that signal RNA polymerase where to begin transcription Terminators - Sequences in the RNA products that tell RNA polymerase where to stop (encoded by DNA) NO CODONS IN TRANSCRIPTION mRNA is the same as the coding strand and opposite of the template strand (the strand its working on)

Describe splicing

RNA splicing - removal of introns Exons - sequences found in both a gene's DNA, primary transcript and in the mature mRNA. Exons are (expressed) coding sequences for the protein product Introns - sequences found in a gene's DNA and primary transcript but NOT in the mature mRNA. They are removed (intervenous) from the primary transcript. Splicing is usually carried out by a complex known as the spliceosome, although some RNA transcripts are self-splicing Some genes are spliced out and some aren't, there's variability that differs based on cell and organism.

What is transcription

The process by which the polymerization of ribonucleotides guided by complementary base pairing produces an RNA transcript of a gene Nucleotides are added in the 5'-to-3' direction Uracil is incorporated in place of Thymine in RNA (both pair with Adenine) It's done simultaneously as soon as template available

Why do Eukaryotes do modifications?

To be recognised in translation by ribosomes These processes start as soon as mRNA is available doesn't wait till done