Meiosis and genetic variation

Mutation

A change in the DNA base sequence of chromosomes.

Explain what is meant by a deletion mutation

A deletion mutation is when a base is removed from the DNA sequence. This changes the number of bases present and this causes a frame shift in all the base triplets after it

Explain what is meant by a substitution mutation.

A substitution mutation is when one base is replaced by another base. If the triplet code codes for the same amino acid then the amino acid sequence won't change but if it codes for a different amino acid then it will change.

What is the pair of the homologus chromsomes called

Bivalent

Independent assortment of homologous chromosomes might result in several different phenotypes among the offspring of two parents.Explain what is meant by homologous chromosomes.

Capable of pairing during meiosis. Similar sequence of genes.

The faulty allele that causes Ellis-van Creveld syndrome is the result of a mutation of a gene called EVC. This mutation leads to the production of a protein that has one amino acid missing.Suggest how the production of a protein with one amino acid missing may lead to a genetic disorder such as Ellis-van Creveld syndrome.

Change in tertiary structure of active site so a non functional protein is produced

Figure 3 summarises the process of meiosis. The circles represent cells and the structures within each cell represent chromosomes. Describe and explain the appearance of one of the chromosomes in cell X.

Chromosome is formed of two chromatids because DNA replication has occurred. Sister chromatids held together by centromere.

What is meant by homologous chromosomes?

Chromosomes of the same size with the same genes at the same loci

Variation is all the differences which exist between members of the same species. There are two kinds of variation. What are they?

Continuous and discontinuous

Continuous variation

Continuous variation is variation that has no limit on the value that can occur within a population. A line graph is used to represent continuous variation.

State three things that occur which increases variation

Cross over and recombination independent segregation random fertilisation

Figure 3 shows a pair of chromosomes at the start of meiosis. The letters represent alleles.Some of the gametes formed during meiosis have new combinations of alleles. Explain how the gametes with the combinations of alleles Ef and eF have been produced.

Crossing over (chiasma). Exchange of alleles between chromosomes.

Only a few gametes have the new combination of alleles Ef and eF. Most gametes have the combination of alleles EF and ef. Suggest why only a few gametes have the new combination of alleles, Ef and eF

Crossing over is infrequent

Describe and explain the processes that occur during meiosis that increase the genetic variation.

Crossing over leads to exchange of parts of alleles between homologous chromosomes and both chromosomes create a new combination of alleles. Homologous chromosomes pair up. Independent segregation. Maternal and paternal chromosomes are reshuffled in any combination.

How does crossing over and independent segregation increase variation?

Crossing over of material between chromatids. Different combinations of alleles. Independent segregation of homologous chromosomes. Different combinations of maternal and paternal alleles.

Explain the process of meiosis 1

DNA coils to form chromosomes, nucleus breaks down, spindle fibres form, and crossing over occurs.The homologous pair of chromosomes line up in middle of cell and attach to spindle fibre via centromere. Spindle fibres pull, homologous pair of chromosomes separate to opposite ends of the pole by independent assortment. Chromosomes uncoil, and nucleus reforms.

What must happen before meiosis starts.

DNA unravels and replicates so that there are two copies of each chromosome. These are called chromatids.DNA condenses to form double armed chromosomes made from two sister chromatids which are attached together by the centromere..

A species of snake breeds on the shore of a very large lake and on the shore of an isolated island in the lake. There are two forms, a banded form (B) and an unbanded form (U). These are shown in the drawing.Name the type of variation shown. Explain your answer.

Discontinuous variation because there is only two discrete classes.

Discontinuous variation

Discontinuous variation is variation that has distinct groups for organisms to belong to. A bar graph is used to represent discontinuous variation.

Explain how random fertilisation of haploid gametes further increases genetic variation within a species.

During sexual reproduction, any sperm can fertilise any egg. This means fertilisation is random. Random fertilisation produces zygotes with different combinations of chromosomes to both parents. This mixing of genetic material increase varation and therefore increase genetic diversity within a species.

