MCBG session 9 - DNA Structure & Replication

Question 1

What is chromatin, euchromatin and heterochromatin?

Answer 1

Chromatin = the substance within a chromosome that contains DNA & proteins.
Euchromatin = The lightly packaged form of chromatin, that has the "bead on a string" like structure (DNA looped around histones)
Heterochromatin = The tightly packaged form of chromatin, with solenoid fibre structures.

Question 2

What length of DNA is packaged into each 6um cell?

Answer 2

2 meters - although the double helix width is 2 nanometers.

Question 3

What is the 1st level of DNA packaging?

Answer 3

• When DNA is looped around a histone core to form a “bead on a string” like structure to form a nucleosome.
• Histone core + 2 bits of DNA looped around it = nucleosome. Specifically it is a histone octamer with 147bp of DNA.

Question 4

What is the 2nd level of DNA packaging

Answer 4

Compaction of histones to form solenoids (30nm).

Question 5

What does a condensed chromosome contain?

Answer 5

Large amounts of solenoid loops tightly packaged within the arms. Unwind them to become beads on a string (nucleosome) structures.

Question 6

What are genes?
Where are they found?
How many do humans have?

Answer 6

• Genes carry the code for proteins
• Genes are found within chromosomes
• Humans have ~25,000 genes

Question 7

What is the human genome?
How many chromosomes are in the human genome?
How many BP are in the human genome?

Answer 7

• The entire DNA sequence of the human species
• 23 pairs - 22 autosomal pairs + 1 pair sex chromosomes
• 3.2 billion

Question 8

DNA/RNA are polynucleotides - what makes up a nucleotide?

What is the difference in between a nucleotide and nucleoside?

Answer 8

Nucleotide = phosphate + pentose sugar + nitrogenous base
Nucleoside = lacks the phosphate group

Question 9

What is the difference between the pentose sugar in RNA and DNA?

Answer 9

RNA = ribose sugar (lacking O on C2 hydroxyl group)
DNA = deoxyribose sugar

Question 10

What are the 2 main types of nitrogenous bases (give the examples of each) and what are their differences?
What do pyrimidine and purines always pair with?

Answer 10

1) Purines (G + A) - 2 ring structures
2) Pyrimidines (C,T & U) - 1 ring structures
- Pyrimidines must ALWAYS pair with a purine, and vice versa - if there is 100bp in DNA, there is 100 purines + 100 pyridines.

Question 11

Where are nucleotide monomers joined and what bond is formed?

Answer 11

Between the C3 and C5 of 2 pentose sugars to form a phosphodiester bond

Question 12

What are the groups found at the 5’ end and 3’ end of DNA?

Answer 12

5' = phosphate group
3' = hydroxyl group

Question 13

What kind of bonds form between bases and how do they form?

How many of these bonds form for each possible pairing of bases?

Answer 13

• Hydrogen bonds, as areas of electronegativity attract areas of electropositivity on adjacent bases
• G-C = 3 hydrogen bonds
• A-T = 2 hydrogen bonds
• A-U = 2 hydrogen bonds

Question 14

Describe the secondary structure of DNA.

What is each single strand and each double strand held together by?

Answer 14

• Base pairings form, allowing complimentary DNA strands join that are anti-parallel
• Each single strand held together by covalent bonds between sugar and phosphate
• Double strand held together by hydrogen bonds between base pairs.

Question 15

Describe the tertiary structure of DNA

Answer 15

Right handed double helix with anti-parallel strand, which produces a major and minor groove.

Question 16

In which direction are nucleic acids typically depicted?

What does this mean for the complimentary strand?

Answer 16

• From 5’ to 3’

- It’s antiparallel so is depicted from 3’ to 5’

Question 17

When is DNA replication needed?

When does DNA replication take place during the cell cycle?

Answer 17

• During the process of cell division

- During the S phase (of interphase) of the cell cycle

Question 18

Why is DNA replication “semi-conservative”?
What is the reaction catalysed by?
What drives the reaction + what else is required?
In what direction does synthesis occur?

Answer 18

• Each DNA strand produced contains 1 new and 1 old strand of DNA
• DNA polymerase
• Hydrolysis of PPi by pyrophosphate + dNTP’s (free nucleotides) required, therefore seperation of double strand template
• Synthesis occurs from 5’ to 3’, reading off the template strand.

Question 19

What are the 3 steps of DNA replication? Describe the first step.

Answer 19

• Initiation, Elongation, Termination
• Origin of replication is recognised and DNA polymerase is recruited.
• DNA polymerase requires kick-start by DNA primase (making a short RNA primer)
• Results in 2 replication forks (for 1 origin of replication)

Question 20

What occurs during elongation phase of DNA replication

Answer 20

• 2 replication forks are moving
• Helicase unwinds double helix
• DNA polymerase extends 3’ ends, leading to a leading strand (continuous), and a lagging strand (discontinous)
• Okazaki fragments on lagging strand joined via DNA ligase.

Question 21

What occurs during termination phase of DNA replication?

Answer 21

• When 2 facing replication forks meet, DNA ligase joins 2 final fragments
• Results in 2 identical DNA molecules (4 strands in total)