3.3 - nucleic acids

Question 1

the central dogma

Answer 1

flow of genetic info from DNA -> RNA -> proteins

Question 2

each nucleotide contains: (3)

Answer 2

1. 5-carbon sugar (pentose)
2. phosphate (1’-3’)
3. base

Question 3

2 types of nucleotide base: (2)

Answer 3

1. pyrimidine (1 ring) - T/C
2. purine (2 rings) - A/G

Question 4

RNA nucleotide bases (4)

Answer 4

1. guanine
2. cytosine
3. adenine
4. uracil

Question 5

DNA/RNA backbone

Answer 5

pentose sugar 3’-OH of one nucleotide covalently bonded via phosphodiester bond to adjacent phosphate group 5’-OH

Question 6

what ends to nucleic acids have? (2)

Answer 6

1. 5’
2. 3’

Question 7

difference between RNA and DNA sugars? (2)

Answer 7

1. RNA - ribose sugar (2’-C of sugar linked to 2’-OH group)
2. DNA - deoxyribose sugar (2’-C of sugar linked to H atom)

Question 8

why is DNA much more stable than RNA?

Answer 8

absence of 2’-OH increases resistance to hydrolysis (deoxyribose sugar bonded to H atom not -OH)

Question 9

what creates the DNA double helix? (2)

Answer 9

1. sugar-phosphate backbone
2. bases H-bonded together

Question 10

orientation of 2 DNA strands (2)

Answer 10

1. anti-parallel (opposite)
2. 5’-3’ orientation opposite

Question 11

what direction do DNA polymerase and RNA polymerase operate in?

Answer 11

5’-3’ direction

Question 12

what non-covalent interactions stabilise the DNA double helix (excluding H-bonds) (2)

Answer 12

1. van der waals
2. hydrophobic effect (phosphate + sugar - hydrophilic/ base - hydrophobic)

Question 13

RNA structure (2)

Answer 13

1. single strand - can form 2x helix (RNA/RNA) and heteroduplex (RNA/DNA)
2. can form other more complicated base-pair structures (due extra oxygen in ribose sugar compares to deoxyribose)

Question 14

what feature of RNA supports the RNA world hypothesis? (2)

Answer 14

1. dual capacity to act as catalyst and carrier of genetic info
2. can catalyse chemical transformations using many same strategies as protein - based enzymes

Question 15

catalytic RNA

Answer 15

ribozymes

Question 16

ribozyme fundamental reactions (2)

Answer 16

1. transesterification
2. phosphodiester bond hydrolysis (cleavage)

Question 17

main types of RNA (2)

Answer 17

1. informational - mRNA (encodes proteins)
2. functional - tRNA, rRNA and others

Question 18

% of cell RNA made up by rRNA?

Answer 18

75%

Question 19

what encodes RNA?

Answer 19

DNA-based genes

Question 20

transfer RNA (2)

Answer 20

1. involved in protein synthesis at ribosome and forms ‘clover leaf’ structure
2. function as adaptor molecules - link amino acids for proteins with mRNA codons

Question 21

DNA at normal/elevated temps (3)

Answer 21

1. double helix at normal temp
2. strands disassociate at elevated temps
3. re-anneal when temp lowered

Question 22

how can DNA strands disassociate/re-anneal?

Answer 22

H-bonds broken but phosphodiester bonds aren’t

Question 23

PCR - polymerase chain reaction (3)

Answer 23

1. open DNA - denaturing
2. primers find target - annealing (must know target DNA sequence)
3. fill in complementary bases (complete copy) - DNA polymerase - extension

Question 24

organisation of genome: prokaryotes (2)

Answer 24

1. each cell contains single circular genome, highly folded and compacted (supercoiled) to fit in small space
2. may also have small circular plasmids, can replicate independently

Question 25

organisation of genome: eukaryotes (2)

Answer 25

1. cells contain genome within nucleus, organised as linear double stranded chromosomes (homologous pair)
2. mitochondria (and chloroplasts) contain own double stranded circular genome

Question 26

chromatin (2)

Answer 26

1. eukaryotic DNA packaged together with histones -> nucleosomes
2. chromatin further condensed into loops/coils, ultimately packaged into chromosomes

Question 27

what determines the accessibility of DNA to cellular machinery?

Answer 27

how tightly packed the nucleosome is