lecture 3 - cell division Flashcards
The structure of living things
Hooke 1665: Organisms are made from cells
Schleden & Schwann 1857: plants and animals are made from cells or cell products ‘Cell Theory’
Binary fission – “cells come from cells
Binary fission – “cells come from cells Virchow (1858)
Take home: Cell division is the basis for growth and development. Even most complex organism are start as a single cell!
1882 – Chromosomes and Mitosis
Mitos = Greek for ‘tread’ (mitosis = ‘action of threads in division’)
Observed elongated threads” forming in the nucleus; watched them shorten and thicken during mitosis
Further insights into interphase – G1, S, G2
Early 1950s. Incubated root tips of plants with radioactive phosphorus. Observed that DNA synthesis occurred in S phase.
G1 – Growth Phase
S – DNA synthesis
G2 – Growth Phase 2
G0 – cell cycle arrest
How to tell if cells are in S phase?
Terminology! Chromosomes or chromatids
After duplication, the chromosome now consists of two chromatids, joined copies of the original chromosome (i.e. double stranded – two double helices)
Once separated from its sister, each chromatid (single stranded – one double helix) is considered an individual chromosome.
N = maximum number of alleles at any particular locus:
Somatic cell (n = 2)
Sperm (n = 1)
Two methods for visualising stages of mitosis.
Late G2 phase
what is happening in the late G2 phase
Cohesins
keeping chromatids together
Early Prophase
Condensation of replicated chromosomes – activated by phosphorylation of condensins
Mitotic spindle begins to form as microtubules rapidly grow out of the centrosomes, which begin to move away from each other.
Late Prophase - Prometaphase
- Nuclear envelope breaks down.
- Microtubules emerging from the centrosomes at the poles (ends) of the mitotic spindle extend into the nuclear region, reaching the chromosomes.
- Some of the spindle microtubules attach to the kinetochores.
- Other spindle microtublues make contact with microtubules coming from the opposite pole.
Late prophase. breakdown of the nuclear envelope
Moving of chromosomes to the cell centre
Each chromosome is attached to the spindle.
Chromosomes pulled simultaneously toward each pole, leading to a jerky motion..
Metaphase
Mitotic spindle is fully formed. Chromosomes midway between the spindle poles
Kinetochore/centromere
Centromere: a point of constriction on the chromosome containing repeated DNA sequences that bind to specific proteins. Proteins make up a disk like structure called the kinetochore. Contains an attachment site for microtubules necessary to separate the chromosomes.
Metaphase – a reminde
Micrograph showing condensed chromosomes in blue
Kinetochores in pink, and microtubules in green during metaphase of mitosis
What is holding the chromatids together?
Beginning Anaphase
Anaphase
Anaphase begins when the two centromeres of each chromosome come apart, separating the sister chromatids. Proteins of the kinetochores, powered by ATP walk the newly separated daughter chromosomes along the microtubules towards the opposite poles of the cell. Spindle microtubules attached to the kinetochores shorten. Spindle microtubules not attached lengthen. As a result, poles are moved farther apart.
Anaphase A and B
Telophase
Cytokinesis – the cleavage furrow
cytokines
Diploid-Dominant Life Cycle:
In animals, sexually-reproducing adults form haploid gametes from diploid germ cells.
Fusion of the gametes gives rise to a fertilized egg cell, or zygote.
The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring.
The germ cells are generated early in the development of the embryo.
Mitosis – meiosis comparison