Which type of gene mutation is likely to be the most damaging and why?

Frame shift mutations

Explain how independent segregation might result in several different phenotypes in the offspring of two parents.

Gametes have different allele combinations as chromosomes are combined in different ways.

Compare and contrast gene and chromosomal mutations.

Gene mutation is a change in the base sequence, in a particular gene, whereas chromosomal mutation is a change in several genes, in the chromosome. Gene mutations can sometimes be corrected, but chromosomal mutations are hardly corrected. Gene mutation is only a slight structural alteration, whereas chromosomal mutations are either numerical or structural changes in the entire DNA strand.Gene mutation is a small scale alteration, but chromosome mutation can be considered as a serious alteration.

How do gene mutations arise

Gene mutations arise spontaneously during DNA replication and include base addition, base deletion and base substitution.

The chromosome number is halved during meiosis. Describe how.

Homologous chromosomes form pairs. One of each pair goes to each cell produced.

Synapsis

Homologous chromosomes pair together, during Prophase 1 of meiosis.

State 2 similarities and one difference between homologous pairs.

Homologous pairs are the same size and carry the same genes however they may have different alleles for that gene.

Schizophrenia is a mental illness. Doctors investigated the relative effects of genetic and environmental factors on the development of schizophrenia. They used sets of identical twins and non-identical twins in their investigation. At least one twin in each set had developed schizophrenia.Explain why both types of twin were used in this investigation.

Identical twins show genetic influence.Differences between them show environmental influence. Non identical twins show an environmental influence.

What's the main difference between meiosis 1 and meiosis 2.

In meiosis 1 the homologous pairs line up at the equator and are separated. In meiosis 2 the chromatids line up at the equator and are separated.

Describe how crossing over occurs.

In meiosis 1, the homologous pairs of chromosomes come together and pair up. The chromatids twist around each other and bits of chromatids swap over. The chromatids now have different combinations of alleles.

Explain the process of meiosis II

In prophase DNA coils to form chromosomes, nucleus breaksdown, spindle fibres form. In Metaphase chromosomes line up in middle of cell and attach to spindle fibre via centromere. In anaphase spindle fibres pull, centromere splits, sister chromatids move to opposite sides by independent assortment. In telophase chromatids uncoil, nucleus reforms and 4 genetically different daughter cells are produced.

Explain the effect of developing this golf course on the index of diversity of insects.

Increase in variety of plants. More niches.Greater variety of food sources.

Explain how meiosis results in variation

Independent segregation and crossing over result in genetically different daughter cells.

Explain three ways in which meiosis leads to genetic variation in gametes.

Independent segregation causes different combinations of maternal and paternal alleles. Crossing over causes different combinations of alleles. Mutation results in the formation of new alleles.

Name 3 ways in which meiosis produces genetic variation.

Independent segregation of homologous chromosomes. Crossing over and recombination. Formation of chiasmata. Random assortment

There was a time lag between the introduction of Bt crops and the appearance of the first insect species that was resistant to the Bt toxin. Explain why there was a time lag.

Initially only a few insects with favourable mutation. Individuals with favourable mutation will have more offspring. It takes many generations for favourable mutation to become the most common allele of this gene.

Is interphase part of meiosis?

Interphase is not part of meiosis. It is the stage prior to it.

The faulty allele that causes Ellis-van Creveld syndrome is the result of a mutation of a gene called EVC. This mutation leads to the production of a protein that has one amino acid missing.Suggest how a mutation can lead to the production of a protein that has one amino acid missing.

Loss of 3 bases

Compare and contrast the similarities and differences between mitosis and meiosis.

Meiosis and mitosis are both types of cell division. Mitosis involves one cell division whilst meiosis involves 2 cell divisions. Mitosis produces cells with the same number of chromosomes as the parent cell whilst meiosis produces half the number of chromosomes as the parent cell. There's no pairing or separating of homologous chromosomes in mitosis and so no crossing over of independent segregation of homologous chromosomes. Mitosis produces daughter cells that are genetically identical to each other and the parent cells whilst meiosis produces daughter cells that are genetically different from each other and the parent cells. Mitosis produces 2 daughter cells whilst meiosis produces 4.

