Sexual and Asexual Reproduction
Only gametes (sperm and ovum) contain chromosomes not paired. They contain 23 single
chromosomes.
Gametes are made by a type of cell division called meiosis. Unlike mitosis, meiosis produces non-
identical cells. Meiosis also takes place in flowering plants (the gametes being pollen and egg cells).
Both animals and flowering plants carry out sexual reproduction. Sexual reproduction involves the
fusion of male and female gametes. In sexual reproduction there is mixing of genetic information.
(the offspring receives genetic information from both the male and female parents), which leads to
variation in the offspring.
Asexual reproduction involves only one parent. Asexual reproduction does not involve gametes.
There is no mixing of genetic information, this leads to genetically identical offspring (clones). As it
doesn’t involve gametes, meiosis does not take place. Asexual reproduction only involves mitosis.
Meiosis and Fertilisation
A human gamete contains 23 single chromosomes.
Meiosis only takes place in reproductive organs. In humans that is the testes in males and ovaries in
females.
Stages of Meiosis:
1. All the chromosomes are copied.
2. The cell divides into two.
3. Both of these cells divide one more time forming the gametes.
Meiosis halved the number of chromosomes. Meiosis produces four gametes, which are genetically
different from each other, from one original cell. Each gamete has different alleles.
In sexual reproduction a sperm and ovum (gametes) fuse together in a process called fertilisation.
After fertilisation, the cell has the normal number of chromosomes as the chromosomes from the
sperm and ovum are paired. After fertilisation the new cell divides by mitosis, producing a clump of
identical cells called an embryo. As the embryo develops, the cells differentiate forming different cell
types. In animals, these include nerve cells and muscle cells.
Advantages and Disadvantages of Sexual and Asexual Reproduction
Advantages of Sexual Reproduction
1. Gives a species survival advantage by natural selection due to the variation in offspring.
Humans take advantage of this variation when carrying out selective breeding.
Advantages of Asexual Reproduction
1. Only one parent is needed therefore more efficient in both time and energy as a mate is not
needed. This makes asexual reproduction faster than sexual reproduction.
2. Because it is fast, it is extremely useful when conditions are favourable.
3. Allows an organism to produce many genetically identical offspring rapidly.
Disadvantages
, 1. Very risky as all the offspring can die if conditions become unfavourable as they are
genetically identical.
Examples of sexual and asexual reproduction
Malaria: In the human host, the malaria parasite reproduces asexually. However, inside the
mosquito, it uses sexual reproduction.
Fungi: Many species of fungi reproduce asexually by producing spores. They can also reproduce
sexually, producing variation in offspring.
Plants: All flowering plant can reproduce sexually to produce seeds e.g., strawberry flower. Some
plants can reproduce asexually e.g., a strawberry plant reproduces asexually by sending out runners,
where the runner touches the soil it can develop into a new plant, genetically identical to the parent.
Daffodils can reproduce asexually by bulb division, the parent plant has an underground bulb which
produces buds which eventually form new offspring plant genetically identical to the parent.
DNA and the Genome
Chromosomes contain the molecule DNA.
DNA is the genetic material as it determines our inherited features.
DNA consists of polymer two strands wrap around each other to form a double helix.
A gene is a small section of DNA on a chromosome. Each gene encodes for a specific sequence of
amino acids to make a specific protein.
The genome id the entire genetic material of an organism e.g., the human genome is the entire
genetic material that makes a human.
Scientists have now studied the entire human genome. Understanding the human genome will help
to search for genes that are linked to disease e.g., genes that increase the risk of developing cancer
or Alzheimer’s disease. It will also help to understand and treat inherited disorders e.g., cystic
fibrosis. The human genome can be used to trace human migration patterns from the past which
helps people to discover their ancestry.
DNA Structure
DNA is a polymer of molecules called nucleotides. Draw diagram of structure of nucleotide.
Nucleotides have got three main parts: a phosphate group attached to a sugar molecule and the
sugar is attached to a molecule called a base. In DNA the phosphate group and sugar molecule never
change however there are four different bases: adenine, thymine, guanine and cytosine.
DNA strands are complementary as the same bases always pair together on the opposite strands: A
to T and C to G.
Protein Synthesis
Proteins are polymers of amino acids. In humans there are 20 different amino acids.
The specific order of the amino acids determines the shape of the protein. The shape determines the
function. The order of amino acids in a protein is determined by the sequence of bases in the gene
for that protein.
