Genetic Diversity: Mutations & Meiosis

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Notas de aula

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 amino acid sequence of a protein, so mutations can change the polypeptide.
Most mutations do not alter the polypeptide, or only alter it slightly, because the genetic code is degenerate.
Deletion of nucleotides: a nucleotide is randomly deleted, causing a frameshift mutation that changes all subsequent triplets and may dramatically change the amino acid sequence.
Substitution of nucleotides: a base is randomly swapped for a different base; this only affects the triplet where the mutation occurs and has no knock-on effect.
Substitution mutations can be silent (no amino acid change due to degeneracy), alter a single amino acid (e.g., sickle cell anaemia), or create a premature STOP codon (e.g., cystic fibrosis).
Mutations can alter protein function: e.g., changing an enzyme's active site prevents substrate binding, or a structural protein like collagen may lose strength.

Mutagenic Agents

Cells have natural mechanisms to ensure DNA replication accuracy, including proofreading and repairing damaged DNA.
When the mutation rate rises above normal, these mechanisms become ineffective.
Mutagenic agents are environmental factors that increase the mutation rate of cells.
Examples include high-energy radiation such as UV light, ionising radiation such as X-rays, and toxic chemicals such as peroxides.

Mutations in Chromosome Number

Chromosome mutations involve a change in the number of chromosomes and involve non-disjunction.
Non-disjunction occurs when chromosomes fail to separate during meiosis, and it occurs spontaneously.
Gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome.
These gametes have a different number of chromosomes compared to the normal haploid number.
If abnormal gametes take part in fertilisation, the resulting diploid cell will have the incorrect number of chromosomes.
An example is Down's syndrome, where individuals have 47 chromosomes instead of 46, due to three copies of chromosome 21.

Meiosis: Independent Segregation

Meiosis produces daughter cells that are genetically different from each other and from the parent cell.
Meiosis has two divisions: meiosis I and meiosis II.
During meiosis I, homologous chromosomes line up in the centre of the cell and are then separated into different cells — this is segregation of homologous chromosomes.
Each pair of homologous chromosomes lines up randomly, so the way one pair segregates does not affect how another pair segregates.
This is called independent segregation (also known as independent assortment), and it leads to genetically different daughter cells.

Meiosis: Crossing Over

Crossing over is the process where homologous chromosomes exchange genetic material during meiosis I.
Homologous chromosomes pair up and form bivalents.
The chromatid breaks and rejoins to the chromatid of its homologous chromosome, so alleles are exchanged.
Crossing over leads to new combinations of alleles on each chromatid; this is called recombination.
Crossing over results in further genetic variation among daughter cells.

Meiosis in Animal & Plant Cells

Meiosis is a form of nuclear division that produces haploid cells from diploid cells, forming gametes in plants and animals.
It has two divisions, meiosis I and meiosis II, each with prophase, metaphase, anaphase and telophase.
Meiosis I separates homologous chromosomes and reduces the chromosome number by half, producing two haploid cells.
In prophase I, DNA condenses, homologous chromosomes pair side by side, crossing over may occur, the spindle forms, and the nuclear envelope breaks down.
In metaphase I, homologous pairs line up randomly along the equator; independent segregation occurs.
In anaphase I, homologous pairs are separated as whole chromosomes are pulled to opposite poles; centromeres do not divide.
In telophase I, chromosomes arrive at poles, nuclear envelopes form, and cytokinesis produces two haploid daughter cells (each with duplicated chromosomes).
Meiosis II is almost identical to mitosis; there is no interphase, so DNA is not replicated.
In prophase II, nuclear envelope breaks down, chromosomes condense, and a new spindle forms at right angles.
In metaphase II, chromosomes (two sister chromatids) line up in single file along the equator.
In anaphase II, centromeres divide and individual chromatids are pulled to opposite poles; each chromatid is now an individual chromosome.
In telophase II, nuclear membranes form, cytokinesis occurs, and four haploid cells are produced.

