L8 - DNA replication, transcription and translation Flashcards
What are facts about DNA replication?
- copying of DNA to create a new identical strain
- has to be copied before a cell divides
- copied during synthesis phase of interphase
- new cells will need identical DNA strands
- happens at the nucleus of eukaryotes
What is the first step of DNA replication?
- 2 strands are open forming replication forks (Y-shaped)
- using DNA helicase to break the H bonds between the bases
What happens as the DNA strands open at the origin?
replication bubbles form
- prokaryotes have a single bubble
- eukaryotic chromosomes have many bubbles
= the bubbles end up joining together
How are the strands kept separate?
enzyme topoisomerase
single-strand binding proteins attach
- binds further down, chops a strand of DNA which releases stress
- keeps it untwisted
What binds before new DNA strands can form?
primase
- synthesises RNA primers
- allows DNA polymerase to bind
What way does DNA polymerase add nucleotides?
- 5’ to 3’ direction
- antiparallel
What is the leading strand?
The strand that forms a continuous strand of DNA from the point of origin toward the opening replication fork
What is the lagging strand?
strand that forms short small sequences
- as it has to be formed 5’ to 3’
- short strands are called okazaki fragments
What enzyme joins the okazaki fragments together?
DNA ligase
Features of proofreading new DNA
- DNA polymerase makes about 1 in10,000 base pairing errors
- enzymes proofread
- error becomes 1 in 1 billion
How does the enzyme proofread DNA?
- incorporates of incorrect nucleotides at 3’ end
- exonuclease removes mis-paired bases
- continues of DNA replication
What is the semi-conservative model of replication?
- 2 strands of parental molecule separate
- each acts as a template for a new complementary strand
- New DNA consists of 1 parental and 1 new strand of DNA
What is the first function of DNA?
store genetic information
- replication passes the information to daughter cells and new organisms
What is the second function of DNA?
Direct synthesis of proteins
- determines cell structure and function
What are the 2 stages of synthesis?
- transcription
- translation
What is gene expression?
the transformation of DNA information into functional molecules
What is transcription?
production of a section of DNA carrying information related to one or more genes
- as mRNA
Key points for transcription:
- RNA polymerase separates strands
- doesn’t need a primer
- doesn’t need proof reading
- mRNA synthesis starts at many places and stops at predetermined places
- when mRNA molecule complete, it’s released as a single stranded molecule
What is the Eukaryotic promoter site?
section of DNA that acts as a start signal
- CAAT box (sometimes present)
- TATA box (Hogness box)
What is the initiation step in transcription?
- polymerase binds to promoter sequence in duplex DNA”closed complex”
- Polymerase melts DNA near transcription site (transcription bubble) “open complex”
- polymerase catalyses phosphodiester linkage of 2 initials rNTPs
What is the elongation step in transcription?
- polymerase advances 3’ to 5’
- melting duplex DNA and adding rNTPs to growing RNA
What is the termination step in transcription?
at the transcription stop site, polymerase releases completed RNA and dissociated from DNA
What is the stop signal in transcription?
self complementary sequence in the mRNA forms hairpin loop
- recognised by RNA polymerase
= stops synthesis
What is translation?
using the sequence of information on the MRNA molecule to construct a protein molecule by assembling a/a in the correct order
- takes place in the ribosome
- involved tRNA
What are the stop/nonsense codons?
UAA
UAG
UGA
What is the start codon?
AUG
Features of the triplet code:
- degenerate: some a/a are coded for by more than one codon
- universal: in all organisms (except a few exceptions)
What is the structure of tRNA?
- all fold into 4 base paired stems and 3 loops
- the CCA sequence at the 3’ end is common
- some of the CGAU residues are modified
What is aminoacyl-tRNA?
aminoacyl-tRNA synthetase (enzyme)
- links each tRNA with its appropriate a/a
What is the enzyme that forms the peptide bond?
peptidyl transferase
- brought about in ribosomes
What are ribosomes like in Eukaryotes?
Made of a num of dif rRNA
- 2 subunits: 60S and 40S subunits
- S: rate of sedimentation during centrifugation
What is the initiation step in translation?
- starts at AUG codon closest to 5’ end
- initiator tRNA binds to small subunit of the ribosome (methionyl-t-RNA^Met)
- binds to mRNA, moves alond to find AUG
- initiation factors dissociate, large rRNA binds
- 2 binding sites peptidyl site (P) or aminoacyl site (A)
- second a/a binds to A
What is the elongation process in translation?
- tRNA bearing growing peptide is at the P site
- next a/a moves into A site
- peptide bond formed by the enzyme peptidyl transferase
- complex moves along mRNA, leaves A site for the next a/a
What is the termination step in translation?
- moves along mRNA until stop codon reached
- release factor bings in the A site
- has no a/a, so terminates chain = COOH
- stimulates separation of the subunits of the ribosome