, The cell reads the DNA sequence as triplets of bases. Each triplet encodes for a specific amino acid in
the protein.
Protein Synthesis contains 2 stages: first stage takes place in the nucleus and second in the
cytoplasm.
1. Transcription: the base sequence of the gene is copied into a complementary template
molecule called messenger RNA or mRNA for short. mRNA is a single-stranded molecule. The
mRNA passes out of the nucleus and into the cytoplasm.
2. Translation: the mRNA molecule attaches to a ribosome. Amino acids are brought to the
ribosome on carrier molecules called transfer RNA or tRNA for short. The ribosome reads the
triplets of bases on the mRNA and uses this to join together the correct amino acids in the
correct order. Once the protein is complete, it folds into its unique shape.
Mutations
A change to a base is called a mutation. Mutations happen all the time.
Different base triplets can sometimes encode for the same amino acid, so sometimes a mutation has
no effect on the protein’s shape or function. However, a mutation can alter the shape of the protein
and this can have drastic effect on its function e.g., the active site of an enzyme may change shape
so it can no longer attach to the substrate. If a mutation changes the shape of a structural protein
such as collagen then it may lose its strength.
Chromosomes contain non-coding parts of DNA which switch genes on and off, they tell genes when
to produce proteins. Mutations in these non-coding regions can affect how genes are switched on or
off e.g., a gene may be turned on when it should be turned off therefore the cell would produce a
protein that it is not meant to have at that time. This could have a very significant effect on a cell
e.g., uncontrolled mitosis leading to cancer
Alleles
Alleles are different versions of a gene.
The genotype of a person tells the alleles present.
The phenotype tells the characteristics caused by the alleles present (genotype).
If two alleles present are the same the organism is homozygous for that trait, if the alleles are
different they are heterozygous.
A dominant allele is expressed in the phenotype even if only one copy is present. A dominant allele
is shown as a capital-case letter.
A recessive allele is only expressed in the phenotype if two copies are present (therefore no
dominant allele present). A recessive allele is shown as a lower-case letter.
Most characteristics are a result of multiple genes interacting, rather than a single gene.
Punnet Square
Inherited Disorders
Cystic Fibrosis
Only gametes (sperm and ovum) contain chromosomes not paired. They contain 23 single
chromosomes.
Gametes are made by a type of cell division called meiosis. Unlike mitosis, meiosis produces non-
identical cells. Meiosis also takes place in flowering plants (the gametes being pollen and egg cells).
Both animals and flowering plants carry out sexual reproduction. Sexual reproduction involves the
fusion of male and female gametes. In sexual reproduction there is mixing of genetic information.
(the offspring receives genetic information from both the male and female parents), which leads to
variation in the offspring.
Asexual reproduction involves only one parent. Asexual reproduction does not involve gametes.
There is no mixing of genetic information, this leads to genetically identical offspring (clones). As it
doesn’t involve gametes, meiosis does not take place. Asexual reproduction only involves mitosis.
Meiosis and Fertilisation
A human gamete contains 23 single chromosomes.
Meiosis only takes place in reproductive organs. In humans that is the testes in males and ovaries in
females.
Stages of Meiosis:
1. All the chromosomes are copied.
2. The cell divides into two.
3. Both of these cells divide one more time forming the gametes.
Meiosis halved the number of chromosomes. Meiosis produces four gametes, which are genetically
different from each other, from one original cell. Each gamete has different alleles.
In sexual reproduction a sperm and ovum (gametes) fuse together in a process called fertilisation.
After fertilisation, the cell has the normal number of chromosomes as the chromosomes from the
sperm and ovum are paired. After fertilisation the new cell divides by mitosis, producing a clump of
identical cells called an embryo. As the embryo develops, the cells differentiate forming different cell
types. In animals, these include nerve cells and muscle cells.
Advantages and Disadvantages of Sexual and Asexual Reproduction
Advantages of Sexual Reproduction
1. Gives a species survival advantage by natural selection due to the variation in offspring.
Humans take advantage of this variation when carrying out selective breeding.
Advantages of Asexual Reproduction
1. Only one parent is needed therefore more efficient in both time and energy as a mate is not
needed. This makes asexual reproduction faster than sexual reproduction.
2. Because it is fast, it is extremely useful when conditions are favourable.
3. Allows an organism to produce many genetically identical offspring rapidly.
Disadvantages
, 1. Very risky as all the offspring can die if conditions become unfavourable as they are
genetically identical.
