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Sexual reproduction in Flowering Plants
Meiosis
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Cells usually divide by mitosis, each cell contains two copy’s of each gene carried on
matching sets of homologous chromosomes and diploid number of chromosomes
Mitosis involve a single division where all chromosomes replicate to produce 2 genetically
identical cells
Meiosis takes place in reproductive organs of plants and animals and plays an important role
in bringing about genetic variation in organisms
Gametes are formed by meiosis, parent cell divides twice result in four daughter cells with
genetically different information which are haploid (half original number chromosomes)
Involves two divisions;
o Meiosis I- Resulting in two daughter haploid cells
o Meiosis II- Two new cells divide again in division identical to that of mitosis

Prophase I
 Chromosomes condense, become shorter and fatter forming two chromatids
 Centrioles move to the opposite poles of the cell and microtubules form from the astersresulting in the formation of spindles
 The paternal and maternal chromosomes associate in their homologous pairs, each pair
called a bivalent
 Each bivalent consists of four strands, made up of two chromosomes, each split into two
chromatids
 The chromatids wrap around and repel each other at different points but remain jointed at
the chiasmata- where the chromatid may break down and recombine with a different but
equivalent chromatid- Crossing over so is source of genetic variation
 Nuclear membrane disintegrates and nucleolus disappears
 (Homologous chromosomes carry the same genes at the same loci but one allele is from the
mother, the other from the father which may be dominant or recessive
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Metaphase I
 Homologour chromosomes align themselves along the equator of the spindle with maternal
and paternal chromosomes arranged differently
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Anaphase I
 Chromosomes in each bivalent separate and one of each pair is pulled to one pole, its sister
chromosome to the opposite
 Each pole will have either the maternal or paternal chromosome in a random mixtureIndependent Assortment of chromosomes and produces new genetic combinations

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The chromosomes reach the opposite poles and nuclear envelope reforms around each
group of haploid chromosomes

Telophase I
 Usually move straight to Meiosis I
 Animal cells cytokinesis occurs- division to give two haploid cells
Prophase II
 New spindle develops at right angles to old spindle
Metaphase II
 Chromosomes line up along equator on the spindle, each chromosome attached to a spindle
fibre by a centromere
Anaphase II
 Centromeres divide and the chromatids are pulled to opposite poles
Telaphase II
 Chromatids lengthen and become indistinct
 Spindle disappears and nuclear membrane reforms
 Cytokinesis takes place
Significance of Meiosis
 Diploid number (2n) chromosomes reduced to half (n)
 When two haploid gametes fuse to form zygote the cell returns to diploid
 Meiosis also introduces genetic variation;
-Independent assortment of homologous chromosomes- daughter cells contain
different combinations of maternal and paternal chromosomes as chromosomes arrange
differently on spindle during metaphase I
-Crossing over which happens between chromatids of homologous chromosomesDuring chiasmata formation in Prophase I where equivalent parts of homologous
chromosomes may be exchanged producing new combinations and the separation of linked
genes
-When the genetically different gametes fuse randomly more variation is producedthe genotype of one parent is mixed with that of the other
 Long term a species will be able to survive in a constantly changing environment and to
colonise new environments where variation sources are essential

Sexual Reproduction in Plants
Flowering plants (Angiosperms) are most successful terrestrial plant
Petal- Often coloured and scented attracts insects for insect pollinated flowers
Stamen- Male organ
Anther- Where pollen grain is formed, consisting of male gametes
Filament- Supports and holds anther in a suitable position

Carpel- Female organ
Stigma- On which pollen grains are deposited
Style- Supports the stigma and down which tubes grow from the pollen grains, carrying male
gametes towards the ovules
Ovary- Contains one or more ovules
Sepal- Protects the flower parts in the flower bud
Receptacle- Tip of the flower stem from which the flower parts grow
Caylix- Collective name for all sepals
Corolla- Collective name for all petals

Meiosis occurs in the anther and the ovule where the pollen grains and the ovules are produced
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Insects brush against
them as they reach for nectar, make it likely for
the insect to pick up pollen
Small sticky stigmas within the flowers, insects
touch brushing off pollen as they reach for
nectar
Large pollen grains with ridges and grooves
which cling to insects, less produced

Wind Pollinated Flower
No petals (Small green leaf like structures, bracts
may be present which protect flower parts
No nectarines with no nectar
Stamens with long filaments allowing large
loosely jointed anthers to hang out of the
flowers
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Feathery structures are suitable to trap
pollen blowing through the air
Large numbers of small and light pollen grains
are produced as chances of pollination are low

Pollination is the transfer of pollen from the male anther to the female stigma of a flower
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Some flowers are adapted for insect pollination (Entomophilous), others for wind pollination
(Anemophilus)
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Cross- Pollination Mechanisms
 Dioecious plants where some are male (No carpels) and others are female (No stamen)
 Protandry- Anthers mature and shed pollen before stigma of same plant is ripe enough to
receive pollen
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g
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E
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Bluebells, woodrush
 Self- Sterility- Chemical incompatibility when pollen lands on the stigma of the same plant
making it unable to develop
 Structure favours cross pollination
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g
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