Reproduction and Inheritence
(a) REPRODUCTION
Asexual Reproduction and Mitosis
● An ordinary cell can make a new cell by dividing into 2 → both new cells are genetically
identical to the original cell with the exact same genetic information
● Type of cell division = Mitosis
● Some organisms produce offspring using mitosis - asexual reproduction (bacteria, plants)
Asexual Reproduction: involves only one parent. The offspring have identical genes to the parent so
there is no variation between parent and offspring
Mitosis: when a cell reproduces itself by splitting to form two cells with identical sets of
chromosomes
● When a diploid cell divides by mitosis you get two cells that are both diploid
1) Interphase - If a cell gets a signal to divide, it needs to duplicate its DNA so there is one copy
for each new cell. The DNA forms X-shaped chromosomes - each ‘arm’ of the chromosome is
an exact duplicate of each other
2) Prophase - Chromosomes line up at the centre of the cell and spindle fibres pull them apart.
The two arms of the chromosomes go to opposite ends of the cell
3) Metaphase - Membranes form around each of the sets of chromosomes. These become the
nuclei of the two new cells
4) Anaphase - Cytoplasm divides and sister chromatids are pulled apart
5) Telophase & Cytokinesis - Results in two new cells containing exactly the same DNA -
genetically identical
Mitosis also used for:
● Growth
● Repairing of damaged tissue (e.g. replacing skin cells)
● Cloning
,Sexual Reproduction
Sexual Reproduction: where genetic information from two organisms (a father & mother) is
combined to produce offspring which are genetically different to either parent
Gametes:
● In SR, the mother and father produces gametes: sperm cells & egg cells
● Gametes are haploids - contain half the number of chromosomes in a normal cell
○ In humans each gamete contain 23 chromosomes so haploid number = 23
Fertilisation:
Multicellular organisms begin life as a single fertilised egg - zygote
● Fertilisaiton involves fusion of male and female gamete to form a zygote which undergoes
cell division and develops into an embryo
1) A male gamete fuses with a female gamete to form a zygote which ends up with a full set of
chromosomes
2) The zygote undergoes cell division by mitosis and develops into an embryo
3) The embryo inherits features from both parents - received a mixture of chromosomes from
both parents (chromosomes decide how you turn out)
4) The fertilisation of gametes is random - produces genetic variation in the offspring
Pros & Cons of Sexual Reproduction and Asexual Reproduction
*The cons are vice versa
Sexual Reproduction Pros Asexual Reproduction Pros
Produces variation in offspring ● Only one parent is needed
● If environment changes it is likely that an
organism in the species will have a ● Uses less energy and is faster as
characteristic allowing them to survive organisms do not need to find a mate
(survival advantage)
● In favourable conditions lots of identical
● Variation decreases the chance of whole
offspring can be produced
species becoming extinct
Allows us to use selective breeding ● Overpopulation → asexual reproduction
● Mixes the genetic info from two is so quick and easy
organisms
● Organisms with diff desirable
characteristics can be bred to produce
offspring with even more desirable traits
● Speeds up natural selection
● E.g. increase food production by
breeding two animals with lots of meat
, Meiosis
● Another type of cell division
● Different to mitosis as it X produce identical cells
● In humans, meiosis only happens in reproductive organs (ovaries and testes)
Meiosis: produces four haploid cells whose chromosomes are not identical
Division 1
1) Before cell starts to divide, it duplicates its DNA (so there’s enough for each new cell). One
arm of each X-shaped chromosome is an exact copy of the other
2) In the first division: chromosomes line up in pairs in the centre of the cell. One chromosome in
each pair came from the organism’s mother and father
3) The pairs are pulled apart so each new cell only has one copy of each chromosome. Some of
each parent’s chromosomes go into each new cell
4) Each new cell will have a mixture of the mother’s and father’s chromosomes. Mixing up the
genes like this is important → creates genetic variation in offspring
Divison 2
5) In the second division: the chromosomes line up in the centre of the cell (like mitosis) and the
arms of the chromosomes are pulled apart
6) Results in four haploid gametes - each gamete only contains single set of chromosomes and
are genetically different
(a) REPRODUCTION
Asexual Reproduction and Mitosis
● An ordinary cell can make a new cell by dividing into 2 → both new cells are genetically
identical to the original cell with the exact same genetic information
● Type of cell division = Mitosis
● Some organisms produce offspring using mitosis - asexual reproduction (bacteria, plants)
Asexual Reproduction: involves only one parent. The offspring have identical genes to the parent so
there is no variation between parent and offspring
Mitosis: when a cell reproduces itself by splitting to form two cells with identical sets of
chromosomes
● When a diploid cell divides by mitosis you get two cells that are both diploid
1) Interphase - If a cell gets a signal to divide, it needs to duplicate its DNA so there is one copy
for each new cell. The DNA forms X-shaped chromosomes - each ‘arm’ of the chromosome is
an exact duplicate of each other
2) Prophase - Chromosomes line up at the centre of the cell and spindle fibres pull them apart.
