Genetic code: DNA & RNA

Question 1

Nirenberg-Matthaei Poly-U 1961

Answer 1

• determined which RNA coded for which protein
• found a codon was 3 nucleotides
• deduction: in order for the 4 nucleotides to account for all 20 amino acids, a minimum of 3 base pairs are required

Question 2

Degenerate code

Answer 2

multiple codons code for the same amino acid

Question 3

neutral/silent mutation

Answer 3

mutation of nucleotide order/ substitution - but does not influence aa encoded

Question 4

neutral amino acid substitution

Answer 4

mutation in different aa produced - but does not affect protein function

Question 5

how many stop codons are there? codons coding to stops translation

Answer 5

3 - no aa is produced - UAG, UAA, and UGA

Question 6

how many start codons?

Answer 6

1 - methionine made

Question 7

3 types of RNA

Answer 7

mRNA - carries transcripts DNA from nucleus to cytoplasm for protein synthesis
tRNA - transfers aas to mRNA to build proteins
rRNA - ribosomal RNA - make up ribosomes w protein

Question 8

Nucleoside

Answer 8

sugar(ribose) + nitrogenous base

Question 9

Nucleotide

Answer 9

nucleoside + phosphate molecule

Question 10

DNA vs RNA ribose structure

Answer 10

Deoxyribose - DNA - H instead of OH on 2nd prime (attached to 2nd carbon in ribose pentagon) = better at long term storage
Ribose RNA - OH = less stable - better for short time/ dynamic use

Question 11

DNA polymerisation (chain making) phosphate structure?

Answer 11

New joining nucleotides with 3 phosphates lose 2 when joining to make phosphodiester bonds - much like ATP (adenosine triphosphate) gives out energy, NTP (nucleoside triphosphates) provides energy for DNA synthesis.

Question 12

Properties of B-DNA (typical double helix)

Answer 12

• 10 base pairs per turn
• right handed helix (rotates in clockwise direction)
• minor and major grooves
• very -ve charge due to phosphate groups

Question 13

2 flavours of nitrogenous bases

Answer 13

Purines: Adenine, Guanine (double carbon rings)
Pyrimidine: Thymine, Cytosine, Uracil (single carbon rings)

Question 14

why is Thymine used in DNA over Uracil?

Answer 14

• U makes more stable U-U bonds in mutation under UV radiation - t-t bonds break easier- less likely to be transcribed
• U paired with A causes kink in double helix backbone
• Cytosine easily mutates (through deamination- hydrolysis) to U - if U bases also existed in DNA, wouldn’t be able to tell the correct base

Question 15

dNTPs

Answer 15

deoxynucleotide triphosphates = single nucleotides with 3 phosphates (free nucleotides)

Question 16

chromatin

Answer 16

material of chromosomes - consists of: DNA, RNA, Proteins

Question 17

nucleus dna types in microscope images

Answer 17

euchromatin - light coloured, sparsely packed dna, 90% of genome, expressed genes

heterochromatin - dark coloured, tight packed - supercoiled, silent genes

each nucleus contains 2m of DNA = supercoiling to fit

Question 18

nucleosome

Answer 18

one “bead” of wrapped DNA along strand around histones (linker DNA with histone 1 H1 spans between nucleosomes - space for restriction enzymes to bind)
per nucleosome:
8 histones = “octamers”
147 base pairs
DNA wound 1.6 wraps around histone protein core

supercoil to form cremation fibres 30um

Question 19

chromatin remodelling factors

Answer 19

protein complexes that travel along DNA to alter histones:
exchange, remove histones
alter nuclear or histone structure

Question 20

histone tails

Answer 20

stick out of nucleosomes to alter gene expression, can be modified with:
methyl groups, acetyl groups, lysine

Question 21

histone methylation

Answer 21

decreased DNA expression - inhibits binding of transcription factors by making histone tails wrap around DNA - often methylation can be reversed

Question 22

DNA methylation

Answer 22

alters gene expression through modification of DNA - linked to disease - irreversible

Question 23

chromatin fibre

Answer 23

Nucleosomes fold up to form a 30-nanometer chromatin fiber, condense into loops to form protein scaffold,

Question 24

facultative heterochromatin

Answer 24

• heterochromatin = tightly packed
• but still able to be expressed - switches between hetero. and euchromatin states e.g, through histone tail modification

Question 25

constitutive heterochromatin

Answer 25

• always condensed
• consists fo repetitive sequences, e.g, telomeres, centromeres (connecting sister chromatid)
• play role in structuring chromosomes

Question 26

telomeres

Answer 26

• repetitive sequences at end of chromosomes
• shorten with age (oxidisation?) 11 - 4 killobaes thru life
• to protect genes

Question 27

cause of telomere shortening

Answer 27

in DNA replication, RNA primer on lagging strand (is 3-5 so 3’ end of new strand was a primer and has nothing before it so is not replaced by ligase = gap

telomeres will keep shortening every cell division until none left = hayflick limit - cell division stops

solution = telomerase enzyme! - only some cells: germ cells (eggs, sperm), some cancer cells, embryonic stem cells - high volume of telomerase present

Question 28

telomerase

Answer 28

enzyme uses reverse transcription from mrna template to dna
on lagging strand, telomere sequence is recognised and dna is added to the parent strand. an RNA primer binds to added length and synthesises 5-3’ to meet original end of RNA = no lost length of telomere, telomerase synthesised extra and primer is then removed.

Question 29

chromosome replication origins

Answer 29

• multiple origins on each chromosome

- each origin only used once per cell cycle - no duplicated dna

Question 30

timing of replication origins

Answer 30

euchromatin origins copied earlier than heterochromatin origins