Protein synthesis

what is transcription

the synthesis of RNA molecules using DNA as a template, production of mRNA

where does transcription take place (euk/pro)

eukaryote: in the nucleus

prokaryote: in the cytoplasm

describe the three stages of transcription

initiation: RNA polymerase binds to DNA at the start of the gene and unwinds the DNA in the section

elongation: RNA polymerase builds a strand of mRNA in the 5’ to 3’ direction using complementary base pairing

termination: a terminator sequence is detected, and mRNA releases and the DNA closes together

what strand does mRNA build onto

The template/ antisense strand

what happens when the right RNA nucleotide is placed?

• it forms a temporary hydrogen bond with the complementary base on the template strand

  • adenine and thymine : two bonds

  • guanine and cytosine: three bonds

why is it important that DNA is stable and doesn’t degrade

• because if it degraded, proteins wouldn’t be able to be produced from it which would stop cell functions

what is translation?

• the synthesis of polypeptides (proteins) from reading mRNA

• mRNA is read by the ribosome and amino acids are added to build a polypeptide

where does translation occur

• once the mRNA moves into the cytoplasm, it is translated at ribosomes which catalyze reactions

  • ribosomes brings mRNA and tRNA together which makes the process happen

what is tRNA

• a single stranded RNA molecule that fold on itself to make to a clover shaper with three loops

• each tRNA attaches to an amino acid

what is an anticodon?

region at the end of the tRNA that pairs to the mRNA because it is the opposite of the codon on the mRNA (complementary)

what is a codon

any three RNA bases in a sequence that codes for the placement of a specific amino acid

how many possible combinations are their of codons

4³ (64)

what happens during initiation in translation

• the mRNA binds to the small subunit of the ribosome

• tRNA charging: it collects the right amino acids in the cytoplasm (enzyme aminoacyl-tRNA sythesase catalyses the binding)

• mRNA has a start codon that links to initiator tRNA that carries methonine

translation - elongation

• ribosome moves along the mRNA one codon at a time

• tRNA attaches at the P site on the start codon

• new tRNA attaches at the A site and a peptide bond forms between the amino acid and transfer to the new tRNA

• the new tRNA moves to the P site

• the old tRNA moves to the E site and exits

what reaction causes bonds in the polypeptide chain

condensation reaction forms the chain of polypeptides (protein)

what is a point mutation

swapping of one base during transcription or translation

example of a point mutation

• sickle cell anaemia: point mutation in the gene that produces a polypeptide in haemoglobin

where does silent mutation occur/ wobble hypothesis

• most likely at the third base in the codon

• some codons code for the same amino acid so it doesn’t matter

• a tRNA molecule can bind to the to the codon based on the first 2 out of 3 bases

what direction do transcription and translation occur in

• transcription

  • RNA polymerase can only build in a 5’ to 3; direction and catalyze the bond between the 3’ end and 5’ end of another

• translation

  • mRna moves through the ribsome in a 5’ to 3’ direction and it only fits to the biding site of the ribsome if oriented that way

what is the promoter

• the region just before a gene

• where RNA polymerase binds in transcription at initiation

• called the TATA box, which only has 2 hydrogen bonds so its easier to break apart and expends less energy

what are transcription factors

proteins that bind to the promoter

• RNA polymerase can’t bind without them and the. gene wouldn’t be expressed

what are enhancers/ silencers

• DNA regions that don’t code for proteins

• they act as biding sites to increase/ decrease rate of transcription

what is an intron

a DNA base sequence only in eukaryotes that gets removed after transcription

• non coding DNA

• they can have controlling sequences

telomeres

repetitive sequences of DNA that protect the ends of chromosomes and ensure DNA is replicated correctly

• they fall off a bit each cell division

• non coding DNA

genes for tRNA and rRNA

• non coding DNA

• code for RNA molecules that fold to form tRNA or rRNA (part of ribosome structure)

exons

• gene regions that code for polypeptides

what are some post TRANSCRIPTIONAL modifications?

• synthesis of pre-mRNA (transcrition)

• addition of a 5’ cap and a poly-A-tail to protect from degradation

• splicing intons and ligating exons to make mature mRNA

what is a spliceosome and how does it work

• snRNP

• base pairs with ends of introns

• cuts out introns by making a loop out of them and then exising it

• ligating (joining) the exons together

what is an example of alternative splicing

• different exons can be ommited and combined

• produces various proteins

• genes that produce troponin T which is for muscle contraction

• in babies the protein in spliced on way which gives it a high sensitvity ro CA2+ but after a few weeks it becomes spliced differently and loses the sensitivity

what is the large ribosomal subunit

• it joins in initiation to complete the assembly of the translation complex

POSt-translation modification

• after polypetides are created they are sent in vesicles to the golgi apparatus

describe how insulin is post-translationally modified

• it begins as pre-proinsulin

• when it is on the rough endoplasmic reticulum (bond ribosome) the signal peptide is removed

• disulfide bridges form between the A and B chain

• proinsulin is moves to the golgi apparatus which removes the c-peptide

proteasomes

• hydrolises proteins by breaking peptide bonds between amino acids

• breaks apart used or damaged proteins to maintain a constant supply of amino acids

when does transcirption occur

interphase

what is the central dogma

• dna can make more dna (replication)

• dna can make rna (transcription)

• rna can make proteins through translation

is mature mRNA shorter or longer?

shorter because you cut out all the introns

translational termination

a stop codon is detected by a protein release factor and the translational apparatus comes apart and releases the finished polypeptide

what is a framshift mutaion

• insertion or deletion of one base that changes all subsequent codons