Nucleic acids and chromosomes

Question 1

Components of nucleotides

Answer 1

Base, sugar, phosphate

Question 2

Components of nucleosides

Answer 2

Base, sugar, no phosphate

Question 3

Structure of DNA/RNA

Answer 3

Chain of nucleotides linked by phosphodiester bonds
3’ OH of sugar linked to phosphate group attached to 5’ carbon of next nucleotide
First nucleotide in sequence has free phosphate attached to 5’ carbon
Last nucleotide in sequence has a free hydroxyl group attached to the 3’ carbon.

Question 4

Direction of DNA/RNA writing

Answer 4

5’ end to 3’ end

Question 5

Features of the double helix

Answer 5

• 10 base pairs per helical turn
• Major and minor grooves
• Hydrogen bonds between bases on the two strands give helix stability
• Chains are antiparallel (they run in opposite directions)
• Negative charges on phosphate group make the sugar-phosphate backbone run on the outside with the bases on the inside.

Question 6

DNA bases

Answer 6

Adenine
Thymine
Cytosine
Guanine

Question 7

RNA bases

Answer 7

Adenine
Uracil
Cytosine
Guanine

Question 8

Purine bases

Answer 8

Adenine

Guanine

Question 9

Pyrimidine bases

Answer 9

Thymine
Cytosine
Uracil

Question 10

Difference between purine and pyrimidine

Answer 10

Purine = double ring
Pyrimidine = single ring

Question 11

Watson-Crick base pairs

Answer 11

• Adenine with Thymine/Uracil = forms two H bonds between bases
  Cytosine with Guanine = forms three H bonds between bases

Question 12

Conditions used for denaturing/melting (separation) of strands

Answer 12

Heat

Low salt

Question 13

Conditions used for re-annealing of strands

Answer 13

Cool

High salt

Question 14

Process of hybridisation in NA analysis

Answer 14

• Target DNA from agar plate/gel immobilised on solid support
• DNA then readily binds single-stranded nucleic acid (denatured DNA/RNA)
• Hybridised with solution of radioactively or fluorescently labelled probe

Question 15

What is hybridisation

Answer 15

A method for detecting specific nucleic acid sequences, homologous single-single stranded DNA/RNA molecules combine to form double-stranded molecules. Probe used to show sequence presence.

Question 16

What is PCR

Answer 16

Polymerase Chain Reaction, an in vitro (outside of the cell) method which allows the selective amplification of a specific target DNA within a mixed collection of DNA sequences.

Question 17

Process of PCR

Answer 17

• 2 primers designed to be complementary to target sequences
• Primers annealed to denatured DNA by lowering the temperature
• TAQ polymerase and free nucleotides (dNTPs) extend the chain
• Denature and repeat the cycle several times to generate many copies of target DNA

Question 18

Temperatures for denaturing, annealing, and extension in PCR

Answer 18

• Denaturing = 94 degrees
• Annealing = 50-60 degrees
• Extension = 72 degrees

Question 19

What types of restriction enzymes are used and what do they do

Answer 19

Type 2 restriction endonucleases which cleave the DNA at specific recognition sequences

Question 20

What types of ends do restriction enzymes produce and what is the difference between them

Answer 20

Blunt ends - both strands cut at the same nucleotide.

Sticky ends - Each strand cut at different nucleotides, two nucleotides apart. Ends overlap.

Question 21

Association between recognition site length and occurrence frequency in DNA

Answer 21

The longer the recognition sequence, the less frequently it occurs in DNA

Question 22

What is a nucleosome

Answer 22

Structural unit of human chromosomes, consists of length of DNA coiled around a core of histones.

Question 23

Packaging of DNA

Answer 23

DNA wrapped around histone proteins. These are positive proteins which interact with negative phosphates on the DNA. 10000 fold shortening due to histones.

Question 24

What is a karyotype

Answer 24

An organised profile of a person’s chromosomes

Question 25

What is the normal human karyotype

Answer 25

A diploid human has 22 autosomal chromosome pairs and one sex-determining chromosome pair