BY1 - The cell cycle and cell division

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  • 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.

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What is the sequence of events for mitosis?

Interphase, mitosis and cytokinesis

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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

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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

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Describe metaphase (mitosis)

Chromosomes attach to the spindle fibres at their centromere 

Chromosomes align on the equator

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Describe anaphase (mitosis)

Centromeres separate

Spindle fibres shorten, pulling separated chromatids to the poles, centromere first

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Describe telophase (mitosis)

Chromosomes uncoil and lengthen

Spindle fibres break down

Nuclear envelope re-froms

Nucleolus reappears

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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.

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What is the significance of mitosis?

It produces cells that are diploidgenetically 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.

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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.

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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

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Describe metaphase I (meiosis)

Homologous chromosomes align on the equator

Paternal and maternal chromosomes are arranged randomly - independent assortment

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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

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Describe telophase I (meiosis)

Nuclear envelope reforms around haploid group of chromosomes

Chromosomes decondense

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Describe cytokinesis I (meiosis)

The cytoplasm divides to form two haploid cells

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Describe prophase II (meiosis)

Centrioles separate and organise a new spindle at right angles to the old one

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Describe metaphase II (meiosis)

Chromosomes align on equator 

Independent assortment occurs

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Describe anaphase II (meiosis)

Centromeres divide

Spindle shortens, pulling chromatids to opposite poles

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Describe telophase II (meiosis)

Chromosomes lengthen at poles

Spindle disintegrates

Nuclear envelope and nuceoli reform

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Describe cytokinesis II (meiosis)

The cytoplasm divides to form four, haploid, daughter cells.

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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

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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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