What does meiosis produce?

Meiosis produces 4 genetically unique daughter cells.

Apart from producing gametes that are genetically different, meiosis is important in sexual reproduction. Explain why

Meiosis produces haploid cells so that when the sperm cell and egg cell fuse the zygote produced has a full set of chromosomes. Meiosis maintains chromosome number.

Compare and contrast meiosis and mitosis

Mitosis divides once, meiosis divides twice. Snyapsis occurs in meiosis but not mitosis.Mitosis produces 2 daughter cells that are genetically identical to the parent cell, meiosis produces 4 cells that have genetic variation. 46 chromosomes produced after mitosis but 23 after meiosis. Mitosis produces somatic cells, meiosis produces gametes.

What is mitosis and where does it take place?

Mitosis is a type of cell division and it takes place in the repoductive organs.

What are mutagenic agents

Mutagenic agents can increase the risk of gene mutation.

A mutation can lead to the production of a non functional enzyme. Explain how.

Mutation is a change in base sequence of DNA. This causes a change in the primary structure of the enzyme. This leads to a change in the position of the hydrogens bonds which results in a change in the tertiary structure. Change in active site means substrate not complementary and so cannot bind to the active site of enzyme so no enzyme-substrate complexes form.

How do chromosomal mutations occur?

Mutations in the number of chromosomes can arise spontaneously by chromosome non disjunction during meiosis.

To reduce the damage caused by insect pests, some farmers spray their fields of crop plants with pesticide. Many of these pesticides have been shown to cause environmental damage. Bt plants have been genetically modified to produce a toxin that kills insect pests. The use of Bt crop plants has led to a reduction in the use of pesticides. Scientists have found that some species of insect pest have become resistant to the toxin produced by the Bt crop plants. One farmer stated that the increase in the use of Bt crop plants had caused a mutation in one of the insect species and that this mutation had spread to other species of insect. Was he correct? Explain your answer.

No. Mutations are spontaneous. Only the rate of mutation is affected by environment. Different species do not interbreed to produce fertile offspring so mutations cannot be passed from one species to another.

Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis.Explain what a non-disjunction event is and how they occur

Non-misjunction is a type of chromosome mutation and it is when the chromosomes fail to separate properly.

The table shows some differences between three varieties of banana plant. In the table variety B has 33 chromosomes in a leaf cell. Variety B cannot produce fertile gametes. Use information in the table to explain why.

Odd number of chromosomes in leaf cell. Chromosomes cannot pair up and so cannot undergo meiosis and so cannot form haploid cells.

Stages of meiosis

Prophase 1: DNA condenses to become visible as chromosomes, the spindle is formed from centrioles at the poles of the cell and the nuclear envelope disintegrates. In late prophase chiasmata are formed, points where the DNA from non-sister chromatids crosses over, resulting in the exchange of genetic material. Metaphase 1: Homologous pairs of chromosomes line up along the equator of the cell Anaphase 1: Homologous pairs of chromosomes are separated and pulled to opposite poles of the cell and the spindle disintegrates. Telophase 1: New nuclear envelopes are formed and the chromosomes unravel to form chromatin Cytokinesis: The division of the cytoplasm to complete the formation of 2 cells from the first division of meiosis Prophase 2: DNA condenses once again to become visible as chromosomes, the spindle is formed from centrioles at the poles of the cell and the nuclear envelope disintegrates. Metaphase 2: Chromosome existing as sister chromatids line up along the equator of the cell Anaphase 2: Sister chromatids are separated and pulled to opposite poles of the cell as the centromeres split. After the sister chromatids have been separated they can be referred to as chromosomes Telophase 2: New nuclear envelopes are formed and the chromosomes unravel to form chromatin. Cytokinesis: The division of the cytoplasm to complete the formation of 2 cells from the first division of meiosis. 4 genetically different haploid ('n') cells are formed from one diploid ('2n') cell.

