Genetic diversity: mutations and meiosis

genetic mutations

• A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide

• Mutations occur continuously and at random

• The DNA base sequence determines the sequence of amino acids that make up a protein; therefore, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for

• Most mutations do not alter the polypeptide, or only alter it slightly, so that its structure or function is not changed

  • This is because the genetic code is degenerate 

deletion of nucleotides

• A deletion mutation occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence

• A deletion mutation changes the amino acid that would have been coded for

• These mutations have a knock-on effect by changing the triplets of three bases further on in the DNA sequence

  • This is sometimes known as a frameshift mutation

• This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function

substitution of nucleotides

• A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base

• Unlike a deletion mutation, a substitution mutation will only change the amino acid for the triplet in which the mutation occurs; it will not have a knock-on effect

• Substitution mutations may result in three possible outcomes:

  • The mutation may be silent if it does not alter the amino acid sequence of the polypeptide because many codons code for the same amino acid as the genetic code is degenerate

  • The mutation may alter a single amino acid in the polypeptide chain (e.g., sickle cell anaemia is a disease caused by a single substitution mutation changing a single amino acid in the sequence)

  • The mutation creates a premature STOP codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function (e.g., cystic fibrosis is a disease caused by a nonsense mutation, although this is not always the only cause)

effect of gene mutations of polypeptides

• Most mutations do not alter the polypeptide or only alter it slightly, so that its appearance or function is not changed

• However, a small number of mutations code for a significantly altered polypeptide with a different shape

• This may affect the ability of the protein to perform its function. For example:

  • if the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site

  • a structural protein (like collagen) may lose its strength if its shape changes

mutagenic agents

• There are natural mechanisms that take place within cells to ensure the accuracy of DNA replication

  • These mechanisms involve proofreading and repairing damaged DNA

• When the mutation rate of a cell rises to above a normal (usually low) rate, then these mechanisms become ineffective

• Mutagenic agents are environmental factors that increase the mutation rate of cells

examples of mutagenic agents

• high-energy radiation such as UV light

• ionising radiation such as X-rays

• toxic chemicals such as peroxides

mutations in chromosome number

• Mutations can occur at different levels, within chromosomes or at the genetic level

• Chromosome mutations involve a change in the number of chromosomes and involve non-disjunction

• Non-disjunction occurs when chromosomes fail to separate during meiosis

  • This occurs spontaneously

• The gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome

• These gametes will have a different number of chromosomes compared to the normal haploid number

• If the abnormal gametes take part in fertilisation, then a chromosome mutation occurs, as the resulting diploid cell will have the incorrect number of chromosomes

• An example of a chromosome mutation is Down’s syndrome

  • Individuals with this syndrome have a total of 47 chromosomes (instead of 46) in their genome, as they have three copies of chromosome 21

meiosis

• Meiosis produces daughter cells that are genetically different from each other and to the parent cell

• This is due to the processes of independent segregation and crossing over

meiosis - independent segregation

• The independent segregation of homologous chromosomes leads to genetically different daughter cells

• This happens during meiosis:

  • Meiosis is the process that forms gametes (sperm or egg), and it has two divisions: meiosis I and meiosis II

• During meiosis I, homologous chromosomes line up in the centre of the cell

• The chromosomes are then separated and pulled into different cells — this is called segregation of homologous chromosomes

• Each pair of homologous chromosomes lines up randomly

  • That means the way one pair segregates does not affect how another pair segregates

  • This is what makes it independent — each chromosome pair has its a different path by chance

meiosis - crossing over

• Crossing over is the process where homologous chromosomes exchange genetic material during meiosis I

• This process results in further genetic variation among daughter cells

• Homologous chromosomes pair up and form bivalents

• The chromatid then breaks and rejoins to the chromatid of its homologous chromosome, so that its alleles are exchanged

• Crossing over leads to new combinations of alleles on each chromatid; this is called recombination

meiosis in animal and plan cells

• Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells

• It produces gametes in plants and animals that are used in sexual reproduction

• It has many similarities to mitosis however, it has two divisions: meiosis I and meiosis II

• Within each division, there are the following stages: prophase, metaphase, anaphase and telophase

meiosis I

• Meiosis I is the first division in meiosis

• It separates homologous chromosomes and reduces the chromosome number by half, producing two haploid cells

meiosis - Prophase I

• DNA replication has already occurred

• DNA condenses and becomes visible as chromosomes

• The chromosomes are arranged side by side in homologous pairs

• Crossing over of non-sister chromatids may occur

• The spindle is formed by centrioles

• The nuclear envelope breaks down, and the nucleolus disintegrates

meiosis - metaphase I

• Homologous pairs of chromosomes line up randomly along the equator of the spindle

