Cell cycle

History of cell division

• Hooke 1665 - organisms are made from cells

• Schlegel and Schwann 1857 - plants and animals are made from cells or cell products “cell theory”

• Virchow 1858 - cells come from cells, binary fission,

• 1882 - chromosomes & mitosis, observed elongated threads forming in the nucleus, watched them shorten and thicken during mitosis

Interphase

• G1 = growth phase

• S = DNA synthesis

• G2 = growth phase 2

• G0 = cell cycle arrest

Early 1950s, incubated root tips of plants with radioactive phosphorus. Observed that DNA synthesis occurred in S phase.

• BrdU stain used to tell if a cell is in S.

Chromosomes to chromatids ….

• After duplication, the chromosome now consists of two chromatids, joined copies of the original chromosome (double stranded, two double helices)

• Once separated from its sister, each chromatid (single stranded, one double helix) is considered an individual chromosome

• N = max number of alleles at any particular locus

2 methods for visualising stages of mitosis

1. Fluorescent dyes, fluorescently labelled antibodies

2. Electron microscopy

Late G2 phase

• cell has doubled in size, and much of it original contents

• Cytoplasm now has 2 centrosomes

• Chromosomes have already replicated but cannot be distinguished

• The microtubule organising centre (MTOC) is a structure found in eurkaryotic cells from which microtubules emerge

What is a centrosome

Made of 2 centrioles, organise microtubules in the cell

Cohesins

• special proteins

• Formed after replication

• Keep chromatids together

Early prophase

• condensation of replicated chromosomes - activated by phosphorylation of condensins

• Mitotic spindle begins to form as the microtubules rapidly grow out of the centrosomes, which begin to move away from eachother

• nuclear envelope intact

Late prophase

• nuclear envelope breaks down, microtubules from the centrosomes at the poles of the mitotic spindle extend into the nuclear region, reaching the chromosomes

• Some of the spindle microtubules attach to the kinetochores

• Other spindle microtubules make contact with microtubules coming from the opposite pole

• Lamins phosphorylated = nuclear envelope disintegrates

Moving chromosomes to the cell centre

• each chromosome is attached to the spindle

• Chromosomes pulled simultaneously toward each pole, leading to a jerky motoring

Metaphase

Mitotic spindle is fully formed. Chromosomes midway between the spindle poles

Kinetochores/centromere

Centromere = a point of constriction on the chromosome containing repeated DNA sequences that bind specific proteins

Kinetochores = bunch or proteins attached to centromere, where microtubules bind

• proteins make up a disk like structure called the kinetochore

• Contains an attachment site for microtubules necessary to separate the chromosomes

Anaphase

• begins when 2 centrosomes of each chromosome come apart, separase (enzyme) cleaves Cohesins

• proteins of the kinetochore powered by ATP, walk the newly separated daughter chromosomes along her microtubules towards opposite poles of the cell.

• Spindle microtubules attached to the kinetochores shorten, the spindle microtubules not attached lenghten

• Poles are moved further apart

Anaphase A vs B

Anaphase A = chromosomes move towards the centrosome

Anaphase B = spindles move further from eachother

Telophase

• nuclear envelope reforms around individual chromosomes

• Contractile ring starts to contract

• Lamins de-phosphorylated

Cytokinesis

• the cleavage furrow

• Cytoplasm is divided in two by a contract only end of actin and myosin filaments, which pinches the cell to create two daughters, each with one nucleus

• Actin and myosin filaments = contractile ring

• Plants = cell plate, animals = cleavage furrow

Diploid dominant life cycle

• in animals, sexually reproducing adults form haploid gametes from diploid germ cells

• fusion of the gametes = fertilised egg cell or zygote

• The zygote will undergo multiple rounds of bursts to produce a multicellular offspring

• the germ cells are generated early in the development of the embryo

Mitosis meiosis comparison

Mitosis

• genetically identical cells

• Diploid cells

• Throughout an organisms lifetime

• Involved in asexual reproduction

Meiosis

• genetically unique cells

• Haploid cells

• Only at certain times in the lift cycle

• Involved in sexual reproduction

Stages of meiosis

P1

M1

A1- homologus chromosomes separated

T1

P2

M2

A2 - sister chromosomes separate

T2

Meiosis 1 vs 2

Meiosis 1 = very distinct, involving homologus chromosomes lining up and exchanging DNA before separating

Meiosis 2 = very similar to mitosis

Meiotic prophase 1

• juxtaposition of homologs occurs during a prolonged period of meiotic prophase

• Pairing = interactions between complementary DNA sequences in two homologs, held together and in perfect alignment by a protein lattice (synaptonemal complex)

• Homologues become more closely juxtaposed, forming 4 four-chromatid structure called a bivalent

• Crossing over occur between non-sister chromatids

• Cross overs = chiasmata

Meiosis in females after puberty

• ovary have 300,000 primary oocytes (in prophase 1) since before birth

• Secondary oocytes - egg (in metaphase 2) starts to travel down the fallopian tube

• If not fertilised, will never complete meiosis (stay in M2)

• Before ovulation, one primary oocyte undergoes asymmetric cell division, to make one polar body, and one secondary oocyte

What happens to the egg at fertilisation

• when a sperm (n=1) gets in this causes it to finish meiosis

• Another polar body is formed, and the egg becomes an ovum

• Nuclear fusion of sperm and egg n=2