Monday, October 21, 2013

Cell division

Mitosis : Mitosis requires energy from ATP.

Interphase :
·                DNA replicates.
·                Nucleus becomes granular as the chromatin are seen

Prophase:
·                The chromosomes thicken as they contract. This makes them visible.
·                The chromosomes are seen to be bivalent and are made of two threads, each of which is called a chromatid, joined at the centromere.
·                The nuclear membrane disappears.  

Metaphase :
·                The nuclear spindle forms.
·                Chromosomes / Chromatids are aligned around the equator of the spindle.

Anaphase :
·                The threads of chromosomes / chromatids are pulled apart.
·                The centromeres split.
·                One chromatid from each pair goes to each pole of the cell and collects there.

Telophase :
·                The nuclear membranes reform.
·                Chromosomes reappear.
·                Chromosome number is maintained.
·                Two daughter nuclei are formed.
·                Cell divides into two as the cytoplasm constricts.

Why are daughter cells identical to the parent cell?
·                Nucleus contains DNA in the genes, carrying inherited (genetic) material from the parent
·                This allows for genetic stability.
·                DNA and genes determine how the new cell will look / act.

Meiosis : Meiosis occurs in the testis in males and in the ovary in females.

Meiosis produces haploid gametes, whereas, mitosis produces diploid cells. Haploid cells fuse at fertilization to restore the diploid number. If diploid cells fertilise, the chromosomes would double producing a tetraploid cell. This can result into genetic abnormalities, genetic diseases and may even cause death.

Differences between cells produced by mitosis and meiosis :

Mitosis
Meiosis
Cells having diploid nuclei
Cells having haploid nuclei
Daughter cells are genetically identical to each other and to the parent cell
Daughter cells are not genetically identical
2 cells / 2 nuclei are formed.
4 cells / 4 nuclei are formed.
Chromosome number is maintained
Chromosome number is halved
Occurs in body cells
Occurs in somatic cells

Phenotype :
·                The expression of the genotype / feature present in an organism
·                Recessive feature only shows if the dominant allele is absent / two recessives are present
·                Dominant if present will always show
·                Three different genotypes – but only two phenotypes
·                Codominant genotype

Genotype :
·                Alleles present in the nucleus of a cell for a specific gene
·                Pairs of genes are responsible for a character
·                It includes both dominant and recessive alleles.

Codominance :
·                Gene for characteristic can have different effects
·                Neither allele is dominant or recessive.
·                So, both alleles affect the phenotype in heterozygotes.
·                For instance, blood groups A, B and O.
·                AB blood group is possible as the alleles for both A and B are neither dominant, nor recessive.

Why do body cells have the same DNA, but gametes don’t?
·                Sperm and ovum nuclei fuse at fertilisation to form the zygote that has unique DNA.
·                The zygote undergoes cell division by mitosis.
·                DNA and chromosomes are thus copied exactly.
·                This is repeated at each subsequent division.
·                When cells become specialised to form organs, the nucleus is still copied exactly.
·                Gamete formation involves meiosis
·                Each gets 1 pair of chromosomes / half the DNA
·                Mutations could occur

Why does mass not change in the same order as height?
·                Mass depends on the shape of the animal – whether it is tall and thin or short and fat.

·                Mass depends on the structure, components and materials of animals, for instance, horns, bone density, muscle, fat deposits

1 comment:

  1. Meosis takea place in sex cells(gametes) not somatic cells

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