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