Meiosis Core — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

Section 1 of 22What meiosis is

Meiosis

Meiosis a type of nuclear division that gives rise to genetically different cells.

• It is a reduction division in which the chromosome number is halved from diploid to haploid.

Role

• Role meiosis is involved in the production of gametes (sex cells).

Why halving is needed

• Why halving is needed otherwise the chromosome number would double at fertilisation in the zygote.

Section 2 of 22Process of meiosis

1

Each chromosome copies itself, forming X-shaped chromosomes, which line up in the centre of the cell.

2

First division: chromosomes pair up, recombination occurs, and fibres pull the pairs apart.

3

Second division: chromosomes line up and fibres pull them apart, as in mitosis.

4

Four haploid daughter cells are produced.

5

These cells form the gametes.

Section 3 of 22Why meiosis is important

1Gametes

• Gametes it produces sperm cells and egg cells in animals, and pollen grains and ovum in plants.

2Variation

• Variation it forms new combinations of maternal and paternal chromosomes every time a gamete is made.

3Random fertilisation

• Random fertilisation gametes fuse at random, so each offspring differs from the others.

4Constant chromosome number

• Constant chromosome number fertilisation restores the diploid number.

Section 4 of 22Mitosis and meiosis compared

Feature	Mitosis	Meiosis
Divisions	One	Two
Cells produced	Two	Four
Chromosome number	Diploid (46 in humans)	Haploid (23 in humans)
Genetic make-up	Identical to parent and each other	Different from parent and each other

Section 5 of 22Gamete formation sites

1Animals

• Animals testes and ovaries, the gonads.

2Flowering plants

• Flowering plants anthers and ovules.

Section 6 of 22More from the specification

1

Independent assortment of maternal genes

• Independent assortment of maternal genes random orientation of homologous pairs during metaphase I redistributes parental genes to generate extensive gametic variation.

2

Protein synthesis

• Making proteins from amino acids using the DNA code.

3

Monohybrid inheritance

• Inheritance of one gene.

4

Reductive Division in Meiosis

• Meiosis involves two successive nuclear divisions following a single round of DNA replication,
  • reducing chromosome number from diploid (2n) to haploid (n) in specialized gametes.

5

Generation of Novel Allelic Combinations

• Independent assortment of maternal and paternal homologous chromosomes and crossing over between non-sister chromatids produce unique allele combinations in every gamete.

6

Maintenance of Diploidy Across Generations

• Fusion of male and female haploid gametic nuclei during fertilisation restores the complete diploid chromosome complement in the resulting zygote.

7

Homozygous

• Having two identical alleles.

8

Sequence

• The order of bases in DNA or amino acids in a protein.

9

Phenotype

• The observable characteristic.

10

Specific

• Able to act on only one type of substrate.

11

Heterozygous

• Having two different alleles.

12

Probability

• The chance that an offspring has a given genotype or phenotype.

13

Punnett square

• A grid predicting offspring genotypes.

14

Blood group

• A phenotype set by alleles I-A, I-B and I-O.

15

Allele

• A version of a gene.

16

Inheritance

• Transmission of genetic information from generation to generation.

17

Recessive

• Expressed only when no dominant allele is present.

18

Dominant

• Expressed when present in the genotype.

19

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

20

Genotype

• The alleles an organism has.

Section 7 of 22Why meiosis is needed

Gametes

• Gametes must have half the usual chromosome number so the zygote has the full diploid number.

Otherwise

• Otherwise the chromosome number would double each generation.

Meiosis

Meiosis a reduction division in which a diploid cell forms four genetically different haploid cells.

Humans

• Humans a diploid cell has 46 chromosomes in 23 pairs and each gamete has 23, one from each pair.

Section 8 of 22Where it happens

1Animals

• Animals in the testes, forming sperm, and in the ovaries, forming eggs.

2Flowering plants

• Flowering plants in anthers and ovules.

