Genetic Diversity
Changes in DNA can arise spontaneously during replication and any change in
the base sequence or quantity of DNA is called a mutation. A change in the
base sequence of a gene can change the sequence of amino acids. Sometimes
this can result in a mutation that is harmful, however due to the genetic code
being degenerate the amino acid sequence may not always be changed. Any
change to one or more nucleotide bases, or a change in the sequence of the
bases, in DNA is known as a gene mutation. There are two key types of gene
mutation, these are:
1. Substitution - this is when one nucleotide in the DNA sequence is
replaced by another. The effect of the change in an amino acid depends
on the role of the original amino acids in the overall shape and function
of the protein. If it codes for another amino acid then the polypeptide
will differ in a single aa, the significance of this difference will depend on
the role of the original aa; if it is important in forming bonds that
determine the tertiary structure of the final protein, then the
replacement aa may not form the same bonds, the protein may then be
a different shape and not function properly. A substitution may not
always be harmful as the substituted nucleotide may code in that triplet
for the same amino acid.
2. Deletion - a deletion event is when a nucleotide in the DNA sequence is
lost. The loss of a single nucleotide can have a significant impact as it
leads to a frame shift, resulting in completely different amino acids being
coded for.
Mutations may also occur in chromosomes for which there are two forms:
1. Polyploidy - changes can occur in the whole set of chromosomes so that
an individual has three of more sets of chromosomes instead of two.
This is common in plants, with many modern wheats arising to be
polyploidy.
2. Non-disjunction - non-disjunction occurs when individual homologous
pairs of chromosomes fail to separate correctly in meiosis. As a result
the gametes and any zygotes formed will have one more or one less
chromosome than they should in every cell of the body. Down’s
syndrome is the result of non-disjunction where individuals have an
extra chromosome 21.
, Meiosis and Genetic Variation
Meiosis is a form of cell division that gives rise to four daughter cells that are
all genetically different and have half the number of chromosomes found in
the parent cell. The main role of meiosis is the production of haploid gametes
as cells produced by meiosis have half the number of chromosomes otherwise
the number would keep doubling in the formation of offspring. When gametes
fuse, they produce a diploid number (two complete sets of chromosomes; one
from each parent). This is necessary to maintain a stable number of
chromosomes.
In most animals meiosis occurs in the formation of gametes, however in some
plants such as ferns, gametes are produced by mitosis. In the fern life cycle
meiosis occurs in the formation of spores.
Stages of Meiosis
The stages of meiosis can be split into two nuclear divisions:
- Meiosis 1 - homologous chromosomes pair up and their chiasma wrap
around each other whereby crossing over at the chiasmata may take
place and exchange of alleles may happen. The cell then divides into two
whereby each of the 2 daughter cell contains one chromosome from
each homologous pair.
- Meiosis 2 - the chromatids of each chromosome are separated
producing 4 haploid daughter cells.
Meiosis produces genetically different cells, genetic variation is achieved by:
- Independent segregation of homologous chromosomes – there are
various combinations of chromosome arrangement. During metaphase 1
homologous chromosomes line up in pairs, the arrangement of these
pairs is random, meaning that the division into the daughter cells is also
random.
- Independent assortment of chromatids – during metaphase 2 the
chromatids of each chromosome line up randomly on the equator of the
cell.
- Crossing over of chromatids - When pairs of chromosomes line up, they
can exchange some of their genetic material. Crossing over occurs in
prophase 1 when one chromosome may swap places with the same part
of its homologous pair leading to a different combination of alleles on
the gene.
- Random fertilisation of haploid gametes – random gametes fuse
resulting in a diploid cell
Changes in DNA can arise spontaneously during replication and any change in
the base sequence or quantity of DNA is called a mutation. A change in the
base sequence of a gene can change the sequence of amino acids. Sometimes
this can result in a mutation that is harmful, however due to the genetic code
being degenerate the amino acid sequence may not always be changed. Any
change to one or more nucleotide bases, or a change in the sequence of the
bases, in DNA is known as a gene mutation. There are two key types of gene
mutation, these are:
1. Substitution - this is when one nucleotide in the DNA sequence is
replaced by another. The effect of the change in an amino acid depends
on the role of the original amino acids in the overall shape and function
of the protein. If it codes for another amino acid then the polypeptide
will differ in a single aa, the significance of this difference will depend on
the role of the original aa; if it is important in forming bonds that
determine the tertiary structure of the final protein, then the
replacement aa may not form the same bonds, the protein may then be
a different shape and not function properly. A substitution may not
always be harmful as the substituted nucleotide may code in that triplet
for the same amino acid.
2. Deletion - a deletion event is when a nucleotide in the DNA sequence is
lost. The loss of a single nucleotide can have a significant impact as it
leads to a frame shift, resulting in completely different amino acids being
coded for.
Mutations may also occur in chromosomes for which there are two forms:
1. Polyploidy - changes can occur in the whole set of chromosomes so that
an individual has three of more sets of chromosomes instead of two.
This is common in plants, with many modern wheats arising to be
polyploidy.
2. Non-disjunction - non-disjunction occurs when individual homologous
pairs of chromosomes fail to separate correctly in meiosis. As a result
the gametes and any zygotes formed will have one more or one less
chromosome than they should in every cell of the body. Down’s
syndrome is the result of non-disjunction where individuals have an
extra chromosome 21.
, Meiosis and Genetic Variation
Meiosis is a form of cell division that gives rise to four daughter cells that are
all genetically different and have half the number of chromosomes found in
the parent cell. The main role of meiosis is the production of haploid gametes
as cells produced by meiosis have half the number of chromosomes otherwise
the number would keep doubling in the formation of offspring. When gametes
fuse, they produce a diploid number (two complete sets of chromosomes; one
from each parent). This is necessary to maintain a stable number of
chromosomes.
In most animals meiosis occurs in the formation of gametes, however in some
plants such as ferns, gametes are produced by mitosis. In the fern life cycle
meiosis occurs in the formation of spores.
Stages of Meiosis
The stages of meiosis can be split into two nuclear divisions:
- Meiosis 1 - homologous chromosomes pair up and their chiasma wrap
around each other whereby crossing over at the chiasmata may take
place and exchange of alleles may happen. The cell then divides into two
whereby each of the 2 daughter cell contains one chromosome from
each homologous pair.
- Meiosis 2 - the chromatids of each chromosome are separated
producing 4 haploid daughter cells.
Meiosis produces genetically different cells, genetic variation is achieved by:
- Independent segregation of homologous chromosomes – there are
various combinations of chromosome arrangement. During metaphase 1
homologous chromosomes line up in pairs, the arrangement of these
pairs is random, meaning that the division into the daughter cells is also
random.
- Independent assortment of chromatids – during metaphase 2 the
chromatids of each chromosome line up randomly on the equator of the
cell.
- Crossing over of chromatids - When pairs of chromosomes line up, they
can exchange some of their genetic material. Crossing over occurs in
prophase 1 when one chromosome may swap places with the same part
of its homologous pair leading to a different combination of alleles on
the gene.
- Random fertilisation of haploid gametes – random gametes fuse
resulting in a diploid cell