• Independent segregation occurs in metaphase I

meiosis - anaphase I

• The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle

  • The centromeres do not divide

meiosis - telophase I

• The chromosomes arrive at opposite poles

• Spindle fibres start to break down

• Nuclear envelopes form around the two groups of chromosomes, and nucleoli reform

• The cell then divides by cytokinesis, forming two haploid daughter cells — each with half the original chromosome number (but still with duplicated chromosomes)

  • These cells are haploid as they contain half the number of centromeres

meiosis II

• There is no interphase between meiosis I and meiosis II, so the DNA is not replicated

• The second division of meiosis is almost identical to the stages of mitosis

meiosis - prophase II

• The nuclear envelope breaks down and chromosomes condense

• A spindle forms at a right angle to the old one

meiosis - metaphase II

• Chromosomes (still made of two sister chromatids) line up in a single file along the equator of the spindle

meiosis - anaphase II

• Centromeres divide, and individual chromatids are pulled to opposite poles

  • Each chromatid is now an individual chromosome

• This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell

meiosis - telophase II

• Nuclear membranes form around each group of chromosomes

• Cytokinesis occurs

  • Cytoplasm divides as new cell surface membranes are formed, creating four haploid cells

meiosis: source of genetic variation

• Having genetically different offspring can be advantageous for natural selection

• Meiosis has several mechanisms that increase the genetic diversity of gametes produced

• Both crossing over and independent segregation result in different combinations of alleles in gametes, which creates genetic variation

• This means each gamete carries substantially different alleles

• During fertilisation, any male gamete can fuse with any female gamete to form a zygote

• This random fusion of gametes at fertilisation creates genetic variation between zygotes, as each will have a unique combination of alleles

• The presence of genetically diverse zygotes contributes to the genetic diversity of a species

• There is almost zero chance that individual organisms resulting from successive sexual reproduction will be genetically identical

meiosis - chromosome combinations after meiosis

• The number of possible chromosomal combinations resulting from meiosis is equal to 2ⁿ

  • n is the number of homologous chromosome pairs

• For humans:

  • The diploid number for humans is 46, then the haploid number or number of homologous chromosomes is 23, so the calculation would be:

    • 2²³ = 8 388 608 possible chromosomal combinations

meiosis - chromosome combinations after fertilisation

• In random fertilisation, any two gametes may combine

• Therefore, the formula to calculate the number of combinations of chromosomes after the random fertilisation of two gametes is (2ⁿ)²

  • n is the haploid number

  • ² is the number of gametes

• In humans:

  • The haploid number is 23, so the number of combinations following fertilisation is (2²³)^2­­  = 70368744177664

• This explains why relatives can differ from one another

  • Even with the same parents, individuals can be genetically distinct due to variation at the meiosis and fertilisation stage (as well as other possible mutations and crossing-over)

looking at meiosis under a microscope - meiosis I or meiosis II

• Homologous chromosomes pair up side by side in meiosis I only

• This means if there are pairs of chromosomes in a diagram or photomicrograph meiosis I must be occurring

• The number of cells formed can help distinguish between meiosis I and II

  • If two new cells are formed, it is meiosis I, but if four new cells are formed, it is meiosis II

• During meiosis II, single chromosomes may be observed

looking at meiosis under a microscope - prophase I

Homologous pairs of chromosomes are visible

looking at meiosis under a microscope - metaphase I

Homologous pairs are lined up side by side along the equator of spindle

looking at meiosis under a microscope - anaphase I

Whole chromosomes are being pulled to opposite poles with centromeres intact

looking at meiosis under a microscope - telophase I

There are two groups of condensed chromosomes around which nuclear membranes are forming

looking at meiosis under a microscope - prophase II

Single whole chromosomes are visible

looking at meiosis under a microscope - metaphase II

Single whole chromosomes are lined up along the equator of the spindle in a single file

looking at meiosis under a microscope - anaphase II

Centromeres divide, and chromatids are pulled to opposite poles

looking at meiosis under a microscope - telophase II

Nuclei are forming around the 4 groups of condensed chromosomes

mitosis vs meiosis - mitosis

• Mitosis and meiosis are both forms of cell division

• Mitosis ends with two daughter cells genetically identical to each other and the parent cell

  • This is important so that growth and cell replacement can occur within the body continually

  • Every cell in an organism's body (other than gametes) contains the same genetic material - the full genome

mitosis vs meiosis - meiosis

• Meiosis ends with four daughter cells all of which contain half the genetic material of the parent cell and are all different from each other and the parent

  • This is important for genetic variation within families and the population

  • Genetic variation can reduce the risk of inheriting genetic diseases