Meiosis: Sources of Genetic Variation

Crossing over and independent segregation result in different combinations of alleles in gametes, creating genetic variation.
During fertilisation, any male gamete can fuse with any female gamete to form a zygote; this random fusion creates genetic variation between zygotes.
The number of possible chromosomal combinations resulting from meiosis is 2ⁿ, where n is the number of homologous chromosome pairs.
For humans, n = 23, so 2²³ = 8,388,608 possible chromosomal combinations.
After random fertilisation, the number of combinations is (2ⁿ)², where n is the haploid number.
For humans, (2²³)² = 70,368,744,177,664 possible combinations.
This explains why relatives can differ from one another, and why individuals can be genetically distinct even with the same parents.

Meiosis Under a Microscope

Cells undergoing meiosis can be observed using specialised microscopes, and different stages have distinctive characteristics.
Homologous chromosomes pair up side by side in meiosis I only, so if pairs are seen, meiosis I is occurring.
The number of cells formed helps distinguish meiosis I and II: two new cells indicate meiosis I, four new cells indicate meiosis II.
During meiosis II, single chromosomes may be observed.
In prophase I, homologous pairs of chromosomes are visible.
In metaphase I, homologous pairs are lined up side by side along the equator.
In anaphase I, whole chromosomes are pulled to opposite poles with centromeres intact.
In telophase I, two groups of condensed chromosomes are surrounded by forming nuclear membranes.
In prophase II, single whole chromosomes are visible.
In metaphase II, single whole chromosomes line up in single file along the equator.
In anaphase II, centromeres divide and chromatids are pulled to opposite poles.
In telophase II, nuclei form around four groups of condensed chromosomes.

Comparing Meiosis & Mitosis

Mitosis ends with two daughter cells genetically identical to each other and the parent cell; it is used for growth and repair.
Meiosis ends with four daughter cells, each with half the genetic material of the parent cell and all genetically different; it is used for production of gametes.
Mitosis: one cell division; meiosis: two cell divisions.
Mitosis: daughter cells are diploid (46 chromosomes in humans); meiosis: daughter cells are haploid (23 chromosomes in humans).
Mitosis: two daughter cells; meiosis: four daughter cells.
Mitosis occurs all over the body; meiosis occurs in the sex organs.
In mitosis, homologous chromosomes do not pair up and crossing over does not occur; in meiosis, homologous chromosomes pair up and crossing over occurs.
Meiosis always involves a reduction division, halving the chromosome number.

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Questões de prática

Prévia grátis — 8 de 62 perguntas. Cadastre-se para ver todas.

1.Which of the following is the correct definition of a mutation?
Easy
- AA change in a gene to adapt to changes in the environment
- BA random copying error in the DNA base sequence
- CAn adaptation that causes variation within a species
- DA deliberate change made by an organism to suit its environment
2.During which phase of the cell cycle do most mutations occur?
Easy
- AProphase
- BMetaphase
- CInterphase
- DCytokinesis
3.The DNA triplet TGA codes for the amino acid threonine. A base substitution mutation changes this triplet to TGT, which also codes for threonine. What is the effect of this mutation on the phenotype of the organism?
Medium
- ANo effect, because the amino acid sequence is unchanged
- BA different amino acid is inserted, altering the protein
- CA premature stop codon is created, shortening the protein
- DThe reading frame is shifted, changing all subsequent amino acids
4.Which type of mutation involves a change in the number of chromosomes?
Easy
- ASubstitution mutation
- BDeletion mutation
- CChromosome mutation
- DSilent mutation
5.Which of the following processes introduce genetic variation during sexual reproduction? (Select all that apply)
Medium
- AIndependent segregation of chromosomes in meiosis
- BRandom fertilisation
- CCrossing over
- DDNA replication
- EMitosis
6.Meiosis produces four daughter cells that are genetically identical to the parent cell.
Easy
True or false?
7.Crossing over occurs during meiosis I.
Easy
True or false?
8.Homologous chromosomes pair up during meiosis II.
Easy
True or false?

