Topic 1: Nucleic Acids

Question 1

Describe the primary structure of DNA

Answer 1

chains of nucleotides held together by nucleotide bonds

each DNA molecule is arranged into one chromosome

Question 2

Describe the secondary structure of DNA

Answer 2

base pairing between the bases in the nucleotides

the two DNA strands run in opposite directions (anti-parallel)

Question 3

What is base pairing?

Answer 3

pair between purine and pyrimidine

specific pairs depending on number of hydrogen bonds

Question 4

Which baes are purines?

Answer 4

Adenine

Guanine

Question 5

Which bases are pyrimidines?

Answer 5

Cytosine
Thymine
Uracil

Question 6

Describe DNA replication

Answer 6

1) DNA strands are unwound by DNA helicase
2) Single-stranded binding proteins bind to each strand to prevent reassembling
3) RNA primase makes a short strand of complementary RNA which acts as the primer
4) DNA polymerase III reads parent strand in 3’ to 5’ direction
5) The lagging strand is made by the synthesis of Okazaki Fragments
6) DNA polymerase I then replaces the RNA primer with DNA
7) DNA ligase joins the fragments together

Question 7

Which protein controls the accuracy of DNA replication?

Answer 7

DNA polymerase III (checks nucleotides and replaces them if they are not correct)

Question 8

What are the two mechanisms that contribute to DNA repair?

Answer 8

1) DNA glycosylase (small lesions)- checks DNA for mispaired bases, modified bases or excess bases; then fixes them
2) DNA helicase and DNA ligase (larger lesions)- helicase removes section of DNA containing damage, ligase attaches a new section which had been made by polymerase

Question 9

What is the approximate size of the human genome?

Answer 9

30,000 genes

3 trillion base pairs

Question 10

Why is the genetic code similar for all sepcies?

Answer 10

the genetic code is degenerate- the same codons code for the same amino acids in all organisms. (some amino acids may be represented by multiple codons, some only by one)

Question 11

What are VNTRs?

Answer 11

Variable Number Tandem Repeats
Have repeated sequences that are tens of base pairs long (minisatellites)
Repeated sequences that are 2-6 base pairs (microsatellites)

Question 12

What can occasional polymerase slippage of VNTRs cause?

Answer 12

• addition/elimination of a sequence

- disease e.g. Huntington’s

Question 13

What can be associated with disease risk?

Answer 13

Single Nucleotide Polymorphisms (SNPs)

can have a very varied effect based on polymorphism

Question 14

What can VNTRs be useful in?

Answer 14

forensic and paternity testing

Question 15

How is DNA transcription started?

Answer 15

• antisense strand is read from 3’ to 5’ end
• RNA polymerase binds to initiation site (normally TATA boxes)
• promoter regions (upstream and downstream of gene) control rate of transcription through binding with trans-acting factors

Question 16

How is RNA polymerase bound during DNA transcription?

Answer 16

• binding of RNA polymerase separates strands of DNA
• first exposed base binds with complementary free nucleotide
• RNA polymerase travels along the DNA adding complementary bases to the mRNA chain

Question 17

How is DNA transcription terminated?

Answer 17

RNA polymerase encounters a stop codon and is dislodged from DNA
mRNA has been formed

Question 18

What is capping of mRNA?

Answer 18

5’ end of mRNA is capped by the addition of 7-methylguanosine to the triphosphate group
Capping is thought to protect the mRNA from destruction by nuclease enzymes

Question 19

What is polyadenylation of mRNA?

Answer 19

the 3’ end is cleaved at the poly-A site (sequence AAUAAA or AUUAAA)
a sequence of up to 300 A nucleotides is then added to the 3’ end

Question 20

What are CpG islands?

Answer 20

Unmethylated C and G rich regions

associated with 5’ ends