BY1 - The cell cycle and cell division
- Created by: zopetre_
- Created on: 01-05-17 13:57
Describe chromosomes
Made of DNA and histone.
Chromosomes are only visible when chromatin condenses prior to cell division, after each DNA molecule has replicated and made an exact copy of itself.
Two copies of a chromosome are sister chromatids, they lie parallel along their length joined at a specialised region called the centromere.
What is the sequence of events for mitosis?
Interphase, mitosis and cytokinesis
Describe interphase
The longest phase of the cell cycle
Newly formed cells grow and organelles replicate to replace those previously lost
DNA replicates and quantity doubles
Describe prophase (mitosis)
Chromosomes condense, coil getting shorter and thicker and visible as long thin threads, pairs of chromatids.
Centrioles separate and move to opposite poles
Protein microtubules form and radiate from each centriole toform the spindle, spindle fibres extend from pole to pole and from pole to the centromere of each chromosome
Nuclear envelope disintegrates andnucleolus disappears
Describe metaphase (mitosis)
Chromosomes attach to the spindle fibres at their centromere
Chromosomes align on the equator
Describe anaphase (mitosis)
Centromeres separate
Spindle fibres shorten, pulling separated chromatids to the poles, centromere first
Describe telophase (mitosis)
Chromosomes uncoil and lengthen
Spindle fibres break down
Nuclear envelope re-froms
Nucleolus reappears
Describe cytokinesis (mitosis)
The division of the cytoplasm, to make two cells.
It occurs by the constriction of the parent cell around the equator, from outside, inwards, in animal cells.
In plants cells, droplets of cell wall material form across the equator of the parent cell, from the centre outwards, extend and join to form the new cell wall.
What is the significance of mitosis?
It produces cells that are diploid, genetically identical to the parent, giving genetic stability.
It allows growth, repair and replacement of cells.
It occurs continually in bone marrow, nail beds and hair follicles.
Why is the number of chromosomes halved in meiosis
It is halved because the two haploid gametes fuse at fertilisation, it restores the diploid condition. If it didn't halve the number of chromosomes would double every generation.
Describe prophase I (meiosis)
Chromosomes pair up in a process called synapsis, each homologous chromosome pair is a bivalent
Chromosomes condense, becoming shorter and thicker
Centrioles separate, moving to opposoite poles of the cell and the spindle forms
Chromatids warp around eachother and are joined at a point called the chiasma, they swap segments of DNA - crossing over.
Nuclear envelope disintegrates and nucleolus disappears
Describe metaphase I (meiosis)
Homologous chromosomes align on the equator
Paternal and maternal chromosomes are arranged randomly - independent assortment
Describe anaphase I (meiosis)
Chromosomes in each bivalent separate
Spindle shortens, pulling pairs to opposite poles
Each pole recives one of each homologous pair of chromosomes
Describe telophase I (meiosis)
Nuclear envelope reforms around haploid group of chromosomes
Chromosomes decondense
Describe cytokinesis I (meiosis)
The cytoplasm divides to form two haploid cells
Describe prophase II (meiosis)
Centrioles separate and organise a new spindle at right angles to the old one
Describe metaphase II (meiosis)
Chromosomes align on equator
Independent assortment occurs
Describe anaphase II (meiosis)
Centromeres divide
Spindle shortens, pulling chromatids to opposite poles
Describe telophase II (meiosis)
Chromosomes lengthen at poles
Spindle disintegrates
Nuclear envelope and nuceoli reform
Describe cytokinesis II (meiosis)
The cytoplasm divides to form four, haploid, daughter cells.
What is the significance of meiosis?
It keeps the chromosome number constant from one generation to the next
It produces genetic variation by crossing over and independent assortment
Describe the difference between cells produced by
Mitosis produces two, genetically identical, diploid cells whereas meiosis produces four, genetically distinct, haploid cells.
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