Session 9 ILOs - DNA structure and replication

Question 1

How many chromosomes does a human have?

Answer 1

Each cell normally contains 23 pairs of chromosomes, for a total of 46 chromosomes. 
22 of these pairs are called autosomes (look the same in both males and females). 
23rd pair (the sex chromosomes) differ between males and females (males = XY, females = XX)

Question 2

Describe and recognise the structural components of nucleic acids

Answer 2

Nucleic acids are either DNA or RNA and are polynucleotides 
Nucleotides are made up of 3 components:
1. Pentose sugar
2. Base (A, G, C, T, U)
3. Phosphate group

Question 3

Describe the primary, secondary and tertiary structure of DNA molecules

Answer 3

Primary structure:
Long chain of nucleotide monomers, joined by phosphodiester bonds

Secondary structure:
Hydrogen bonds (either 2 or 3 bonds) between complimentary base pairs - DNA strands formed are anti-parallel and complimentary

Tertiary structure:
Right handed double helix with anti-parallel strands (with major and minor grooves) - helix held together by the stacking of the base pairs with Van de Waals forces plus the hydrogen bonding between pairs

Question 4

Outline the quaternary structure of eukaryotic DNA

Answer 4

The large shapes and structures that can be made by nucleic acids e.g. the core histones forming the histone octomer then forming the NDA and forming the ‘beads on a string’ nucleosome array

Question 5

Compare and contrast DNA & RNA

Answer 5

DNA = Deoxyribonucleic acid (bases: A, C, G, T)
RNA = Ribonucleic acid (bases: A, C, G, U)
DNA is double stranded, RNA is single stranded
DNA has an H group on the 2nd carbon, RNA has an OH group on the 2nd carbon

Question 6

Describe the relationship between DNA sequences, DNA molecules, chromatin and chromosomes

Answer 6

COME BACK TO

DNA sequence forms an alpha helix, which then forms ‘beads on a string’ formation with histones, which can then be further condensed to chromatin, which is then further condensed to chromosomes (only in the process of division i.e. mitosis/meiosis)

Question 7

List the 5 bases and whether they are purines or pyrimidines and what combinations of bases they can form

Answer 7

Purines: G & A (Guanine and Adenine)
Pyrimidines: C, T & U (Cytosine, Thymine and Uracil)

Combinations:
G = C
A = T
A = U

Question 8

Describe the general outline of the mechanism of DNA replication

Answer 8

Initiation:
DNA polymerase requires a kickstart by DNA primase

Elongation:
DNA helicase unwinds the DNA and for each new area that’s been unzipped, the DNA polymerase moves back along the DNA towards the 3’ end to do this area. DNA ligase fuses the separate fragments of the lagging strand (Okazaki fragments)

Termination:
Occurs when each of the Okazaki fragments have fused together i.e. one chromosome = one DNA molecule

Question 9

Describe what is meant by semiconservative replication

Answer 9

Means that half of the DNA is kept from the original template i.e. is conserved!

Question 10

Describe the main functions of the enzymes involved in the process of DNA replication

Answer 10

1. DNA primase - ‘kick start’ synthesise very short sequences of RNA to allow DNA polymerase to work
2. DNA polymerase - slides along the single strand template of DNA reading its nucleotide bases as it goes along and inserting new complementary nucleotides
3. DNA Helicase - unwinds/unzips DNA ready for DNA polymerase
4. DNA ligase - joins together Okazaki fragments of the lagging strand by forming a bond between the phosphate group of one strand and the deoxyribose group on another