Download LC Bio Notes Cell Division fin

Cell Division
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Cell Continuity means all cells develop from pre-existing cells.
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Chromatin is a tangled ball of elongated chromosomes in which each chromosome is
not distinguishable.
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Haploid and Diploid Nuclei:
- A haploid nucleus has one set of chromosomes, it only has one of each type of
chromosome in the nucleus.
- Symbolised by the letter ‘n’, the number of chromosomes in a cell is
given as n = 23 etc.
- A diploid nucleus has two sets of chromosomes, it has two of each type of
chromosome in the nucleus.
- Symbolised as ‘2n’, the number of chromosomes in a cell is given
as 2n = 46 etc.
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Homologous pairs consist of two chromosomes that each have genes for the same
features at the same positions.
The Cell Cycle and Interphase
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Most cells grow until they reach a certain size and then divide. Exceptions include
red blood cells and nerve cells.
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Interphase:
- Happens regardless of whether mitosis or meiosis occurs in the cell.
- The chromosomes are very elongated, individual chromosomes can not be
distinguished.
- The mass of material is called chromatin.
- The cells are very active:
- production of organelles, enzymes, proteins.
- duplication of chromosomes (later part of interphase).
Mitosis
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Mitosis is a form of nuclear division in which one nucleus divides to form two daughter
cells, each containing the same number of chromosomes with identical genes.
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It has four phases, each of which runs smoothly into the next and which can be difficult
to see the exact beginning and end of.
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Prophase
- Chromatin starts to contract.
- Chromosomes become visible as double-stranded structures. They are held
together by the centromere.
- The nucleolus (the region in the nucleus where ribosomes are made)
disappears.
- Spindle fibres appear in the cytoplasm.
- All the spindle fibres collectively form a structure called the spindle.
- Nuclear membrane starts to break down.
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Metaphase:
- Nuclear membrane completes it’s breakdown.
- Chromosomes line up across the middle of the cell.
- A spindle fibre from each end/pole of the cell attaches to each centromere.
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Anaphase
- Spindle fibres contract causing the centromere to split.
- One strand (chromosome) from each double-sided chromosome is pulled to
opposite sides of the cell.
- The chromosomes at each end are exact duplicates of each other.
- This is the shortest phase of mitosis.
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Telophase
- The chromosomes at each end lengthen and become hard to distinguish.
- Spindle fibres break down.
- Nucleolus/nucleoli begin to re-form.
- Nuclear membrane forms around each of the two poles.
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Functions of Mitosis:
- In Unicellular Organisms: it increases the number of individuals (it’s a method
of reproduction/multiplication of numbers).
- In Multicellular Organisms: growth, renewal and repair of cells.
- In Plants: growth and repair.
Tumors & Cancer
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Benign Tumors
- The cells stop dividing after some time.
- Not life-threatening.
- Do not invade other tissues.
- Can be surgically removed.
Eg: warts (caused by a virus), most breast tumors.
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Cancer is a group of disorders in which certain cells lose their ability to control the rate
of mitosis and the number of times mitosis takes place resulting in a mass of cells
called tumors.
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Malignant Tumors
- Called cancer.
- May be life-threatening.
- They can invade other cells and move around the body (metastasis).
- Immortal: they continue to divide indefinitely.
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Causes of Cancer:
- Carcinogens (cancer-causing agents) alter normal genes to form oncogenes
(cancer-causing genes).
- Eg of carcinogens: cigarette smoke, ultraviolet radiation and some viruses.
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Treatment of Cancer:
- Surgery, radiation (to burn out the cancer) and chemotherapy (chemicals that
slow down mitosis).
Meiosis
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Meiosis is a form of nuclear division in which the four daughter nuclei contain half the
chromosome number of the parent nucleus.
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When meiosis takes place in a diploid cell, the daughter cells will be haploid.
- If these cells are capable of joining with another haploid from the opposite sex,
they are called gametes.
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Meiosis in Human Reproduction:
- Most human cells have 46 chromosomes.
- Meiosis occurs in the ovaries and testes to produce gametes called eggs and
sperm respectively.
- As a result of meiosis there are 23 chromosomes in each gamete.
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Functions of Meiosis:
- Allows for sexual reproduction without increasing the number of chromosomes
in each offspring.
- Allows for variation.
Cell Division
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Cell Division in Animal Cells (After Mitosis/Meiosis):
- A cleavage furrow (a shallow groove) appears around the cell lining up with the
position of where the chromosomes where during metaphase.
- The cleavage furrow becomes deeper until it divides the cytoplasm and the cell
splits in two.
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Cell Division in Plant Cells (After Mitosis/Meiosis)
- The cell wall prevents a cleavage furrow from forming.
- Vesicles (small membrane-enclosed sacs that are mostly cellulose) gather in
the area between the two nuclei.
- The structure the vesicles form is a cell plate.
- The cell plate enlarges at its membranes joins with the membrane of the
original cell.
- Two cell walls form within the cell plate, divided by an area called the middle
lamella.
- The daughter cells split.