Organisation of DNA Flashcards
How much DNA is in our cells?
- 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
Where is DNA found in eukaryotes?
It is distributed among a number of chromosomes found in the nucleus.
In the nucleus, where are the ribosomes synthesized?
They are made in the nucleolus, where it consists of DNA, RNA and proteins.
Where is the genetic material of prokaryotes and viruses found as opposed to eukaryotes?
Prokaryotes - Found free floating in cytoplasm as circular DNA
Viruses - Found in the head of the virus as circular/linear DNA/RNA
Describe the characteristics of viral chromosomes.
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)
Describe the characteristics of the prokaryotes.
- Genetic material floats loose (no nucleus)!
- Usually single chromosome
- Bacteria & Archaea: chromosome arranged in dense clump called the “Nucleoid”
Which enzyme controls the amount and type of DNA supercoiling?
These are called topoisomerases.
Essential for DNA replication as they allow replication to occur smoothly.
Why are bacterial chromosomes organised into looped domians?
- 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.
Which chromosome is bigger; X and Y?
X chromosome
How many chromosomes do humans have?
Humans have 23 pairs of chromosomes so they have 46 in total.
What do you call chromosomes that aren’t sex chromosomes?
Autosomes. There are 22 pairs of these.
Do males have two X chromosomes?
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.
What do you call members of a pair of chromosomes? (contain the same genes)
Homologous chromosomes. Each member of the pair is called a homolog.
How do you classify chromosomes?
- 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.
How is the location of gene on a certain chromosome found out?
Morphology is taken into account.
How does DNA fold up small enough to fit in the nucleus?
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.
What is chromatin?
Combination of DNA and protein.
Is chromatin stainable?
Yes it is. It can stained with acridine orange.
Wat is ration between DNA:Histone:Non-Histone?
1:1:1
1/3 DNA, 1/3 Histone, 1/3 Non-histone
Why is it important for histone to have a small positive charge?
DNA have a small negative charge, this helps the histones to bind to the DNA.
What are the four main types of histone core found as an octamer?
- H2A
- H2B
- H4
- H3
Each type has two copies.
H1 associates with DNA where the DNA enters and leaves the nucleosome.
True or False?
True
How is each nucleosome connected?
Through linker DNA
Why is the binding of H1 important?
Allows DNA to condense even further into a chromatin fibre.
Are nonhistone scaffold proteins more condensed than the solenoid model?
Yeah as it produces aa chromosome that is roughly 10,000X shorter and 400X thicker than naked DNA.
Why is it harder to access dense packaging of chromatin?
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.
What are centromeres?
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.
Why are centromeres constricted in appearance?
Surrounded by constitutive heterochromatin hence tightly packaged.
What is the role of telomeres in chromosomes?
- Provide stability of the chromosome
- Required for replication
- Shields chromosome from fusion or degradation
- Shares a unique DNA sequence that is repeated many times
What happens when telomeres shorten over time?
Eventually cell division stops.
What is high telomerase activity characteristic of?
Masses of tumour cells. The enzyme telomerase maintain optimal number of repeats of telomere.
