MGD S4 - DNA Structure and Chromosome Organisation

Question 1

What are the two types of nitrogenous bases?

Answer 1

1) Pyramidines - one ring structure (C, T and U)
2) Purines - Two ring structure (G and A)

Question 2

State three differences between DNA and RNA

Answer 2

DNA - Deoxyribose , ACGT, double stranded molecule

RNA - Ribose (extra oxygen at C2’), ACGU, single stranded molecule

Question 3

What makes DNA chains polar?

Answer 3

Phosphate molecule at 5’ end and hydroxyl group at 3’

Question 4

What is a purine?

Answer 4

2 ring structure (hexagon and pentagon) - adenine and guanine

Question 5

Describe the bonds found within and between chains of DNA in the double helix

Answer 5

Phosphodiester bonds between nucleotides to form sugar phosphate backbone of the double helix Hydrogen bonds form between complementary base pairs on opposite strands of the double helix

Question 6

List the key features of the DNA double helix

Answer 6

• 0.34nm
• Antiparallel strands which are complimentary
• Bases on the inside of helix
• Deoxyribose and phosphate molecules on the outside of helix

Question 7

What is a histone?

Answer 7

Positively charged protein molecule that helps to package DNA as negatively charged DNA molecules coil around the histone core to form a nucleosome giving the ‘beads on a string’ structure

Question 8

What is a solenoid? When would it be found?

Answer 8

• 30nm fibre of densely coiled ‘beads on a string’
• DNA found in this structure when it is not being expressed

Question 9

Define a chromosome

Answer 9

Structure found in the cell nucleus containing one double stranded DNA molecule or two identical double stranded DNA molecules after DNA replication

Question 10

Define the term gene

Answer 10

A unit of heredity, a length of DNA on a chromosome that contains the code for a protein to be transcribed

Question 11

List the enzymes involved in DNA replication

Answer 11

DNA Helicase, primase, DNA polymerase, DNA ligase

Question 12

How is DNA replication terminated?

Answer 12

Two replication forks meet and the leading and lagging strands of each template strand are joined by ligase

Question 13

Why is the process of DNA replication termed semi-conservative?

Answer 13

Each strand of DNA from the parent molecules are conserved in the two daughter helices as each new helix produced contains one parent strand and one newly synthesised strand

Question 14

Summarise the process of DNA replication

Answer 14

1) DNA helicase unwind the parental double helix
2) Single-strand binding proteins stabilise the unwound parental DNA
3) The leading strand is synthesised continuously in the 5’ to 3’ direction by DNA polymerase
4) The lagging strand is synthesised discontinuously. Primase synthesised a short RNA primer, which is extended by DNA polymerase to form an Okazaki fragment
5) After the RNA primer is replaced by DNA, DNA ligase joins the okazaki fragment to the growing strand
6) Replication is terminated when two replication forks meet and the leading and lagging strands of each template strand are joined by ligase

Question 15

List the stages of the cell cycle

Answer 15

1) Interphase - G0, G1, S, G2
2) Mitosis
3) Cytokinesis

Question 16

What is the purpose of G0?

Answer 16

Rest phase when cell either permanently (CNS) or temporarily leaves the cell cycle and stops dividing

Question 17

List the differences between mitosis and meiosis

Answer 17

Mitosis - Produces two identical daughter cells, occurs in somatic cells, used for growth and repair of tissues, diploid cells produced

Meiosis - Creates genetic diversity by crossing over and independent assortment, used in spermatogenesis and oogenesis, haploid cells produced which maintains chromosome numbers

Question 18

Name the process of producing egg cells

Answer 18

Oogenesis

Question 19

Define the terms genotype and phenotype

Answer 19

Genotype - genetic makeup of an individual

Phenotype - all observable characteristics of an individual as a result of the genetic makeup of one or more specific genetic locus or loci

Question 20

List the environmental factors that can affect both genotype and phenotype

Answer 20

Mutagens

Radiation

Chemicals that affect cell growth

Diet

Lifestyle

Question 21

What is meant by the term allele?

Answer 21

• An alternative form of a gene
• Each individual has two alleles for every gene which can either be the same or different (diploid organism)

Question 22

Give an example of an X - linked genetic disease

Answer 22

Haemophilia

Question 23

What is complementation?

Answer 23

More than one gene is responsible for the phenotype Eg albinism

Question 24

What can be said about two genes found on the same chromosomes?

Answer 24

Said to be ‘linked’, closer the loci of the genes, the more unlikely crossing over will occur

Question 25

Explain the term dominance and give an example of an autosomal dominant disease

Answer 25

Phenotypic trait expressed in both heterozygous and homozygous so only one allele required in genotype for the trait to be expressed Eg huntingtons