Cell cycle

Haploid (n)

one set of chromosome, sperm and eggs in humans

Diploid (2n)

two sets of chromosomes, somatic cells

Tetraploid (4n)

four sets of chromosomes, ex: trophoblast of placenta

Dna content (C)

amount of DNA/haploid nucleus(1 set of chromosomes), 3.5×10^-12g for humans

Chromatin

complex of DNA and basic proteins, tightly packs DNA

structure of DNA

double helix, purine (G/A) pairs with pyrimidine (T/C)

Human chromosomes

3.2 billion base pairs, 6ft long packeged by binding to proteins, 23 pairs → 22 autosome + 1 pair of sex chromosomes

Nucleosomes

beads on string, DNA wraps around histone octamer (8 basic proteins)

Euchromatin

transcriptionally active DNA, extended, diffusely stained

Heterochromatin

transcriptionally inactive, highly condensed, darkly stained

Constitutive heterochromatin

always inactive, composed of repetitive DNA sequences

Nuclear localization of chromosomes

gene rich chromosomes at interior of nucleus, gene poor chromosomes at periphery

cellular replication health and disease

H - development/ repair, D- excessive (cancer) or insufficient (degenerative) proliferation

Phase of cell cycle

G0, G1, S, G2, M

G0

resting cell, not actively cycling

G1

preparing machinery to duplicate DNA

S

Synthesis, DNA is duplicated

G2

preparing machinery to separate DNA, 2 DNA double helices per chromosomes

M

Mitosis, cell division into 2 new daughter cells

Cell cycle checkpoints

Spindle Assembly, G1, G2

Spindle Assembly Checkpoint

check chromosome attached to spindle

G1 Checkpoint

check cell size/ nutrient/ growth factors/ DNA damage

G2 Checkpoint

check cells size/ DNA replication

Tumour suppressor

gene that inhibit cell cycle (RB pathway or p53), mutation in these can cause cancer, ensure proliferation at correct time and place, prevent DNA mutation from accumulating

Metaphase chromosomal structure

have 2 sister chromatid and centromeres

Centromeres

restriction point that separate chromosomes into long(q) and short (p) arms, where sister chromatid joined, where spindle fibers attach, if missing (acentric) chromosomes lost during cell division

Components of Mitotic spindle

Centrosomes (become spindle poles), Kinetochore microtubule, polar/astral microtubule

Centrosomes

composed of pair of centrioles, duplicated during cell cycle, generates spindle pole (opposite sides of cell), microtubule organizing centre (spindle fibres of alpha + beta tubulin originate from)

Kinetochore

multi protein complex on centromere, fibres attach to link it to spindle poles

Meiosis in female germline

asymmetric, unequal division of cytoplasm, 1 oocyte → 1 mature egg + 2 polar bodies

Meiosis in male germline

symmetric, equal division of chromosomes and cytoplasm, 1 spermatocyte → 4 mature spermatozoa

Genetic diversity in Meiosis I

recombination and random assortment

Chromosomal crossing over

prophase Meiosis I, exchange of genetic material between homologous chromosomes

chromosomal recombination

child receive chromosomes derived from both their grandmother’s and grandfather’s germline, unique

Independent assortment diversity

2^n, random if getting from maternal or paternal chromosome

Telomeres

repetitive DNA sequence at distal end, maintain integrity of DNA ends, allows for full DNA replication, orients chromosome, decrease in length with each cell cycle (except in immortal cells)

Telomerase

enzyme, extends telomere repeats, composed protein and template RNA, activity increased in immortal cells

Chromosomal staining

count number of chromosomes, identify abnormalities (fusions/deletions), used diagnostic, differentiated based characteristic staining pattern, ex: Giesma stain observed under light microscope (G-Banding)

specific fluorescent probes

FISH - identify specific gene by attaching probe, chromosome painting - identify chromosome by labelling entire chromosome, used to identify specific genetic rearrangements (ex: Philadelphia chromosome)

Spectral karyotyping (SKY)

fluorescently labelling all chromosomes

Mixploidy

2+ genetically distinct lineages in one person, Mosaicism or Chimerism

Mosaicism

abnormalities arise in the same zygote

Chomerism

aggregation of 2 zygotes (fraternal twins)

Mosaic

different somatic tissue have different chromosomal complements

Constitutional abnormalities

present in all cell, very early event, (abnormal sperm/egg/fertilization)

Translocation event steps

DNA break occur in 2 separate chromosomes, broken ends associate forming fusion chromosomes

Philadelphia chromosomes

translocation in ~95% of chronic Myeloid leukaemia, leads to activation of kinase now targeted by small molecule inhibitor (Imantinib)

Balanced translocation event/ Balanced carrier

no loss of genetic information, just reorganization

Acentric

Translocation with no centromeres, lost during meiosis/mitosis bc spindle can’t bind

Dicentric

Translocation with 2 centromere, lost during metaphase

Robertsonian Translocation

dicentric that are stable bc centromeres close together and function as 1, lost chromosome does not include essential gene , balanced carrier has 45 chromosomes