Mais perguntas

9.A cell has a diploid number of 16. How many different chromosomal combinations can result from meiosis, assuming no crossing over occurs?
Medium
- A16
- B256
- C65536
- D8
10.Match each stage of meiosis with the correct description.
Medium
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
- Homologous pairs line up side by side along the equator
- Homologous chromosomes separate and are pulled to opposite poles
- Nuclear envelopes form around two groups of chromosomes
- Crossing over occurs and spindle forms
11.Arrange the stages of meiosis I in the correct order.
Medium
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
12.Which of the following statements about meiosis is correct?
Medium
- AMeiosis produces two diploid daughter cells.
- BMeiosis produces four haploid daughter cells.
- CMeiosis produces two haploid daughter cells.
- DMeiosis produces four diploid daughter cells.
13.During anaphase I, centromeres divide and individual chromatids are pulled to opposite poles.
Medium
True or false?
14.Which of the following is an example of a mutagenic agent?
Medium
- AUV light
- BWater
- COxygen
- DGlucose
15.Which of the following statements about non-disjunction are true? (Select all that apply)
Medium
- AIt occurs when chromosomes fail to separate during meiosis.
- BIt can result in gametes with an extra chromosome.
- CIt always results in a mutation that is beneficial.
- DIt can lead to conditions such as Down's syndrome.
- EIt occurs during mitosis only.
16.Match each mutagenic agent with its category.
Medium
- UV light
- X-rays
- Peroxides
- High-energy radiation
- Ionising radiation
- Toxic chemicals
17.Match each stage of meiosis with the event that can be observed in it.
Medium
- Anaphase I
- Anaphase II
- Metaphase I
- Metaphase II
- Whole chromosomes pulled to opposite poles with centromeres intact
- Centromeres divide and chromatids are pulled to opposite poles
- Homologous pairs line up side by side along the equator
- Single chromosomes line up in single file along the equator
18.Arrange the events of meiosis in the correct sequence, starting from the earliest.
Medium
- Homologous chromosomes pair up and crossing over occurs
- Homologous pairs line up randomly along the equator of the spindle
- Homologous chromosomes are separated and pulled to opposite poles
- Chromosomes line up in single file along the equator of the spindle
- Centromeres divide and individual chromatids are pulled to opposite poles
19.Arrange the stages of meiosis II in the correct order.
Medium
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
20.Which of the following statements about non-disjunction are correct? (select all that apply)
Medium
- AIt occurs when chromosomes fail to separate during meiosis
- BIt can produce gametes with one extra copy of a particular chromosome
- CIt can produce gametes with no copies of a particular chromosome
- DIt always produces gametes with the normal haploid number of chromosomes
- EIt occurs spontaneously
21.Which of the following are sources of genetic variation in the gametes produced by meiosis? (select all that apply)
Medium
- ACrossing over between homologous chromosomes
- BIndependent segregation of homologous chromosomes
- CDNA replication in interphase producing identical chromatids
- DRandom fertilisation of gametes
- EProofreading of DNA during replication
22.Which of the following statements about meiosis I are correct? (select all that apply)
Medium
- ADNA replication has already occurred before prophase I
- BHomologous chromosomes arrange side by side in pairs
