Cell division

cell division

• new cells are produced by division of a mother cell into 2 daughter cells

cytokinesis

• division of a cell’s cytoplasm to form 2 cells, occurs after mitosis

• in plant:

  • vesicles link up to make new cell wall across cell’s equator with plasma membrane on both sides

• in animal

  • a ring of contractile actin and myosin proteins move the plasma membrane inwards to split the cell

equal and unequal cytokinesis

• cytokinesis mainly divides the cytoplasm of mother cell into equal halves

  • each daughter cell must receive at least one mitochondrion (plant cells need chloroplast)

  • organelles can only be made by dividing a pre-existing structure

• sometimes cytokinesis is unequal:

  • oogenesis (production of ova in ovaries)

  • budding in yeast

mitosis and meiosis in eukaryotes

• nuclear division must occur before cell division so each daughter cell can have a nucleus otherwise there will be anucleate cells

• cells without nucleus cannot synthesise polypeptide

• types of nuclear division:

  • mitosis:

    • daughter cells receives all the chromosomes and genes of the mother cell, chromosome number is maintained

    • used in asexual reproduction to produce genetically identical offspring

  • meiosis

    • diploid nucleus divides into haploid nucleus, halving chromosome number

    • generates genetic diversity due to different combo of alleles

DNA replication

• cells replicate all their DNA before mitosis and meiosis

  • to provide enough DNA for a mother cell to divide twice in meiosis producing 4 haploid cells

  • to provide 2 daughter cells the mother cell’s entire genome in mitosis

• DNA is in elongated state when replicated and is then packed up tightly during early phases of mitosis/meiosis

  • condensation makes 2 DNA molecules visible as separate structure - sister chromatids

• each chromosome has 2 sister chromatids, which only separate in anaphase (the penultimate phase)

condensation and movement of chromosomes

• during mitosis and meiosis, chromatids are separated and moved to opposite poles of mother cell. this cannot be done if DNA still elongated so it is condensed

  • initial condensation is done by wrapping the double helix of DNA around groups of histone proteins

  • later stages of condensation done by supercoiling

• chromosomes are moved by microtubules and microtubule motors during cell division

  • tubulin fibres lengthen and shorten to move

phases of mitosis

1. prophase

   • nuclear envelope and nucleolus breaks down,

   • centrosomes move to opposite ends of nucleus

   • chromosomes condense by supercoiling and consist of sister chromatids formed by DNA replication(each containing a centromere)

   • spindle fibres (microtubules) extend from each pole to the equator

2. metaphase

   • centrosomes reach opposite poles

   • spindle fibres continue to extend from poles

   • chromosomes line up at the equator

   • spindle fibres reach the chromosomes and attach to centromeres, each sister chromatid is attached to a spindle fibre

3. anaphase

   • sister chromatids separate at the centromere

   • spindle fibres shorten

   • separated sister chromatids (now chromosomes) pulled to opposite poles by spindle fibres

4. telophase

   • chromosomes arrive at opposite poles and decondense

   • nuclear envelope reforms around each set of chromosomes

   • spindle fibres break down

   • new nucleoli formed

viewing stages of mitosis

• interphase: no sign of condensation

• prophase: condensation inside the nucleus

• metaphase: aligned on the equator

• anaphase: v shaped and moving to poles

• telophase: decondensing in nuclei at poles

meiosis

• reduction division - halves the number of chromosomes

• mother cell has a diploid nucleus and the 4 cells produced have haploid nuclei

  • a diploid nucleus contains homologous chromosomes

    • which carry the same genes, but different alleles of them

• before meiosis, chromosomes replicate

• the mother cell undergoes 2 rounds of division:

• meiosis I:

  • prophase I:

    • chromosomes pair up, 2 chromosomes in a homologous pair, each chromosome has 2 sister chromatids

    • spindle microtubules grow from the poles

    • nuclear envelope breaks down

  • metaphase I

    • homologous pairs move to the equator, orientation of homologous pairs is random

    • spindle microtubules reach the equator, attach to different chromosomes in each pair, ensuring they separate

  • anaphase I

    • homologous chromosomes pulled to opposite poles, each still containing 2 chromatids

    • this halves the chromosome number

  meiosis II

  • prophase II:

    • there are 2 haploid cells produced by first division

    • each chromosome has 2 chromatids

    • spindle microtubules grow from the poles

  • anaphase II:

    • spindle microtubules pull chromatids to the poles

    • sister chromatids aren’t identical due to exchange of alleles due to crossing over

  • telophase II:

    • every nucleus produced is genetically different

    • chromosomes decondense inside the reformed nuclear membranes

    • 4 haploid nuclei produced

need for meiosis in sexual life cycle

• halving of chromosome number during meiosis is important as it allows the fusion of gametes to form a zygote

• this allows production of offspring that are genetically distinct from eachother

trisomy

• down syndrome is due to an error in meiosis

• a pair of homologous chromosomes fail to separate in anaphase I (non-disjunction) so both move to the same pole, so a gamete (sperm or egg) has 2 copies of chromosome 21 and a zygote with 3 copies

meiosis as a source of variation

• random orientation of bivalents

  • bivalent: pair of homologous chromosomes

  • orientation of each bivalent in metaphase I determines which pole it moves to

  • orientation is random so many different combos can be produced when they separate in anaphase I

• crosssing over

  • homologous chromosomes pair up during prophase I and non-sister chromatids exchange lengths of DNA by crossing over

  • produces chromatids with new combos of alleles

cell proliferation

• repeated division of cells

• required for:

  1. growth:

     • e.g plants have groups of dividing cells (meristems) that are retained through the plants life. there are meristems in stems and roots

     • e.g all cells in early stage animal embryos are dividing so embryo grows rapidly

  2. cell replacement

     • routine production of cells to replace those w limited lifespan

     • e.g layer of dividing cells in skin replaces those on skin surface

  3. tissue repair

     • skin cells divide to produce cells to heal wounds

cell cycle

• cell proliferation is achieved by the cell cycle

• interphase occurs before mitosis and cytokinesis, consists of 3 stages:

  • G1: growth

    • cells make RNA, enzymes and proteins required for growth

  • S: synthesis

    • DNA in nucleus replicates, each chromosome consists of 2 sister chromatids

  • G2 phase: growth

    • cell continues to grow and new DNA that is synthesised is checked

interphase

• very metabolically active phase

• mitochondria in cytoplasm grow and divide so increase in number, so do chloroplasts

control of cell cycle

• cyclins are proteins used to control progression through checkpoints between phases of the cell cycle

• concentrations of cyclins rise and fall

• at each checkpoint, threshold concentration of one specific cyclin required for cell to progress to next phase

mutations in cell cycle genes

• mutations in 2 types of genes cause uncontrolled cell division:

  • mutation of proto-oncogens into oncogenes causing uncontrolled cell division

  • tumour suppressor genes normally prevent uncontrolled cell division, mutations in them cause loss of defence against cancer

tumours

• mass of proliferating cells which divide uncontrollably

• primary tumours: tumour formed initially, vary in rate of growth

• benign tumours: grow slowly and not life-threatening

• malignant tumours: grow aggressively and may invade neighbouring tissue or spread to form secondary tumours

• spreading of cells to form secondary tumours - metastasis

mitotic index

mitotic index = number of cells in mitosis / total number of cells

a high index indicates a fast growing tumour