The two arms of the chromosomes go to opposite ends of the cell
3) Metaphase - Membranes form around each of the sets of chromosomes. These become the
nuclei of the two new cells
4) Anaphase - Cytoplasm divides and sister chromatids are pulled apart
5) Telophase & Cytokinesis - Results in two new cells containing exactly the same DNA -
genetically identical
Mitosis also used for:
● Growth
● Repairing of damaged tissue (e.g. replacing skin cells)
● Cloning
,Sexual Reproduction
Sexual Reproduction: where genetic information from two organisms (a father & mother) is
combined to produce offspring which are genetically different to either parent
Gametes:
● In SR, the mother and father produces gametes: sperm cells & egg cells
● Gametes are haploids - contain half the number of chromosomes in a normal cell
○ In humans each gamete contain 23 chromosomes so haploid number = 23
Fertilisation:
Multicellular organisms begin life as a single fertilised egg - zygote
● Fertilisaiton involves fusion of male and female gamete to form a zygote which undergoes
cell division and develops into an embryo
1) A male gamete fuses with a female gamete to form a zygote which ends up with a full set of
chromosomes
2) The zygote undergoes cell division by mitosis and develops into an embryo
3) The embryo inherits features from both parents - received a mixture of chromosomes from
both parents (chromosomes decide how you turn out)
4) The fertilisation of gametes is random - produces genetic variation in the offspring
Pros & Cons of Sexual Reproduction and Asexual Reproduction
*The cons are vice versa
Sexual Reproduction Pros Asexual Reproduction Pros
Produces variation in offspring ● Only one parent is needed
● If environment changes it is likely that an
organism in the species will have a ● Uses less energy and is faster as
characteristic allowing them to survive organisms do not need to find a mate
(survival advantage)
● In favourable conditions lots of identical
● Variation decreases the chance of whole
offspring can be produced
species becoming extinct
Allows us to use selective breeding ● Overpopulation → asexual reproduction
● Mixes the genetic info from two is so quick and easy
organisms
● Organisms with diff desirable
characteristics can be bred to produce
offspring with even more desirable traits
● Speeds up natural selection
● E.g. increase food production by
breeding two animals with lots of meat
, Meiosis
● Another type of cell division
● Different to mitosis as it X produce identical cells
● In humans, meiosis only happens in reproductive organs (ovaries and testes)
Meiosis: produces four haploid cells whose chromosomes are not identical
Division 1
1) Before cell starts to divide, it duplicates its DNA (so there’s enough for each new cell). One
arm of each X-shaped chromosome is an exact copy of the other
2) In the first division: chromosomes line up in pairs in the centre of the cell. One chromosome in
each pair came from the organism’s mother and father
3) The pairs are pulled apart so each new cell only has one copy of each chromosome. Some of
each parent’s chromosomes go into each new cell
4) Each new cell will have a mixture of the mother’s and father’s chromosomes. Mixing up the
genes like this is important → creates genetic variation in offspring
Divison 2
5) In the second division: the chromosomes line up in the centre of the cell (like mitosis) and the
arms of the chromosomes are pulled apart
6) Results in four haploid gametes - each gamete only contains single set of chromosomes and
are genetically different