- CCentromeres divide during anaphase I
- DThe chromosome number is halved, producing two haploid cells
- EDNA is replicated again between meiosis I and meiosis II
23.A deletion mutation changes only the triplet in which the mutation occurs and has no knock-on effect on later triplets.
Easy
True or false?
24.A substitution mutation creates a premature STOP codon, causing the polypeptide chain produced to be incomplete.
Easy
True or false?
25.In metaphase I, homologous pairs of chromosomes line up randomly along the equator of the spindle.
Easy
True or false?
26.Individuals with Down's syndrome have 47 chromosomes because they have three copies of chromosome 21.
Medium
True or false?
27.In a substitution mutation, the codon UAU is changed to UAC. Both codons code for tyrosine. What is this type of mutation called?
Medium
- ASilent mutation
- BNonsense mutation
- CFrameshift mutation
- DChromosome mutation
28.Which statement correctly describes the daughter cells produced at the end of meiosis I?
Medium
- ATwo haploid cells, each still containing duplicated chromosomes
- BTwo diploid cells, each containing single chromosomes
- CFour haploid cells, each containing single chromosomes
- DFour diploid cells, each containing duplicated chromosomes
29.A structural protein such as collagen loses its strength after a mutation. Which best explains why?
Medium
- AThe mutation significantly altered the polypeptide's shape
- BThe mutation changed the active site of the protein
- CThe mutation prevented the protein from being transcribed
- DThe mutation changed the number of chromosomes in the cell
30.Which of the following is the correct formula for the number of possible chromosomal combinations resulting from the random fertilisation of two gametes?
Medium
- A(2ⁿ)²
- B2ⁿ
- C2^(2n)
- D(2n)²
31.A species has a diploid number of 36. How many different chromosomal combinations can result from meiosis, assuming no crossing over occurs?
Medium
- A262144
- B131072
- C68
- D524288
32.A gamete is produced that contains an extra copy of chromosome 21. Which process most directly caused this?
Hard
- ANon-disjunction during meiosis
- BCrossing over during prophase I
- CIndependent segregation during metaphase I
- DA substitution mutation during DNA replication
33.Under the microscope, a cell shows single whole chromosomes lined up in a single file along the equator of the spindle. Which stage is this?
Hard
- AMetaphase II
- BMetaphase I
- CAnaphase I
- DTelophase I
34.Which observation would allow a scientist to conclude that meiosis II, rather than meiosis I, is occurring?
Medium
- AFour new cells are being formed
- BHomologous pairs are visible side by side
- CWhole chromosomes are being pulled to opposite poles
- DTwo new cells are being formed
35.In a species of plant, a cell in the root tip is observed to have 24 chromosomes. A cell in the flower bud of the same plant is observed undergoing meiosis. What is the haploid number (n) of this plant?
Medium
- A12
- B24
- C48
- D6
36.A human cell has 46 chromosomes. Assuming no crossing over, how many different chromosomal combinations can be produced by meiosis?
Medium
- A2²³
- B2⁴⁶
- C(2²³)²
- D46²
37.A species has a diploid number of 20. Assuming no crossing over, how many different chromosomal combinations can result from the random fertilisation of two gametes?
Hard
- A(2¹⁰)²
- B2¹⁰
- C2²⁰
- D(2²⁰)²
38.A mutation in a gene results in the substitution of a single amino acid in an enzyme. Which of the following is the most likely effect on the enzyme?
Hard
- AThe active site shape may change, preventing the substrate from binding.
