function of nucleic acids

Question 1

what are the components of a nucleotide?

Answer 1

• a phosphate group
• a pentose sugar : deoxyribose or ribose
• an organic nitrogenous base

Question 2

what are the 2 groups of organic nitrogenous base?
write the examples

Answer 2

PYRAMIDINE (single ring) :
thymine
cytosine
uracil

PURINE (double ring):
adenine
guanine

Question 3

identify which nucleotides form complementary base pairs with each other
explain why

Answer 3

C : G
A : T/U
C and G form three hydrogen bonds, A and T/U only form two hydrogen bonds

Question 4

what does DNA stand for?

Answer 4

deoxyribonucleic acid

Question 5

what are the two functions of DNA?

Answer 5

1. DNA replication
   2.

Question 6

outline steps to DNA replication

Answer 6

1. DNA unwinds
2. hydrogen bonds holding complementary base pairs break, catalysed by DNA helicase and the 2 strands separate
3. each DNA strand acts as a template
4. free DNA nucleotides align opposite with their complementary bases
5. DNA polymerase catalyses the condensation reaction between 2 DNA nucleotides. this occurs from the 5 prime to the 3 prime end of the chain
6. each new DNA molecule is made from one original template strand and one newly replicated strand. this is known as semi-conservative replication

Question 7

where does DNA replication, and when?

Answer 7

takes place in the nucleus
during interphase, in the cell cycle

Question 8

why must DNA replicate?

Answer 8

so that each daughter cell receives an exact copy of the genetic information

Question 9

how many theories are there as to how DNA might replicate? what are they?

Answer 9

THREE:
1. conservative
2. semi-conservative
3. dispersive

Question 10

describe the conservative theory

Answer 10

the parental double helix remains intact, completely conserved, and a whole new DNA molecule is made

Question 11

describe the semi-conservative theory

Answer 11

• each strand in the parental double helix acts as a template to synthesise a new polynucleotide strand.
• each new DNA molecule contains one template strand and one newly replicated strand

Question 12

describe the dispersive theory

Answer 12

each new DNA molecules contains fragments of the parental double helix and newly synthesises DNA

Question 13

what is the Meselson-Stahl Experiment?

Answer 13

an experiment which proves evidence for semi-conservative replication

Question 14

outline the steps to the Meselson-Stahl Experiment

Answer 14

1. E.coli was grown in a medium containing amino acids with the heavy isotope 15N. The bacteria produced nucleotides containing 15N. All the bacteria’s DNA contained this heavy isotope. The DNA was extracted and the suspension then centrifuged
2. the bacteria were washed and then transferred to a medium containing amino acids with the lighter isotope - 14N - and allowed to divide once (one generation). The DNA was extracted and the suspension then centrifuged
3. the bacteria were allowed to divide again (two generations) on the 14N medium. Again the DNA was centrifuged.

Question 15

Why is E.coli bacteria used?

Answer 15

it is easy to culture and the DNA is not contained within a nucleus
- the cells divide rapidly

Question 16

Explain why this experiment was conclusive evidence of semi conservative replication rather than conservative or dispersive.

Answer 16

• after one generation, conservative replication would not give a band in the middle, only a band at the top (light) and at the bottom (heavy).
• dispersive would only produce one band which got progessively higher in the tube, rather than two