nucleic acids

Question 1

what are the three components of a nucleotide?

Answer 1

phosphate group.
pentose sugar.
nitrogen-containing base.

Question 2

name the pentose sugars in DNA and RNA.

Answer 2

DNA - deoxyribose.
RNA - ribose.

Question 3

state the role of DNA in living cells.

Answer 3

base sequence of genes codes for functional RNA and amino acid sequence of polypeptides.

genetic info determines inherited characteristics = influences structure and function of organisms.

Question 4

state the role of mRNA in living cells.

Answer 4

complementary sequence to 1 gene from DNA with introns (non coding regions) spliced out. codons can be translated into a polypeptide by ribosmes.

Question 5

state the role of rRNA in living cells.

Answer 5

component of ribosomes (along with proteins)

Question 6

state the role of tRNA in living cells.

Answer 6

supplies complementary amino acid to mRNA codons during translation.

Question 7

how do polynucleotides form?

Answer 7

Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone).

Question 8

describe the structure of DNA.

Answer 8

double helix of 2 polynucleotide strands.

deoxyribose sugar.

hydrogen bonds between complementary purine and pyrimidine base pairs on opposite strands:
adenine (A) and thymine (T).
guanine (G) and cytosine (C).

Question 9

which bases are purine and which are pyrimidine?

Answer 9

purine = A&G
pyrimidine = T&C&U

Question 10

name the complementary base pairs in DNA.

Answer 10

adenine and thymine.
guanine and cytosine.

Question 11

name the complementary base pairs in RNA.

Answer 11

adenine and uracil.
guanine and cytosine.

Question 12

relate the structure of DNA to its functions.

Answer 12

sugar-phosphate backbone and many hydrogen bonds provide stability.
helix is compact for storage in nucleus.
base sequence of triplets code for amino acids.
double-stranded for semi-conservative replication.
weak hydrogen bonds so strands separate in replication.

Question 13

Describe the structure of messenger RNA.

Answer 13

shorter than DNA.
contains uracil instead of thymine.
single-stranded and linear.
codon sequence is complementary to exons of 1 gene from 1 DNA strand.

Question 14

Relate the structure of messenger RNA to its functions.

Answer 14

shorter than DNA - breaks down quickly so no excess polypeptide forms.
single-stranded and linear - ribosomes can move along strand and tRNA can bind to exposed bases.
codon sequence is complementary to exons of 1 gene from 1 DNA strand - can be translated into a specific polypeptide by ribosomes.

Question 15

Describe the structure of transfer RNA (tRNA).

Answer 15

single stranded.
folded into clover shape.
anticodon on one end, amino acid binding site on the other:
- anticodon binds to complementary mRNA codon.
- amino acid corresponds to anticodon.

Question 16

order DNA, mRNA and tRNA according to increasing length.

Answer 16

tRNA
mRNA
DNA

Question 17

Why is DNA replication described as semiconservative?

Answer 17

strands from original DNA molecule act as a template.
new DNA molecule contains 1 old strand and 1 new strand.

Question 18

outline the process of semiconservative DNA replication.

Answer 18

1. DNA HELICASE breaks hydrogen bonds between base pairs.
2. each strand acts as a template.
3. free nucleotides from nuclear sap attach to exposed bases by complementary base pairing.
4. DNA POLYMERASE catalyses condensation reactions that join adjacent nucleotides on new strand.
5. hydrogen bonds form.

Question 19

Describe the meselon-stahl experiment.

Answer 19

1. bacteria were grown in a medium containing heavy isotope 15N for many generations.
2. some bacteria were moved to a medium containing light isotope 14N. samples were extracted after 1 and 2 cycles of DNA replication.
3. centrifugation formed a pellet. heavier DNA (bases made from 15N) settled closer to the bottom of the tube.

Question 20

explain how the meselon-stahl experiment validated semiconservative replication.

Answer 20

1 division - all molecules had 1 strand 15N and 1 strand 14N.

2 divisions - 50% have 2 strands 14N and 50% have 1 strand 15N 1 strand 14N.