module 2 - 3.9 DNA replication and the genetic code

Question 1

what are the 4 requirements for DNA to be a genetic molecule?

Answer 1

1. must carry information (this is the genetic code)
2. must replicate (this is DNA replication)
3. must allow for the information to change (this is mutation)
4. must control the phenotype (this is gene expression using protein synthesis)

Question 2

where is DNA information stored?

Answer 2

in the sequence of nitrogenous bases

Question 3

the sequence of A, C, T, and G encodes information in the form of what to make what?

Answer 3

encodes information in the form of genes to make proteins

Question 4

when are most genes only accessible?

Answer 4

when DNA is unwound (chromatin)

Question 5

why is extreme accuracy of DNA replication necessary?

Answer 5

in order to preserve the integrity of the genome in successive generations

Question 6

in eukaryotes, when does DNA replication occur?

Answer 6

during the S phase of the cell cycle (stationary phase)

Question 7

what is the S phase?

Answer 7

when the cell is doing its job

Question 8

what is the replication rate like in prokaryotes/ eukaryotes?

Answer 8

prokaryotes - faster rate, lower fidelity (accuracy)
eukaryotes - slower rate, higher fidelity (accuracy)

Question 9

what is semi-conservative DNA replication?

Answer 9

• one strand of a double helix is passed on unchanged to each daughter cell
• ‘conserved’ strand acts as a template for synthesis of a new, complementary strand by DNA POLYMERASE

Question 10

what is N14?

Answer 10

non-radioactive isotope of nitrogen

Question 11

what is N15?

Answer 11

radioactive isotope of nitrogen

Question 12

where does DNA replication begin?

Answer 12

at the origin (methionine)

Question 13

what is the origin within prokaryotic cells?

Answer 13

single circular DNA molecule or chromosome and so single origin of DNA replication

Question 14

where is the origin within eukaryotic cells?

Answer 14

eukaryotic cells have multiple, linear DNA molecules/ chromosomes & each DNA molecule has multiple origins of DNA replication

Question 15

where does DNA replication always go from and to?

Answer 15

5’ to 3’ direction

Question 16

what does DNA polymerase require before synthesis can be initiated?

Answer 16

a free 3’ hydroxyl group

Question 17

what is the difference between the old and new DNA strand to do with the way they are read?

Answer 17

the new DNA template strand is read in a 3’ to 5’ direction, whereas the old strand is read from 5’ to 3’ direction

Question 18

why is DNA replication discontinuous?

Answer 18

the DNA forms complementary fragments of the DNA molecule from 5’ to 3’ on the new strand
(synthesises in sections)

Question 19

why is DNA replication continuous?

Answer 19

the DNA forms its complementary strand from 3’ to 5’ continuously on the new strand

Question 20

how are complimentary base pairs paired together?

Answer 20

collision theory

Question 21

what is the enzyme that untwists the DNA?

Answer 21

helicase

Question 22

what is DNA polymerase in a DNA molecule?

Answer 22

the active site

Question 23

where does DNA replication happen?

Answer 23

a replication fork

Question 24

what is a replication fork?

Answer 24

the structure that forms within the long helical DNA during DNA replication

Question 25

what is a replication fork created by?

Answer 25

helicases, which break the hydrogen bonds holding the 2 DNA strands together in the helix

Question 26

what happens once the hydrogen bonds are broken?

Answer 26

causes 2 strands of DNA and has 2 branching 'prongs'

Question 27

what happens when the 2 strands then serve as a template for the leading and lagging strands?

Answer 27

strands will be created as DNA polymerase which matches complementary nucleotides to the templates

Question 28

what happens when the helicase unwinds the DNA at the replication fork? what does this result in?

Answer 28

the DNA ahead is forced to rotate results in a build up of twists and tension in the DNA ahead

Question 29

what are topoisomerases?

Answer 29

- enzymes that temporarily break the strands of DNA, relieving tension caused by unwinding 2 strands of DNA helix - they add negative supercoils to DNA helix

Question 30

what does primase do?

Answer 30

- creates short strands of RNA complementary to DNA strand - allows DNA polymerase to bind to DNA and begin replicating it

Question 31

what happens if you have a purine + purine?

Answer 31

base pair too thin

Question 32

what happens if you have pyrimidine + pyrimidine?

Answer 32

base pair too wide

Question 33

how many mistakes does DNA polymerase make?

Answer 33

1 in every billion base pairs copied

Question 34

what does DNA polymerase do?

Answer 34

proof-reads the base pairs, corrects mistakes in the newly synthesised DNA

Question 35

what happens when an incorrect base pair is recognised?

Answer 35

the DNA polymerase moves backwards by one base pair of a DNA

Question 36

what does the 3'-5' exonuclease activity of the DNA polymerase allow?

Answer 36

allows the incorrect base pairs to be exercised

Question 37

what is a mutation within DNA?

Answer 37

mistakes lead to random and spontaneous changes in the base sequence of the newly synthesised strand

Question 38

what are the differences between polypeptides and proteins?

Answer 38

polypeptides: chain of amino acids proteins: chain of amino acids, virtually all enzymes are proteins, functional molecules

Question 39

what is the genetic code?

Answer 39

set of rules used by all living cells to translate the information encoded in genetic material, into proteins

Question 40

how is translation accomplished?

Answer 40

- by the ribosome which links to amino acids in an order specified by mRNA - uses tRNA molecules to carry amino acids

Question 41

what are 4 features of the genetic code?

Answer 41

- it is a triplet code (codon) - it is universal among all living organisms - it is a degenerate code (more codes than needed) - it establishes the reading frame

Question 42

where does is reading frame always set within polypeptides?

Answer 42

methionine (ATG)

Question 43

what is the degenerate code?

Answer 43

- the redundancy of the genetic code (more codes than needed) - the genetic code has redundancy but no variation

Question 44

how is the reading frame defined/started?

Answer 44

- defined/started by the initial triplet of nucleotides where translation starts - frame is a run of SUCCESSIVE, NON-OVERLAPPING CODONS