- BThe enzyme will be completely non-functional.
- CThe enzyme will have a completely different amino acid sequence.
- DThe enzyme will still function normally because the genetic code is degenerate.
39.A student examines a cell under the microscope and observes homologous chromosomes lined up side by side along the equator of the spindle. Which stage of meiosis is this?
Medium
- AMetaphase I
- BMetaphase II
- CAnaphase I
- DProphase II
40.A cell undergoing meiosis is observed to have single chromosomes lined up in a single file along the equator. Which stage is this?
Hard
- AMetaphase II
- BMetaphase I
- CAnaphase I
- DTelophase I
41.Name the property of the genetic code that means different triplets can code for the same amino acid.
Easy
42.Calculate the number of possible chromosomal combinations following random fertilisation in a plant species with a haploid number of 8. Use the formula (2ⁿ)².
Medium
43.Complete the sentence about meiosis.
Easy
Meiosis is a form of nuclear division that produces ____ cells from ____ cells.
44.A human cell has a diploid number of 46. Calculate the number of possible chromosomal combinations resulting from meiosis, assuming no crossing over. Give your answer in standard form if needed.
Medium
45.Complete the definition of an allele.
Easy
An allele is a different version of a ____.
46.Name the process by which homologous chromosomes exchange genetic material during meiosis I.
Medium
47.Name the structure formed when homologous chromosomes pair up during prophase I.
Easy
48.Give the name of the process in which homologous chromosomes exchange genetic material during meiosis I.
Medium
49.A plant species has a haploid number of 9. Calculate the number of possible chromosomal combinations resulting from meiosis, assuming no crossing over occurs.
Medium
50.A species has a haploid number of 5. Calculate the number of possible chromosomal combinations following the random fertilisation of two gametes.
Hard
51.How many daughter cells are produced at the end of meiosis?
Medium
52.Complete the sentence about crossing over.
Medium
Crossing over leads to new combinations of alleles on each chromatid; this is called ____.
53.Explain how crossing over during meiosis contributes to genetic variation.
Hard
54.Explain why some substitution mutations have no effect on the polypeptide produced.
Medium
55.Explain how a substitution mutation can result in a non-functional enzyme.
Hard
56.Explain how crossing over and independent segregation produce genetically different gametes.
Hard
57.Explain how a deletion mutation could have a greater effect on a polypeptide than a substitution mutation.
Hard
58.Calculate the number of possible chromosomal combinations following random fertilisation in a plant species with a haploid number of 8. Use the formula (2ⁿ)².
Medium
- A58982
- B65536
- C72090
- D78644
59.A human cell has a diploid number of 46. Calculate the number of possible chromosomal combinations resulting from meiosis, assuming no crossing over. Give your answer in standard form if needed.
Medium
- A7549747
- B8388608
- C10066330
- D9227469
60.A plant species has a haploid number of 9. Calculate the number of possible chromosomal combinations resulting from meiosis, assuming no crossing over occurs.
Medium
- A614
- B512
- C563
- D461
61.A species has a haploid number of 5. Calculate the number of possible chromosomal combinations following the random fertilisation of two gametes.
Hard
- A1126
- B922
- C1228
- D1024
62.How many daughter cells are produced at the end of meiosis?
Medium
- A5
- B4
- C3
- D6

