Organisation of DNA

Question 1

How much DNA is in our cells?

Answer 1

• Nearly 2m of DNA is crammed into every cell of our bodies!
• Each of these 2m lengths consists of 3.2 billion nucleotides/bp!
• A ladder made of the DNA in ten cells could get you to Mars

Question 2

Where is DNA found in eukaryotes?

Answer 2

It is distributed among a number of chromosomes found in the nucleus.

Question 3

In the nucleus, where are the ribosomes synthesized?

Answer 3

They are made in the nucleolus, where it consists of DNA, RNA and proteins.

Question 4

Where is the genetic material of prokaryotes and viruses found as opposed to eukaryotes?

Answer 4

Prokaryotes - Found free floating in cytoplasm as circular DNA

Viruses - Found in the head of the virus as circular/linear DNA/RNA

Question 5

Describe the characteristics of viral chromosomes.

Answer 5

T-even bacteriophages - T2, T4, T6

• d/s DNA
• Single linear chromosome within head
• 168 kb (kilobases)

Gamma bacteriophages

• d/s DNA
• Linear (in phage) and circular (occurs when host in infected)

Question 6

Describe the characteristics of the prokaryotes.

Answer 6

• Genetic material floats loose (no nucleus)!
• Usually single chromosome
• Bacteria & Archaea: chromosome arranged in dense clump called the “Nucleoid”

Question 7

Which enzyme controls the amount and type of DNA supercoiling?

Answer 7

These are called topoisomerases.

Essential for DNA replication as they allow replication to occur smoothly.

Question 8

Why are bacterial chromosomes organised into looped domians?

Answer 8

• This compacts the chromosome into a smaller space where there is roughly 100 domains that make up the whole bacterial genome.
• The end of the loops are held by proteins so that there is no influence from DNA replication.
• Compaction is tenfold.

Question 9

Which chromosome is bigger; X and Y?

Answer 9

X chromosome

Question 10

How many chromosomes do humans have?

Answer 10

Humans have 23 pairs of chromosomes so they have 46 in total.

Question 11

What do you call chromosomes that aren’t sex chromosomes?

Answer 11

Autosomes. There are 22 pairs of these.

Question 12

Do males have two X chromosomes?

Answer 12

No, they have one X chromosome and one Y chromosome.

Females have two copies of the X chromosomes.

Males are much more at risk of X-related genetic diseases.

Question 14

What do you call members of a pair of chromosomes? (contain the same genes)

Answer 14

Homologous chromosomes. Each member of the pair is called a homolog.

Question 15

How do you classify chromosomes?

Answer 15

• Each chr has a constriction along its length called a centromere, which is essential for the behaviour of the chr during cell division. The location of the centromere in one of 4 general positions is useful for classifying chrs.
• A metacentric chr has the centromere in the middle and roughly equal arms
• Submetacentrics have one arm longer than the other
• Acrocentrics have a long arm with a bulb or “satellite”
• Telocentric only have one arm because the centromere is at the end.
• Small arm is called p and long arm called q. Numbered and then subnumbered regions then assigned from centromere out.

Question 16

How is the location of gene on a certain chromosome found out?

Answer 16

Morphology is taken into account.

Question 17

How does DNA fold up small enough to fit in the nucleus?

Answer 17

DNA winds around proteins called histones and it also winds around non-histones.

Scaffold proteins, DNA polymerase, Heterochromatin Protein 1 and Polycomb are common non-histone proteins.

Question 18

What is chromatin?

Answer 18

Combination of DNA and protein.

Question 19

Is chromatin stainable?

Answer 19

Yes it is. It can stained with acridine orange.

Question 20

Wat is ration between DNA:Histone:Non-Histone?

Answer 20

1:1:1

1/3 DNA, 1/3 Histone, 1/3 Non-histone

Question 21

Why is it important for histone to have a small positive charge?

Answer 21

DNA have a small negative charge, this helps the histones to bind to the DNA.

Question 22

What are the four main types of histone core found as an octamer?

Answer 22

1. H2A
2. H2B
3. H4
4. H3

Each type has two copies.

Question 23

H1 associates with DNA where the DNA enters and leaves the nucleosome.

True or False?

Answer 23

True

Question 24

How is each nucleosome connected?

Answer 24

Through linker DNA

Question 25

Why is the binding of H1 important?

Answer 25

Allows DNA to condense even further into a chromatin fibre.

Question 26

Are nonhistone scaffold proteins more condensed than the solenoid model?

Answer 26

Yeah as it produces aa chromosome that is roughly 10,000X shorter and 400X thicker than naked DNA.

Question 27

Why is it harder to access dense packaging of chromatin?

Answer 27

It is more difficult for transcrption enzymes to access DNA sequence. Therefore it is transcriptionally active and typically named at heterochromatin as it is difficult to code and express.

Question 28

What are centromeres?

Answer 28

DNA sequence found at the point of attachment of mitotic/meiotic spindle fibres.

• Responsible for segregation of replicated chromosomes to daughter cells.
• Has different chromatin structure and packing to other regions.

Question 29

Why are centromeres constricted in appearance?

Answer 29

Surrounded by constitutive heterochromatin hence tightly packaged.

Question 30

What is the role of telomeres in chromosomes?

Answer 30

• Provide stability of the chromosome
• Required for replication
• Shields chromosome from fusion or degradation
• Shares a unique DNA sequence that is repeated many times

Question 31

What happens when telomeres shorten over time?

Answer 31

Eventually cell division stops.

Question 32

What is high telomerase activity characteristic of?

Answer 32

Masses of tumour cells. The enzyme telomerase maintain optimal number of repeats of telomere.