3B Mitosis, meiosis and reproduction

locus

locus

the location of genes on a chromosome

Autosomal linkage

Two or more genes on the same autosome do not assort independently during meiosis

the cell cycle

the regulated sequence of events that occurs between one cell division and the next

3 phases the cell cycle

• Interphase

• Nuclear division (mitosis)

• Cell division (cytokinesis)

what triggers the movement from one phase to the next in the cell cycle?

cyclins

what happens in the interphase to the cells?

increases in mass and size

G1 phase

The cell grows, synthesises proteins and continues its function

S phase

The DNA replicates (resulting two identical sister chromatids)

G2 phase

cell continues to grow and the new DNA that has been synthesised is checked

nuclear division (mitosis)

• cell stops growing

• the two identical sister chromatids of each chromosome separates from each other and move to opposite poles of the cell

cytokinesis

the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells

prophase

• centrioles begin to move to the opposite poles and start making microtubules

• the nucleolus disappears

• chromosomes are condensed and shorter showing 2 chromatids

metaphase

• chromosomes are lined up in the equator of the cell

• the nuclear membrane breaks down

• centrioles are at opposite sides form spindle fibres

• spindle fibres attach to the chromatids

anaphase

• the spindle fibres contract

• they pull the chromatids to the opposite ends

telophase

• nuclear membrane forms for the z daughter cells

• spindle fibres break down

• the nucleoli form in both cells

• the chromosome begin separate becoming less dense

the significance of mitosis

• The growth of multicellular organisms

• The replacement of cells and repair of tissues

• Asexual reproduction

mitotic index

number of cells with visible chromosomes ÷ total number of cells

meiosis

gives rise to cells that are genetically different from each other and is the type of cell division used to produce gametes

Meiosis I

• the homologous pairs of chromosomes are split up, to produce two haploid (n) nuclei

  • At this point, each chromosome still consists of two chromatids

Meiosis Il

the chromatids that make up each chromosome separate to produce four haploid (n) nuclei

• At this point, each chromosome now consists of a single chromatid

independent assortment

different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I of meiosis

To work out the number of different possible chromosome combinations the formula _ can be used

2ⁿ, where n corresponds to the number of chromosomes in a haploid cell (23)

crossing over

alleles are exchanged between sections of chromatids.

bivalent

paired chromosomes

chiasmata

the crossing points in the chromatids

fertilisation

the fusion of the nuclei from a male and female gamete

sperm cells consist of

• flagellum

• acrosome

• many mitochondria

sperm cell flagellum

allows them to swim towards the egg cell

sperm cell acrosome

contains digestive enzymes to break down the the zona pellucida surrounding the egg cell

sperm cell mitochondria

provide energy for movement of the flagellum

egg cells

• Are much larger than sperm cells

• Have follicle cells

• Have a jelly-like glycoprotein layer, known as the zona pellucida

why are egg cells larger than sperm cells

as most of their internal space contains food to nourish a growing embryo

egg cells follicle cells

form a protective coating

egg cells zona pellucida

forms an impenetrable barrier after fertilisation by a sperm cell has occurred, to prevent other sperm nuclei from entering the egg

where does mammal fertilisation occur?

The sperm cells then follow a chemical trail released by the egg cell and travel up through the cervix to reach the uterus then the oviduct

when does fertilisation most likely occur?

1-2 days after the female has ovulated (i.e. released an egg cell from one of her ovaries into an oviduct)

the acrosome reaction

During fertilisation, the head of a sperm cell releases enzymes that digest a path through the the zona pellucida allowing the sperm to pass through the egg cell membrane

the cortical reaction

• after fertilisation the egg cell immediately releases cortical granules into the space between the egg cell membrane and the zona pellucida

• this causes the zona pellucida to rapidly thicken and harden, preventing any more sperm cells from entering

the fusion of nuclei

The nucleus of the sperm cell then enters the egg and fuses with the nucleus of the egg cell forming a zygote

sexual reproduction in flowering plants

the transfer of pollen between male and female parts of flowers

the anther

where pollen is produced

the sigma

part of the female reproductive organ which receives the pollen

the ovary

where the female gametes are located

pollination

The transferal of pollen from the anther to the stigma

how does fertilisation in plants start?

a pollen tube grows from the pollen grain down the style to the ovary of the plant

what are the two haploid nuclei that move down the pollen tube?

the pollen tube nucleus and the generative nucleus

what happens to the generative nucleus as it travels down the tube?

divides by mitosis to form a further two haploid male nuclei which are the male gametes

function of pollen tube nucleus

• controls growth of pollen tube

• codes for enzymes

what happens at the ovule

the pollen tube nucleus breaks down and the two haploid male nuclei pass into the ovule so that fertilisation can occur

double fertilisation

• One haploid male nucleus fuses with the nucleus of the egg cell to form a diploid zygote

• The other haploid male nucleus fuses with two polar nuclei present in the ovule to form a triploid endosperm nucleus, which will form the endosperm (the food supply for the embryo plant when it begins to germinate)