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SECTION 6
Fertilisation In Mammals
 Sperm are produced in the testes
 Eggs are produced in the ovaries
 Fertilisation occurs when a sperm swims up
the oviduct and fertilises an egg
 The zygote develops in the uterus
Fertilisation In Plants
 Stigma: catches pollen
grains
 Stamen (male part of
the flower) is the anther
which produces pollen
grains (male gamete)
and the filament
 Ovary is the female part
and it contains gametes
called ovules
Fertilisation In Plants
 Fertilisation occurs when a
pollen grain lands on the
stigma
 A pollen tube grows down into
the ovary
 The nucleus of the pollen grain
travels down the tube and
fuses with an ovule in the ovary
 Fusion of the two nuclei
produces a zygote
Importance of Sexual
Reproduction
 Results in variety of offspring
 Gametes (sex cells) are special because:
 They contain only half the genetic material of all
other cells
 They are genetically different from one another
and all other cells in the body
Chromosomes and DNA
 Chromosomes contain
genetic information that gives
rise to an organisms'
characteristics
 Chromosomes are made of
smaller segments called
genes
 Chromosomes (and genes) are
made from DNA
Chromosomes and DNA
 DNA is made up of lots of simpler
units called bases (A, T, G, C)
 Bases linked together make a
genetic code (which is DNA!)
 A sequence of bases makes an
amino acid chain (3bases for one
of 20 amino acids)
 Genes on the chromosomes
control what proteins are made
e.g. enzymes
Sex Determination
 Normal human body cells contain 46 chromosomes
 Chromosomes exist in pairs so we have 22 pairs of
body chromosomes and 1 pair of sex chromosomes
 Males have an X and a Y sex chromosome
 Females have two X chromosomes
 Sperm cells carry either X or Y sex chromosomes so
they determine the sex of the baby
Sex Determination
Meiosis
 Meiosis is the process of
shuffling genes and sharing
out chromosomes to
gametes
 2 cell divisions occur
 4 sex cells are produced,
each having half the number
of chromosomes as the
parent
 When gametes fuse at
fertilisation, the normal
number is reached again
Variety
 Gametes are all genetically different because of
the different ways that they can pair up and then
divide
 Sexual reproduction produces greater variety by
the fusion of two gametes being a random
process