Centromere
= a region of DNA typically at the centre of a chromosome, where 2 identical sister chromatids come in contact
involved in cell division as the point of the mitotic spindle
Allele
= alternative DNA sequences at the same physical locus
it may or may not result in different physical phenotype traits
Heterozygote
= an organism whose cells contain different genes for a characteristic
Structure of a chromosome
in eukaryotic cells, chromosomal DNA forms a complex with RNA and proteins (CHROMATIN)
Chromatin is packed into bead-like structures called nucleosomes
Chromosomes form by the supercoiling and condensation of chromatin.
Genes are specialised functional sites arranged along the chromosomes
Continuously renewing tissues
e.g. epidermis of skin, intestinal epithelium, blood-forming tissue
Conditionally renewing tissues
e.g. liver, kidney, endocrine glands
Static/non-proliferative tissues
e.g. cardiac & nerve cells
Phases of cell division
G1 Phase:
* The first growth or “gap” phase. The G1-S phase transition is an important control point determining whether or not cells enter S phase.
S Phase:
* Involves the replication of DNA, RNA and proteins. It is the sum of many events because there are multiple DNA replication sites.
G2 Phase:
* Mainly concerned with final preparations before mitosis.
M Phase:
*Ensures the accurate separation of two sister chromatids of each chromosome into separate nucleoli, and the division of the parent cell into two daughter cells.
G0 Phase:
* A term applied to cells which divide very infrequently or not at all, or the period at which cells leave the cycle to express their differentiated function.
General process of mitosis
results in nuclear division (karyokinesis), which is followed by the division of the cytoplasm (cytokinesis).
the nuclear division of mitosis has several stages: prophase, prometaphase, metaphase, anaphase and telophase.
Stages of mitosis - prophase
Chromatin condenses to form visible chromosomes.
Each chromosome comprises 2 sister chromatids, joined at the centromere.
Centrioles duplicate and move apart to form 2 poles and act as microtubule-organising centres.
Microtubules form the mitotic spindle apparatus between the centrioles.
Stages of mitosis - prometaphase
Nucleoli regress and nuclear membranes disassemble.
Chromosomes move towards equator and interact with microtubules.
Each pair of sister chromatids has an attachment site (“kinetochore”) which attaches to the spindle.
Stages of mitosis - metaphase
Equatorial region of the cell forms the metaphase plate.
Chromosomes arrange themselves on the as two sister chromatids adjoined at their centromeres, with microtubules attached to the kinetochores.
Stages of mitosis - anaphase
Sister chromatids of each pair move apart to opposite poles of the cell, being pulled along by the microtubules.
Stages of mitosis - telophase
Chromosomes reach poles and start to become less condensed.
Nuclear membranes re-assemble, and nucleoli reappear.
A cleavage furrow forms around the equator.
The spindle disappears.
Stages of mitosis - cytokinesis
cytoplasm divides, giving rise to 2 daughter cells
What is meiosis?
Where does this occur?
= a process of cell division for reducing the somatic quantity of DNA (2c) to that found in sex cells or gametes (1c)
occurs in the ovaries (oogenesis) and testes (spermatogenesis).
there must be two separate reduction divisions to achieve the 1c amount in the gamete.
=> these are called the first and second reduction divisions, or Meiosis I (4c -> 2c) and Meiosis II (2c -> 1c), ending up with a tetrad of haploid daughter cells.
the DNA double helix
consists of two polynucleotide chains coiled round common axis
The chains are anti-parallel, 5’->3’ in opposite directions for stability
Chains held together by hydrogen bonding between base pairs and hydrophobic stacking forces. Base pairing occurs between complementary bases: “Watson-Crick base pairing”:
G—C, A=T (DNA), A=U (RNA)
Genotype vs Phenotype
DNA is the store of genetic information – it is the genotype.
This is inherited from one cell to daughter cells or one organism to progeny, and will be expressed in each cell/organism to control shape, form, function etc. This expressed information is called the phenotype.
what is a eukaryote?
an organism whose cells have a membrane-bound nucleus and other specialised, membrane-bound structures called organelles
Topoisomerase vs. DNA gyrase
Act by breaking, swiveling and re-joining the parental DNA strands ahead of the replication fork, relieving the tension caused by unwinding.
Eukartyotic cells have topoisomerase
Prokaryotic cells (e.g. bacteria) have DNA gyrase
Therefore DNA gyrase can be a target for some antibacterial agents & topoisomerase can be a target for some anti-cancer agents.
Turner’s Syndrome Karyotype
= 45XO
Features of Turner’s syndrome
(45XO)
- short stature
- gonadal dysgenesis
- lymphoedema
Mode of inheritance - achondroplasia
Autosomal Dominant
Mode of inheritance - polycystic kidney disease
Autosomal Dominant
