gene expression / DNA replication

Question 1

how do we inherit characteristics

Answer 1

• genes: inherited ‘units’ of DNA that influence characteristics of an organism
• proteins can be made of more than one polypeptide, and multiple genes coding for a protein
• central dogma: DNA to mRNA to polypeptide to protein

Question 2

what RNA’s are used to make proteins

Answer 2

• mRNA: directs which AA are assembled into polypeptides
• rRNA: the site of polypeptide assembly during polypeptide synthesis
• tRNA: transports and positions AA / complementary base to form a protein

Question 3

what is polypeptide synthesis (brief)

Answer 3

• transcription: DNA becomes RNA, copying ‘recipe’

- translation: RNA becomes protein / amino acid sequence, cooking up protein, constructing machinery

Question 4

describe the steps of transcription

Answer 4

• RNA polymerase: a fundamentally important enzyme, pries the DNA strands apart, adds complementary nucleotides to DNA, RNA ‘grows’ in the 5’ to 3’ direction
  2. promotor: binding site for RNA polymerase, short sequence (not transcribed) location where transcription starts
  3. initiation: specific proteins mediate, binding of RNA polymerase, unwinding of DNA helix begins
  4. elongation: RNA polymerase moves down DNA, nucleotides are added at the 5’ end
  5. termination: RNA transcribes a terminator sequence found at the end of a gene in the DNA, RNA polymerase proceeds beyond the sequence (pre-mRNA is cut free from the enzyme)
  6. 5’ end (1st): nucleotide (guanine-like base), prevents degradation and acts as ‘attach here’ sign
  7. 3’ end (2nd): a ‘tail’ of adenine added, facilitates export of mRNA

Question 5

describe the steps of RNA splicing

Answer 5

• cutting out junk: average protein = 1200 nucleotides, 8000 nucleotides transcribed into mRNA
• introns: long non-coding intervening sequences
• extrons: other sequences eventually expressed
• splicing of entire RNA sequence to remove introns and join axons to form a mRNA molecule with a continuous coding sequence
• genes can be cut in different ways to be made into a different amino acid

Question 6

describe the steps of translation

Answer 6

1. start stop signals: ribosome identifies start (AUG) and finish (UAA, UAG, UGA - no complimentary AA)
2. polypeptide synthesis begins with formation of initiation complex, brings together mRNA, tRNA (+ first AA of polypeptide) and 2 ribosomal subunits (accurate positioning)
3. elongation: mRNA codon adjacent to the ‘start’ codon (AUG) is exposed (A site)
4. codon re=positioned (P site)
5. subsequent tRNA places its AA adjacent to start codon
6. amino acids form a peptide bond, first AA released (E site) by tRNA
7. translocation: ribosome continues to move by 3 nucleotides (one codon)
8. positions the previous tRNA to be ejected and exposes the next codon to interact with a tRNA
9. process repeats itself
10. termination: nonsense codons are recognised by release factors that release newly made polypeptide from ribosome

Question 7

what is chromosome packing

Answer 7

• chromatin: loose, strands / threads, enzymes / substances easily access DNA / genes (transcription, translation, replication)
• chromosome: during mitosis, DNA compacts to aid in accurate division, dense, packing, winding around histones to form nucleosomes

Question 8

how many chromosomes do humans have

Answer 8

• 46 chromosomes
• diploid: 2 sets of chromosomes (23 pairs)
• haploid: 1 set of chromosomes (23 unpaired)

Question 9

how do we store genetic information

Answer 9

• chromosomes: contain DNA, linear, tightly packed
• genes location on p (short arm) and q (long arm)
• 2% is used for coding

Question 10

what is the purpose of DNA replication, why is it semi-conservative / discontinuous

Answer 10

• mitotic cell division
• 2 cells arise from division of parent cell
• daughter cells contain identical DNA to parent
• semi-conservative: complementary image, one new strand, one old
• semi-discontinuous: leading strand (replicates towards fork) and lagging strand (elongates from fork, discontinuously in short pieces)
• can only add from 5’ end of new strand being synthesised

Question 11

describe the steps of DNA replication

Answer 11

1. replication origin: DNA begins replicating at one or more sites on DNA
2. topoisomerase: unwinds DNA helix, manipulation of strand to break up backbone, release tension, rejoin DNA
3. single stranded (ss) binding proteins: help stabilise the DNA and prevent cleavage and rewinding of DNA strands
4. helicase: unzips DNA by breaking H bonds between N bases
5. form replication bubbles which meet and fuse to form new DNA strands
6. leading strand: DNA primase adds an RNA primer to DNA, polymerase (3) adds complimentary nucleotides as new DNA is exposed at fork from 5’ to 3’ (new strand)
7. lagging strand: elongates away from fork as nucleotides cannot be added from 3’ to 5’
8. RNA primase: lays down a primer, gives polymerase a starting point to work backwards from fork and attach ~10 RNA primers
9. DNA polymerase (3): add nucleotides in the 5’ to 3’ direction away from the fork, skip a section and then repeat
10. okazaki fragments: synthesised in small bursts, eventually put together / attached by DNA ligase, DNA polymerase (1) replaces the RNA primers with DNA and strand is replicated