Chromosomes and DNA packaging Flashcards
Why do we need DNA packaging?
The human genome is 2.2 metres (2,200,000 micrometer). The diameter of a nucleus is 11 micrometers. This creates a size issue as we need to fit such a large DNA molecule inside a nucleus of such a size. Therefore, the packaging and access to genetic info must be regulated.
What is the solution to the size issue?
Proteins called Histones (via DNA packaging)
what are histones?
Proteins that DNA wrap around.
What are the 4 main core histones?
H2A, H2B, H3, H4
H1 = linker
why do basic amino acids need to assist in these histone proteins?
Due to the amino acid charge - it is acidic so we need basic amino acids to bind together (25% of amino acids in these proteins are basic - arginine and lysine)
what do histones form?
octamers
what is the solenoid?
Packing of DNA as a 30nm finer of chromatin and results in the helical winding of nucleosome strands.
what are the levels of DNA compaction?
- Naked DNA - sequence of base pairs - double helix, 2 chromatin strands - 2nm
- Nucleosomes - octamers of histones, linker DNA = 10nm fiber
- Solenoid - 30nm fiber - several nucleosomes
- Chromatin fiber - super helically arranged solenoids (300nm)
- Chromosome - viewed during metaphase in mitosis
What is euchromatin?
form of chromatin that is uncoiled and active (high transcriptional activity loosely packed)
What is heterochromatin?
form of chromatin that is condensed and inactive (low transcriptional activity densely packed)
what are the 2 types of DNA modification (chromatin modification)?
Acetylation and methylation
What is acetylation?
Histones in euchromatin acetylated. Keeps structure open and prevents further condensation. (doesn’t occur in heterochromatin)
What is methylation?
Opposite to acetylation during condensing DNA. Histones in euchromatin are not methylated. Histones in heterochromatin have high methylation (compact)
what is a chromosome?
single long strand of DNA containing a linear array of genes.
What are karyotypes?
The arrangement of a full chromosome set
what is the method to Karyotype?
- add tissue sample and add a chemical that stimulates mitosis (culture in growth medium)
- incubate for 2-3 days
- Add chemical to stop mitosis in metaphase.
- Transfer cells to tube and centrifuge to concentrate in layers.
- transfer cells to new tube containing a fixative
- Put cells onto a microscope and add stain to enhance the chromosomes
- identify and photograph
- Cut out pictures and arrange into a karyotype
What are the methods to generate karyotypes?
- G-banding / giemsa stain
2. FISH - fluorescent in situ hybridisation
What is G - banding?
- heterochromatic regions e.g adrenine/thymine are gene poor and stain darker.
- Each chromosome has a specific banding pattern as we have different sets of genetic information on each chromosome. Thus, specific banding patterns can be seen (only occur on 2 sister chromatids)
What is FISH?
- Can visualise where a gene/DNA sequence is within a genome.
- uses specific probes for defined DNA sequences. Then hybridise the defined DNA sequences - hybridisation probe is a short fragment of DNA that has a fluorescent dye attached.
- Can then arrange chromosomes based on size and specific fluorescence
What is a centromere?
Each pair of chromatids is attached at a centromere - DNA hidden beneath kinetochore proteins. They divide chromosomes into 2 arms - the short arm (p arm) and long arm (q arm).
What is another name for centromeres?
Primary constrictions (based on microscopic appearance)
What are telomeres?
Caps at the end of chromosomes. They contain unique repetitive sequences called telomere tandem repeats.
Why do telomeres shorten with each cell division?
Because DNA polymerases only work in 1 direction. We have to deal with discontinuous replication at the lagging strand of the DNA. This leaves an unprotected end of DNA which is prone to being digested by enzymes.
What types of cell avoid cellular ageing?
Stem cells - regenerate cells & tissues
Tumour cells - generate new telomeric sequences - immortal
What enzyme do the cells use to avoid cellular ageing? and what is this?
Telomerase - Has DNA polymerase activity and contains RNA sequence that provides a template for synthesis of telomeric repeat DNA
If chromosomes fail to divide properly - what does this lead to?
- Numerical aberrations - caused by the failure of chromosome division - cells with extra chromosome or a deficiency.
- Structural aberrations - due to a loss of genetic material, or rearrangement in the location of genetic material. Includes- duplications, deletions, inversions, translocations & ring formations (5)
what is trisomy 21?
Down syndrome. Have reduced muscle tone (hypotonia), slanted eyes upwards and outwards. Flat back of head. Small mouth with protruding tongue. Have some degree of learning disability
What is XXY trisomy?
Klinefelter syndrome. Sex chromosome abnormality - occurs in males with 2 or more X chromosomes instead of Male norm of XY. The more X chromosomes = the more severe version of the syndrome.
Symptoms include: Taller, less muscular, larger breasts, weaker bonds, delay in puberty and less hair.
What is Turner syndrome?
One normal X instead of the female norm of XX.
Smaller than average, underdeveloped ovaries, lack of monthly periods and infertility.