Resumos de revisão

## Genetic Mutations - **Gene mutation** = 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**; most do not alter the polypeptide (or only slightly) because the genetic code is **degenerate**. - **Deletion** – a nucleotide/base is randomly deleted; changes the amino acid coded for and has a **knock-on (frameshift)** effect on later triplets, potentially dramatically changing the amino acid sequence and protein function. - **Substitution** – a base is randomly swapped for a different base; only changes the triplet where it occurs (no knock-on effect). Three outcomes: - **Silent** – no amino acid change (code is degenerate). - **Alters a single amino acid** – e.g. sickle cell anaemia. - **Creates a premature STOP codon** – incomplete polypeptide, e.g. cystic fibrosis (nonsense mutation). - Effect on protein: altered shape may stop substrate binding to an enzyme's active site, or weaken a structural protein like collagen. - **Mutagenic agents** = environmental factors that increase the mutation rate: high-energy radiation (UV light), ionising radiation (X-rays), toxic chemicals (peroxides). ## Mutations in Chromosome Number - **Chromosome mutations** involve a change in the number of chromosomes and involve **non-disjunction**. - Non-disjunction = chromosomes fail to separate during meiosis; occurs spontaneously. - Gametes may get an extra copy or no copy of a chromosome; if such a gamete is fertilised, the diploid cell has the wrong chromosome number. - Example: **Down's syndrome** – 47 chromosomes instead of 46 (three copies of chromosome 21). ## Meiosis - Produces **haploid cells from diploid cells**; produces gametes in plants and animals for sexual reproduction. - Two divisions: **meiosis I** and **meiosis II**; each has prophase, metaphase, anaphase, telophase. - **Meiosis I** separates homologous chromosomes, halving the chromosome number → two haploid cells. - Prophase I: DNA already replicated; chromosomes condense; homologous pairs side by side; crossing over of non-sister chromatids may occur; spindle forms; nuclear envelope breaks down. - Metaphase I: homologous pairs line up randomly along the equator; **independent segregation** occurs. - Anaphase I: homologous pairs separated; whole chromosomes pulled to opposite poles; centromeres do not divide. - Telophase I: chromosomes at poles; nuclear envelopes form; cytokinesis → two haploid cells (still with duplicated chromosomes). - **Meiosis II** – no interphase, DNA not replicated; almost identical to mitosis. - Prophase II: envelope breaks down; spindle forms at right angles. - Metaphase II: chromosomes (two sister chromatids) line up single file at the equator. - Anaphase II: centromeres divide; chromatids pulled to opposite poles (each now an individual chromosome). - Telophase II: nuclear membranes form; cytokinesis → **four haploid cells**. ## Sources of Genetic Variation - **Crossing over** (meiosis I): homologous chromosomes form bivalents; chromatids break and rejoin, exchanging alleles → new combinations (**recombination**). - **Independent segregation** of homologous chromosomes (metaphase I). - **Random fertilisation**: any male gamete can fuse with any female gamete. - Chromosome combinations after meiosis = **2ⁿ** (n = number of homologous pairs). For humans: 2²³ = 8 388 608. - After random fertilisation: **(2ⁿ)²** (n = haploid number). For humans: (2²³)² = 70 368 744 177 664. ## Meiosis Under a Microscope - Meiosis I: homologous chromosomes pair up side by side; two new cells form. - Meiosis II: single chromosomes visible; four new cells form. - Metaphase I – pairs line up side by side; Metaphase II – single file. Anaphase I – whole chromosomes pulled, centromeres intact; Anaphase II – centromeres divide, chromatids pulled apart. ## Comparing Meiosis & Mitosis | Feature | Mitosis | Meiosis | |---|---|---| | Purpose | growth and repair | production of gametes | | Divisions | once | twice | | Daughter cells | 2, diploid (46), genetically identical to parent and each other | 4, haploid (23), genetically different from parent and each other | | Location | all over the body | sex organs | - Meiosis is always a **reduction division** – ploidy is halved. - Chromosomes are counted by **centromeres**; after replication there are still 46 chromosomes but 92 chromatids.

Gene mutation: A change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide.
Deletion mutation: A mutation in which a nucleotide (and therefore its base) is randomly deleted from the DNA sequence; it changes the amino acid coded for and has a knock-on (frameshift) effect on later triplets.
Substitution mutation: A mutation in which a base in the DNA sequence is randomly swapped for a different base; it only changes the amino acid for the triplet in which the mutation occurs, with no knock-on effect.
Degenerate (genetic code): The property of the genetic code whereby many codons code for the same amino acid, so some mutations are silent.
Mutagenic agents: Environmental factors that increase the mutation rate of cells, e.g. UV light, X-rays, peroxides.
Non-disjunction: The failure of chromosomes to separate during meiosis, which can result in gametes with an incorrect number of chromosomes.
Chromosome mutation: A change in the number of chromosomes, involving non-disjunction.
Meiosis: A form of nuclear division that results in the production of haploid cells from diploid cells, producing gametes in plants and animals.
Independent segregation: The random lining up and separation of homologous chromosomes during meiosis I, leading to genetically different daughter cells.
Crossing over: The process where homologous chromosomes exchange genetic material during meiosis I, producing new combinations of alleles (recombination).
Recombination: The new combinations of alleles on each chromatid produced by crossing over.
Allele: A different version of a gene.
Haploid: A cell containing half the number of chromosomes (e.g. 23 in humans).
Diploid: A cell containing the full number of chromosomes (e.g. 46 in humans).
Random fertilisation: The fusion of any male gamete with any female gamete to form a zygote, creating genetic variation between zygotes.

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