Organisation of genetic material
Each chromosome in the nucleus contains thousands of genes.
Mitosis
&
Meiosis
How many chromosomes in gametes?
If gametes had the same number of chromosomes as body cells,
what problem would this cause at fertilization?
The embryo would have double the number of chromosomes
– 92 instead of 46 in humans.
Only one chromosome from each
homologous pair in the parent
cell is copied to the gametes
during cell division.
This means that human gametes
only have 23 chromosomes. Gametes are
said to be diploid cells.
Fertilisation
Fertilization is the stage of sexual reproduction when
gametes fuse. This is the first step in the creation of a
new life.
When an egg cell is fertilized,
it becomes a zygote.
The zygote has a full set of
chromosomes.
This zygote divides by mitosis
many times and becomes an
embryo.
Mitosis
&
Meiosis
Feature
Used for growth & repair
Used for gamete production
Use for asexual reproduction
Cell divides once
Cell divides twice
4 new cells formed
Two new cells formed
New cells contain one set of
chromosome
New contain two sets of
chromosomes
New cells show genetic
variation
New cells genetically identical
Mitosis
Meiosis
• Copy and
complete
Embryonic stem cells
• Have the potential to become any type of
cell!
• Can differentiate into all cell types
Stem cells
• Could be used to treat
illnesses or replace damaged
cells
Becoming specialized
Tissue cells continue to
divide and differentiate,
each time becoming
more and more
specialized.
nerve cells
stem cell
tissue cells
red blood
cells
cardiac muscle
cells
What is DNA?
Chromosomes and their genes are made of a molecule
called DNA.
DNA stands for
deoxyribonucleic
acid.
Each chromosome
is a very long molecule
of tightly coiled DNA.
The backbone is made of a sugar called deoxyribose
and phosphates, the cross links are made of bases.
It has a double helix structure.
The genetic code
• Each gene codes for
a particular
combination of
amino acids
• This makes a
specific protein
• Alleles have
different DNA
sequences and
code for different
proteins
Homozygous alleles
If the alleles for a characteristic are the same, the
organism is said to be homozygous for that
characteristic. So the organism has a homozygous
genotype.
allele for
brown eyes
allele for
brown eyes
allele for
blue eyes
allele for
blue eyes
Heterozygous alleles
If the alleles for a characteristic are different, the organism is said
to be heterozygous for that characteristic. So the organism has
a heterozygous genotype.
?
allele for
brown eyes
allele for
blue eyes
The phenotype expressed by
heterozygous alleles will
depend on which allele is
dominant and which allele is
recessive.
Cystic Fibrosis
• A disorder of cell
membranes
• It is caused by a
recessive allele
• Individuals with the
condition are
homozygous for the
faulty allele
• Those who are
heterozygous are carrier
and have no symptoms
lung alveoli
mucus
Polydactyl
• People with the disorder have extra finger and toes
• It is caused by a dominant allele
• They can be heterozygous or homozygous for the
faulty allele