Chromosomes Flashcards
Describe the structure of a chromosome.
Two sister linear chromatids which are held together by the centromere.
What are the ends of the chromosomes known as?
Telomeres
What are the functions of telomeres?
They protect the chromosome ends from degradation in the cell and also potentially from inappropriate repair.
What are heterochromatins?
Large amounts of DNA in the eukaryotic genome that is not active.
How is euchromatin characterised?
By the ability to be expressed
Why does the entire genome need to get replicated?
So that a complete copy of the DNA can be passed to each of the daughter cells.
What is the DNA synthesis reaction carried out by?
DNA polymerase
What direction does DNA polymerase synthesis DNA?
In a five prime to three prime direction due to the anti parallel nature.
So the replication can stay in the same direction, how do the second strand (lagging strand) get replicated?
In short fragments known as Okazaki Fragments
What enzyme stitches the shorter fragments together in DNA replication?
Ligase
What is the end replication problem?
RNA primers get removed in the lagging strand so the fragments can join together.
However, at the end, the removal of a primer leaves a gap meaning this could lead to degradation and loss of DNA.
What is the solution to the end replication problem?
Expression of the enzyme telomerase which adds repeats and resynthesizes and fills in the end of DNA during DNA replication.
G1=?
S=?
G2?
M+?
G1=growth phase
S=replication of DNA
G2=cells grow and prepare to divide
M= mitosis
Name the order of the five stages of mitosis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Describe prophase.
Chromosomes condense
Nuclear membrane disappears
Spindle fibres form from the centriole
Describe metaphase
Chromosomes aligned at the equator of the cell
Attached by fibre to each centriole
Maximum condensation of chromosome
Describe anaphase
Sister chromatids separate at centromere
Separate longitudinally
Move to opposite ends of cell
Describe telophase.
New nuclear membranes form
Each cell contains 46 chromosomes (diploid)
Describe cytokinesis.
Cytoplasm separates
Two new daughter cells
What is the centromere?
Constricted region joining sister chromatids
What are centromeres characterised by?
Repetitive DNA sequences
->like satellite DNA
What are centromeres the site of?
The kinetochore which is a protein complex that binds to microtubules.
What is the kinetochore required for?
Required for chromosome separation during cell division.
What stages of the cell cycle make up interphase?
G1, S, G2
Describe the structure and genes of Heterochromatin.
-Condensed structure
-Silenced genes
Describe the structure and genes of Euchromatin
-Open structure
-Active genes
What percentage of our DNA is protein coding?
2%
What is chromatin?
DNA protein assembly into which our DNA is assembled and packaged to make it possible to fit it within the nucleus
What is the central component of chromatin.
Histone proteins
Why are histones well suited to packaging DNA?
They are positively charged and the phosphate backbone is negatively charged.
What are the four main histone particles called?
Histone 3
Histone 4
Histone 2A
Histone 2B
What does Histone 1 do?
Binds to the base of the nucleosome and helps to hold DNA in place.
What charge do histones have?
Positive
Describe how DNA gets wrapped up into chromosomes.
-Short region of DNA double helix wrapped around nucleosomes
-These then get assembled into a chromatid fibre
-Chromatid fibres become attached to a scaffold complex so that the fibre is assembled in a series of loops on a chromatin scaffold
-DNA condensed
Give some of the purposes of the packaging of DNA
-Negatively charged DNA neutralised by positive charged histone proteins
-DNA takes up less space
-Inactive DNA can be folded into inaccessible locations until required
-Inactive chromatin characterised by specific histone convalent modification (e.g. methylation)
What side of the chromosome is known as the P side?
The shorter side
P= petite arm
Which side of the chromosome is known as the Q arm?
The longer side
Contains more DNA
Give an example of a chromosome in which the P arm is so small that it doesn’t have any function.
Chromosome 21
Describe simply the process of chromosome analysis
-Blood sample taken
-Want to separate off the white blood cells through incubation.
-Cells put on slide and stained
-Photographed
-Chromosomes assembled into a karyotype
What does a karyotype show?
All the chromosomes lined up in their pairs
In each chromosome pair, how many strands of DNA are there?
4
Define metacentric.
The centromere constriction is close to the centre of the the chromosome
Define sub-metacentric.
Centromere is displaced from the centre of the chromosome.
Define acrocentric.
Q arm is so small that it doesn’t consist of any functional DNA.
Name the stain which can be used to highlight the differences in chromosomes.
Giemsa
Name four type of FISH (Fluorescent in situ hybridisation) probes
Unique sequence probes
Centromeric probes
Whole chromosome probes
Telomeric probes
Why are centromeric probes useful?
Useful for determining chromosome number
Why are telomeric probes useful?
Useful for detecting subtelomeric rearrangements
Often present in children with unexplained mental retardation
Where would you find satellite DNA?
Centromeres
Telomeres
What type of DNA is used for finger printing?
Minisatellite DNA