Name the stages in meiosis

Prophase I, metaphase I, Anaphase I, Telophase I, Metaphase II, Anaphase II, Telophase II

It is important that gametes contain the haploid number of chromosomes. Explain why

Restores diploid number at fertilisation. Maintain chromosome number from one generation to next.

Figure 4 shows the same chromosomes on the spindle during meiosis. One chromosome has two copies of allele T. What occurs during meiosis which results in only one copy of the allele T being present in a gamete?

Separation of chromatids.

Identify two events that occur during division 2 but not during division 1 of meiosis

Separation of sister chromatids. Division of centromere

Explain how independent segregation leads to variation in gametes.

Shuffling of maternal and paternal chromosomes produces different combination of alleles. Random alignment of chromosomes produces different combination of alleles.

What's the difference between somatic cells and sex cells?

Somatic cells have a diploid number of chromosomes. This means they have 2 sets of chromosomes, one from each parent. Sex cells on the other hand have a haploid number of chromosomes which means they only have one copy of each chromosome.

Deletion of the sixth base (G) in the sequence shown in Figure 3 would change the nature of the protein produced but substitution of the same base would not. Use the information in Table 1 and your own knowledge to explain why.

Substitution would result in CCA/CCC/CCU which all code for the same amino acid. Deletion would cause frame shift which causes a change in all following codons e.g. it would change next codon from UAC to ACC

What is the result of crossing over?

The crossing over of chromatids in meiosis one, means that each daughter cells formed from meiosis contains chromatids with different alleles.

Why don't all substitute mutations bring about a change in the amino acid sequence?

The degeneracy of the genetic code means that not all base substitutions cause a change in the amino acid sequence.

Why do gene mutations affect the sequence of amino acids?

The order of DNA bases determines the order of the amino acids in a particular protein.

Chiasmata

The point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover in meiosis. Sister chromatids also form chiasmata between each other but because their genetic material is identical, it does not cause any noticeable change in the resulting daughter cells.

Recombination

The process by which offspring get new combinations of alleles, which differ from the parents.

What does the process of meiosis involve and what does it produce?

The process of meiosis involves two nuclear divisions and forms four haploid daughter cells.

Explain the role of independent segregation in meiosis

To provide genetic variation. Allows different combinations of maternal and paternal chromosomes. To produce haploid cells. Allows homologous pairs to arrange randomly at the equator of the cell and separate.

Figure 3 shows a pair of chromosomes at the start of meiosis. The letters represent alleles. Explain the appearance of one of the chromosomes in Figure 3.

Two sister chromatids due to DNA replication, joined by a centromere.

Examples of mutagenic agents.

UV light, ionising radiation, toxic chemicals, viruses

What is intraspecific variation?

Variation within the same species.

Explain the importance of meiosis in a life cycle

When gametes fuse diploid number is restored. Meiosis produces variation.

Meiosis produces cells that are genetically different. Explain how independent segregation of chromosomes increases variation.

When homologous pairs are separated in meiosis 1, it is completely random which chromosome from each pair ends up in which daughter cell. The 4 daughter cells produced by meiosis all have completely different combinations of those paternal and maternal chromosomes

A zeedonk is the offspring produced from breeding a mountain zebra with a donkey. The body cells of a mountain zebra contain 32 chromosomes.The body cells of a donkey contain 62 chromosomes. Use this information to suggest why zeedonks are usually infertile

Zeedonk has 47 chromosomes. Chromosomes cannot pair as they are not homologous and chromosomes cannot be halved. If meiosis cannot occur then gametes can't be produced.

Suggest why there are fewer gametes with genotypes Gd and gD than GD and gd.

crossing over is infrequent between close genes.

Figure 5 shows another pair of chromosomes from the same plant cell. The table shows the numbers of gametes with each genotype produced by this plant.Describe what happens during meiosis, which results in the new combinations of alleles, Gd and gD.

crossing over occurs between D and G. Sections of chromosomes are exchanged.