Examples of sexual and asexual reproduction
Malaria: In the human host, the malaria parasite reproduces asexually. However, inside the
mosquito, it uses sexual reproduction.
Fungi: Many species of fungi reproduce asexually by producing spores. They can also reproduce
sexually, producing variation in offspring.
Plants: All flowering plant can reproduce sexually to produce seeds e.g., strawberry flower. Some
plants can reproduce asexually e.g., a strawberry plant reproduces asexually by sending out runners,
where the runner touches the soil it can develop into a new plant, genetically identical to the parent.
Daffodils can reproduce asexually by bulb division, the parent plant has an underground bulb which
produces buds which eventually form new offspring plant genetically identical to the parent.
DNA and the Genome
Chromosomes contain the molecule DNA.
DNA is the genetic material as it determines our inherited features.
DNA consists of polymer two strands wrap around each other to form a double helix.
A gene is a small section of DNA on a chromosome. Each gene encodes for a specific sequence of
amino acids to make a specific protein.
The genome id the entire genetic material of an organism e.g., the human genome is the entire
genetic material that makes a human.
Scientists have now studied the entire human genome. Understanding the human genome will help
to search for genes that are linked to disease e.g., genes that increase the risk of developing cancer
or Alzheimer’s disease. It will also help to understand and treat inherited disorders e.g., cystic
fibrosis. The human genome can be used to trace human migration patterns from the past which
helps people to discover their ancestry.
DNA Structure
DNA is a polymer of molecules called nucleotides. Draw diagram of structure of nucleotide.
Nucleotides have got three main parts: a phosphate group attached to a sugar molecule and the
sugar is attached to a molecule called a base. In DNA the phosphate group and sugar molecule never
change however there are four different bases: adenine, thymine, guanine and cytosine.
DNA strands are complementary as the same bases always pair together on the opposite strands: A
to T and C to G.
Protein Synthesis
Proteins are polymers of amino acids. In humans there are 20 different amino acids.
The specific order of the amino acids determines the shape of the protein. The shape determines the
function. The order of amino acids in a protein is determined by the sequence of bases in the gene
for that protein.
, The cell reads the DNA sequence as triplets of bases. Each triplet encodes for a specific amino acid in
the protein.
Protein Synthesis contains 2 stages: first stage takes place in the nucleus and second in the
cytoplasm.
1. Transcription: the base sequence of the gene is copied into a complementary template
molecule called messenger RNA or mRNA for short. mRNA is a single-stranded molecule. The
mRNA passes out of the nucleus and into the cytoplasm.
2. Translation: the mRNA molecule attaches to a ribosome. Amino acids are brought to the
ribosome on carrier molecules called transfer RNA or tRNA for short. The ribosome reads the
triplets of bases on the mRNA and uses this to join together the correct amino acids in the
correct order. Once the protein is complete, it folds into its unique shape.
Mutations
A change to a base is called a mutation. Mutations happen all the time.
Different base triplets can sometimes encode for the same amino acid, so sometimes a mutation has
no effect on the protein’s shape or function. However, a mutation can alter the shape of the protein
and this can have drastic effect on its function e.g., the active site of an enzyme may change shape
so it can no longer attach to the substrate. If a mutation changes the shape of a structural protein
such as collagen then it may lose its strength.
Chromosomes contain non-coding parts of DNA which switch genes on and off, they tell genes when
to produce proteins. Mutations in these non-coding regions can affect how genes are switched on or
off e.g., a gene may be turned on when it should be turned off therefore the cell would produce a
protein that it is not meant to have at that time. This could have a very significant effect on a cell
e.g., uncontrolled mitosis leading to cancer
Alleles
Alleles are different versions of a gene.
The genotype of a person tells the alleles present.
The phenotype tells the characteristics caused by the alleles present (genotype).
If two alleles present are the same the organism is homozygous for that trait, if the alleles are
different they are heterozygous.
A dominant allele is expressed in the phenotype even if only one copy is present. A dominant allele
is shown as a capital-case letter.
A recessive allele is only expressed in the phenotype if two copies are present (therefore no
dominant allele present). A recessive allele is shown as a lower-case letter.
Most characteristics are a result of multiple genes interacting, rather than a single gene.
Punnet Square
Inherited Disorders
Cystic Fibrosis
