week 19 Flashcards
what two factors damage DNA
high energy radiation and chemicals (mutagens)
describe mitochondrial DNA
mtDNA is found near the membrane, is double-stranded and circular and codes for proteins in the respiratory chain. It is passed through generations through maternal lineage.
describe the difference between RNA and DNA
RNA has ribose instead of deoxyribose and uracil instead of thymine.
how is the RNA transcript modified before it leaves the nucleus
A cap (modified form of G) is added to 5’. A polyA tail is added to the 3’. This protects mRNA from degradation, aids export from the nucleus and helps the mRNA to anchor to ribosomes.
what do nuclear ribonucleoproteins (snRNPs) do
splice out the non-coding regions (introns)
what marks the splice sites on introns
short nucleotide sequences at the end of introns
how is DNA replication made to be a rapid and accurate process
specific base pairing (a-t and g-c) and proofreading. All DNA polymerases have proofreading and detect and remove incorrectly added bases.
what catalyses transcription
the enzyme RNA polymerase, which unwinds the double helix structure and makes a complementary copy
what benefit does splicing have
it helps to increase the range of possible protein products from a gene
what are the stages of transcription
initiation, elongation and termination
describe initiation in transcription
initiation is a site known as a promoter containing a particular sequence of bases. RNA polymerase binds to the promoter.
describe elongation in transcripton
unwinding of the DNA as nucleotides are added and rewinding of DNA after added nucleotides.
describe termination of transcription
appears to be signalled by a sequence of the RNA transcript. after the termination sequence is transcribed, proteins bind to the transcript and cut it from the polymerase. the transcript is released, and the RNA polymerase detaches from the DNA.
what are the types of RNA
mRNA (mitochondrial), rRNA (ribosomal), tRNA (transfer) and small nuclear RNA
what does tRNA do
transfers amino acids to the site of protein synthesis, a ribosome. there are different types of tRNA for each of the 20 amino acids.
describe initiation of translation
30 S subunit of ribosome binds to mRNA at the start codon. an initiator tRNA carrying amino acid Met binds to start codon on mRNA. 50 S ribosome subunit binds forming initiation complex. protein initiation factors + energy are required.
describe elongation of translation
amino acids added one at a time, peptide chain grows from amino towards carboxyl end.
1. codon recognition- bringing correct aminoacyl-tRNA
2. peptide bond formation- new amino acid is joined to growing peptide chain, catalysed by rRNA
3. translocation- ribosome moves tRNA in A site to the P site
what is translocation is translation
stage in elongation. ribosome moves tRNA in A site to the P site. discharged tRNA in P site is moved to E site to leave ribosome requiring energy (GTP)
describe termination of translation
elongation continues until a stop codon appears in A site of the ribosome. the peptide chain is released.
what is the composition of ribosomes
2 subunits- large (50 S) and small (30 S). The two subunits only join when mRNA is present. rRNA is a key component of ribosomes.
what are the binding sites on ribosomes used for translation.
- mRNA binding site (30 S)
- P site (50 S)= binds the growing peptide chain
- A site (50 S)= binds tRNA bound to an amino acid
- E site (50 S)= exit site which discharges the tRNA to leave ribosome
what is aminoacyl- tRNA synthase
required enzyme to load the correct amino acid onto the correct tRNA.
what do polyribosomes do
multiple ribosomes bound to one mRNA allowing multiple copies of polypeptide chains to be made simultaneously from the same mRNA transcript.
what is the genetic code
used for translating nucleotide language into amino acid language, showing the different amino acid possibilities which could be produced from each mRNA sequence
