Transcription and translation

What does gene expression involve (3)

transcription

pre-mRNA splicing

translation

where does transcription begin

at the promoter section of the gene where the RNA polymerase binds

promotion recognition sequence

TATA

where does transcription end

termination sequence

which way is upstream

5'

which way is downstream

3'

where is DNA found

in the nucleus

form of RNA that carries information

mRNA

what does transcription require (3)

energy + RNA nucleotides (ATP, CTP, GTP, UTP)

DNA helicase (unwinds DNA)

RNA polymerase (reads DNA)

DNA helicase

an enzyme that unzips the double-stranded DNA helix by breaking the hydrogen bonds between the bases, converting it into single-stranded DNA

RNA polymerase

crucial enzyme that transcribes genetic information from DNA into RNA

6 steps of transcription

DNA is unwound by DNA helicase

3' - 5' strand (transcription strand) read by DNA polymerase

RNA polymerase synthesises a complimentary 5' - 3' RNA strand (using the law of base pairing)

primary product (pre-mRNA) formed

Introns are cut from pre-mRNA and a 5' cap and a poly-A tail are added as a post-transcription modification

RNA strand leaves the nucleus through the nuclear pore

what is transcription initiation

the first stage of a process where a gene's DNA sequence is copied into an RNA molecul

steps of transcription initiation (2)

transcription factors (proteins) and RNA polymerase bind to the promoter region

then RNA polymerase binds to DNA, unwinds and unzips (the weak H bonds between base pairs)

purpose of unzipping the DNA

exposes the bases on the template / coding strand

what is transcription elongation

the stage of gene expression where RNA polymerase moves along a DNA template to synthesize a complementary messenger RNA (mRNA) strand

what happens in transcription elongation (2)

RNA polymerase moves along the template strand unzipping (forming the transcription bubble) and unwinding

RNA polymerase begins to attach complementary nucleotides to the template strand of DNA. this builds the pre-mRNA strand

what is the transcription bubble

the uncoiled section of the DNA

what direction does the RNA molecule go

5'-3' (complementary strand to the 3'-5' direction of the template strand)

what is transcription termination

final step in transcription, where the RNA polymerase stops transcribing a DNA sequence and releases the new RNA transcript

what happens in transcription termination (2)

RNA polymerase reaches the termination sequence site. This region contains a STOP codon (triplet)

the STOP codon attracts release factors (proteins) that cause RNA polymerase to detach from the template strand

product of transcription

pre-mRNA

Post transcriptional modifications (3)

introns are spliced out

5' end methyl cap is added (helps the molecule bind to the ribosomes)

Poly-A-tail is added to the 3' end (increases stability of the molecule)

5' methyl cap

a modified guanine nucleotide that is added to the beginning of eukaryotic messenger RNA (mRNA) during transcription

purpose of 5' methyl cap

his cap is crucial for the mRNA's stability, protection from degradation by enzymes, and its proper function in protein synthesis, as it helps with splicing, nuclear export, and ribosome binding

poly-A-tail

a chain of adenine nucleotides added to the 3' end of messenger RNA (mRNA) molecules in eukaryotes

purpose of poly-A tail

crucial for mRNA stability, transport from the nucleus to the cytoplasm, and efficient translation into protein.

what is alternate splicing

genes can be regulated in different ways so that more than one protein can be produced

2 occurences of different genes being produced

genes can produce different proteins at different stages of development

genes can produce different proteins in different tissues

what percentage of genes are regulated by alternate splicing

30%

how is the production of different proteins triggered

alternative splicing of pre-mRNA molecules in a single gene

what is translation initiation

the first step of protein synthesis where the ribosomal machinery assembles on an mRNA molecule to prepare for protein creation

what happens in translation initiation (2)

a small ribosome attaches to the 5' end of the mRNA molecule and moves along until it reaches the START codon

a tRNA molecule with a complementary anticodon sequence binds to the START codon within the ribosome

what is translation elongation

the stage of translation where the polypeptide chain lengthens by adding amino acids one by one, following the sequence of the mRNA template

what happens in translation elongation (3)

after the tRNA molecule binds, another can bind to the next codon along the mRNa strand that is complementary to the sequence

once two are bound, the ribosome catalyses a reaction in which the amino acids (on top of the tRNA) can join and form a peptide bond

the first tRNA molecule is then ejected from the ribosome and the ribosome and the other tRNA molecule move forward one codon

what is translation termination

the final stage of protein synthesis where the ribosome releases the newly synthesized polypeptide chain

what happens in transcription termination (2)

assembly of the amino acid chain ends when the ribosomes reach a STOP codon

this causes the ribosome to detach and the amino acid chain to be reduced

triplet

set of 3 nucleotides in DNA

codon

set of 3 nucleotides in mRNA

difference between triplet and codon

one in DNA, one in mRNA

what is degeneracy

multiple mRNA codons may encode for the same amino acid

what enzyme ensures the correct AA is bound to the tRNA molecule

highly specific synthetase enzymes

what does binding of AA to tRNA require

ATP

what does the ribosome act as

enzyme - catalyses protein synthesis when reading codons

how many and what binding sites does the ribosome have

3, A site, P site, E site (exit)

what is the A-site

one of three binding sites on a ribosome where transfer RNA (tRNA) molecules attach during protein synthesis (translation)

what is the P-site

a key binding site on the ribosome for tRNA during protein synthesis, specifically holding the tRNA that carries the growing polypeptide chain

how do tRNA molecules interact with the A and P sites (5)

once A site is occupied by tRNA molecule its AA peptide chain is transferred from the P site to the A site

the tRNA molecule in the P site leaves the ribosome

P site is now empty

Ribosome moves along a codon. tRNA with growing peptide chain now occupies the P-site

A site is now empty and can receive the next activated tRNA with its AA

where does the STOP codon occupy

A site

what does the peptide chain take up once it is released

tertiary site

Post translational modifications (4)

folding the amino acid chain into specific shapes

adding new functional groups

chain cleaved and cut into smaller pieces

bonds between R-groups can be catalysed to be generated

types of specific post translation modifications (5)

Proteolysis - breaking off some

phosphorylation - adding phosphate

glycosylation - adding carbohydrate

ubiquination - ubiquitin added

lipidation / prenylation - lipid added

polysome (polyribosome)

length of mRNA with many (100+) ribosomes attached to it

why are polysomes important

allows hundreds of copies of a particular peptide to be made in a short period of time

differences in gene expression between prokaryotes and eukaryotes (6)

circular vs linear

DNA naked vs DNA complexed with histone proteins

comprises unique nucleotide vs DNA contains many repeated sequences

DNA free in cell vs DNA in nucleus

no introns vs yes introns

additional plasmids vs no plasmids

similarities in gene expression between prokaryotes and eukaryotes

both undergo transcription and translation but prokaryotes do both in the cytoplasm, which is faster