Nucleic acids and chromosomes

Question 1

Explain the structure of DNA

Answer 1

Double helical structure
Each strand consisting of polymerised deoxyribonucleotide units (Nitrogenous base, deoxyribose sugar and one or more phosphate groups)
- 5’ end on phosphate bound to 3’ end of deoxyribose form phosphodiester bonds
Strands run antiparallel held together by hydrogen bonds between base pairs
10 bp per helical turn

Question 2

Explain the structure of RNA

Answer 2

Single stranded, comprised of polymerised ribonucleotide units (nitrogenous base, ribose sugar and one or more phosphate groups)
tRNA
mRNA
rRNA

Question 3

What are the *(Watson-Crick) base pairs found in DNA?

Answer 3

Adenine-Thymine (2 bonds)

Guanine-Cytosine (3 bonds)

Question 4

What are the bases found in RNA?

Answer 4

Adenine
Guanine
Cytosine
Uracil

Question 5

Which bases are purines?

Answer 5

(double ring)
Adenine
Guanine

Question 6

Which bases are pyramidines?

Answer 6

(single ring)
Cytosine
Thymine
Uracil

Question 7

Explain melting and re-annealing of complementary stands

Answer 7

Heat/put in low salt conc solution to melt/denature the DNA

Can be reversed by cooling/place in high conc salt solution due to complimentary base pairing

Question 8

Explain hybridisation of nucleic acids

Answer 8

Radioactively/fluorescently labelled probes used to indentify target molecules in a mixture of unlabelled NAs
Target DNA immobilised on solid support
probe DNA in sol. added then melting/re-annealing allows probe DNA to bind to target DNA and the rest to be washed away

Question 9

What does energy needed for denaturation depend on?

Answer 9

Strand length
Base composition
Chemical environment e.g. Na+ conc

Question 10

What is the definition of melting temperature?

Answer 10

Midpoint of transition between dsDNA and ssDNA,

hybridisation carried out at 25°C below Tm

Question 11

What is hybridisation stringency?

Answer 11

The power to distinguish between related sequences
At high stringency, only perfectly complimentary strands bind
At low stringency, less perfect matching is allowed

Question 12

What affects hybridisation stringency?

Answer 12

Increases with increase in temperature (only perfect matches) and decease in Na+ conc (only most stable ds form)

Question 13

Explain the polymerase chain reaction

Answer 13

Selective amplication of target DNA within heterogenous collection of DNA sequences
3 stages of cycle that repeats many times:
-Denaturing
-Annealing
-Extending

Question 14

What are the temperatures and other requirements needed for PCR?

Answer 14

95° denature
50-60° anneal
72° extend
Two primers complimentary to each DNA strand
Thermostable Taq polymerase
Added dNTPs (deoxynucleoside triphosphates)

Question 15

What can PCR be used for?

Answer 15

Detection of point mutations
Sequencing
Gene expression
Microarrays

Question 16

What are restriction enzymes used for?

Answer 16

Cleave DNA at specific 4-8bp sequences

Introduces sticky or blunt ends

Question 17

What is the human karyotype?

Answer 17

Organised profile of the 23 pairs of human chromosomes

Question 18

How is DNA packaged into nucleosomes?

Answer 18

DNA wrapped around a +vely charged histone which strongly attracts the -vely charged DNA backbone, causing 7-fold condensing (~150bp per histone)

Question 19

How does DNA form chromosomes?

Answer 19

DNA → Nucleosome → Chromatin fibre → Chromatin fibre folded into loops → Chromosome