2.1.3 Nucleotides and nucleic acids

Question 1

structure of nucleotide

Answer 1

monomer that makes up nucleic acids when joined by phosphodiester bonds
ribose (pentose sugar)
phosphate group (phosphodiester bond with 5 carbon of ribose)
nitrogenous base (phosphodiester bond with 1 carbon of ribose)

Question 2

ADP and ATP structure

Answer 2

phosphorylated nucleotides
ADP = 1 additional phosphate group
ATP = 2 additional phosphate group

Question 3

how nucleic acids / polynucleotides form

Answer 3

condensation reactions to form phosphodiester bonds
between OH group on 3 carbon of pentose sugar and phosphate group on 5 carbon of pentose sugar of another nucleotide
forms sugar phosphate backbone

Question 4

complementary base pairs of nucleotides

Answer 4

adenine+thymine (uracil in RNA), 2 hydrogen bonds

cytosine+guanine, 3 hydrogen bonds

Question 5

pyrimidines

Answer 5

smaller bases
single carbon ring structure
thymine, cytosine

Question 6

purines

Answer 6

larger bases
double carbon ring structure
adenine, guanine

Question 7

DNA molecule structure

Answer 7

2 polynucleotides coiled into DNA double helix
sugar phosphate backbone
2 strands held together by hydrogen bonds between complementary base pairs
strands have 3’ (hydroxyl group) and 5’ (phosphate group) ends
strand run anti parallel (parallel and in opposite directions)

Question 8

RNA structure

Answer 8

single strand
uracil instead of thymine
ribose sugar instead of deoxyribose sugar in DNA

Question 9

DNA extraction method (plant cells)

Answer 9

grind sample with pestle and mortar (break down cell wall)
mix sample with detergent (break down cell membrane)
add salt (breaks hydrogen bonds between DNA and water)
add protease enzyme (breaks down histones associated with DNA)
add ethanol on top of sample (DNA precipitates out of solution)
“spool” DNA (white strands) using glass rod

Question 10

why DNA replication is semi-conservative

Answer 10

2 molecules of DNA produced each time

both consisting of one original strand and one new strand

Question 11

DNA replication method

Answer 11

occurs during synthesis phase of interphase gyrase unwinds double helix
DNA helicase catalysing break down of hydrogen bonds between complementary nitrogenous base pairs and “unzips” double helix
free DNA nucleotides pair with exposed bases of template strand
DNA polymerase catalyses formation of phosphodiester bonds between nucleotides (builds from 5’ to 3’)
leading strand is continuous
lagging strand is discontinuous and forms Okazaki fragments
DNA ligase catalyses joining of Okazaki fragments

Question 12

mutation definition

Answer 12

random and spontaneous errors in the sequence of bases that occur during replication

Question 13

why DNA is genetic code

Answer 13

DNA codes for sequences of amino acids that form different proteins for organism
different proteins are foundation of physical and biochemical characteristics of organisms

Question 14

codon definition

Answer 14

triplet code (3 nitrogenous bases in a row)
codes for an amino acid

Question 15

gene definition

Answer 15

section of DNA consisting of complete sequence of codons to code for a protein

Question 16

roles of different codons

Answer 16

start codon: if at beginning of gene, signalling start of sequence that codes for a protein
“stop” codons: 3 of them, don’t code for any amino acids, signal end of sequence
if start codon is in middle of gene, it codes for amino acid methionine

Question 17

why genetic code is non-overlapping

Answer 17

start codon ensures codons are read “in frame” (in the right order) and won’t overlap with other genes

Question 18

why code is degenerate

Answer 18

many amino acids can be coded by more than one codon

high likelihood that mutation won’t alter sequence of amino acids

Question 19

transcription method

Answer 19

gyrase unwinds double helix
RNA polymerase catalyses break down of hydrogen bonds between complementary nitrogenous base pairs, “unzips double helix”
free RNA nucleotides form hydrogen bonds with exposed complementary bases on template strand
RNA polymerase catalyses formation of phosphodiester bonds between nucleotides
transcription stops at end of gene, mRNA is formed, detached from template strand and leaves through nuclear envelope to ribosome
DNA double helix reformed

Question 20

strands in transcription

Answer 20

sense strand: 5’ to 3’, codes for protein
antisense strand: 3’ to 5’, complementary to sense strand, template strand in transcription
RNA uses antisense strand as a template so has same sequence as sense strand

Question 21

ribosome structure

Answer 21

large and small subunits

made up of protein, ribosomal RNA, site that catalyses formation of peptide bonds

Question 22

tRNA structure

Answer 22

strand of RNA
folded so 3 bases (anticodon) exposed at one end of molecule so can bind to complementary codon on mRNA
carries corresponding amino acid

Question 23

translation method

Answer 23

mRNA binds to small subunit of ribosome at start codon (AUG)
tRNA with complementary anticodon and corresponding amino acid binds to mRNA start codon
another tRNA binds to next codon on mRNA
peptide bond form between amino acids on 2 tRNA (catalysed by peptidyl transferase)
ribosome moves along mRNA, releasing first tRNA, peptide chain begins to peel away from ribosome and fold into protein
repeat until at stop codon
ribosome falls apart, releasing finishes protein

Question 24

how multiple identical polypeptides can be made quickly

Answer 24

polysome
many ribosomes can follow on mRNA behind the first
multiple identical polypeptides can be synthesised simultaneously

Question 25

enzyme in translation

Answer 25

peptidyl transferase