3Men

• Men meiosis produces sperm continually from puberty.

4Women

• Women eggs mature one at a time, usually one per menstrual cycle, and the other cells are small polar bodies that degenerate.

Section 9 of 22Stages in outline

1

DNA is copied in interphase, so each chromosome has two chromatids.

2

First division: homologous chromosomes pair up and are separated into two cells, each with the haploid number.

3

Genetic material may be exchanged between homologous chromosomes (crossing over).

4

Second division: the chromatids separate, as in mitosis.

5

The result is four haploid cells, each genetically different.

Section 10 of 22Chromosome number through meiosis

Stage	Chromosome number per cell	Ploidy
Starting cell (4 chromosomes)	4	Diploid
After first division	2 (each with two chromatids)	Haploid
After second division	2 (single)	Haploid

• The number is halved from 4 to 2 in this model.

Section 11 of 22Fertilisation restores the number

Fusion

• Fusion the sperm nucleus (n = 23) fuses with the egg nucleus (n = 23).

Zygote

• Zygote forms with 2n = 46.

Embryo

• Embryo the zygote divides by mitosis.

Constant number

• Constant number the chromosome number is kept constant from generation to generation.

Variation

• Variation each gamete carries a random mixture of the parent's chromosomes, so every gamete is different.

Section 12 of 22Specialised gametes and errors

Sperm

• Sperm small and mobile with many mitochondria.

Egg

• Egg large with food stores.

Both

• Both haploid.

Non-disjunction

• Non-disjunction failure of chromosomes to separate, giving a gamete with an extra or missing chromosome.

Down's syndrome

• Down's syndrome caused by an extra chromosome 21.

Section 13 of 22More from the specification

1

Meiotic division and genetic variation

• Meiosis reduces the diploid chromosome number by half to produce four genetically distinct haploid gametes via independent assortment and crossing over.

2

Monohybrid genetic inheritance

• Punnett square diagrams calculate the theoretical probability of offspring inheriting dominant vs recessive phenotypes from heterozygous parental genotypes.

3

Alleles and homologous chromosomes

• Alleles are alternative nucleotide sequences of a gene occupying the same gene locus on maternal and paternal homologous chromosomes.

4

Punnett square monohybrid inheritance

• Punnett square monohybrid inheritance a Punnett square grid predicts genotypes and phenotypes of potential offspring resulting from parental allele combinations.

5

Dominant versus recessive allele expression

• Dominant versus recessive allele expression recessive traits manifest in individual offspring only when both inherited homologous chromosomes carry the recessive allele.

6

Independent assortment of genes

• Independent assortment of genes meiosis segregates maternal and paternal alleles independently into haploid gametes, generating extensive genetic diversity among offspring.

7

Blood group

• A phenotype set by alleles I-A, I-B and I-O.

8

Protein synthesis

• Making proteins from amino acids using the DNA code.

9

Monohybrid inheritance

• Inheritance of one gene.

10

Homozygous

• Having two identical alleles.

11

Sequence

• The order of bases in DNA or amino acids in a protein.

12

Phenotype

• The observable characteristic.

13

Specific

• Able to act on only one type of substrate.

14

Heterozygous

• Having two different alleles.

15

Probability

• The chance that an offspring has a given genotype or phenotype.

16

Punnett square

• A grid predicting offspring genotypes.

17

Daughter cell

• Cell produced by cell division.

18

Allele

• A version of a gene.

19

Inheritance

• Transmission of genetic information from generation to generation.

20

Recessive

• Expressed only when no dominant allele is present.

21

Dominant

• Expressed when present in the genotype.

22

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Section 14 of 22Sources of genetic variation

Independent assortment

• Independent assortment each pair of homologous chromosomes lines up randomly, so gametes get different mixtures of maternal and paternal chromosomes.

Crossing over

• Crossing over homologous chromosomes swap pieces, producing new combinations of alleles.

Random fertilisation

• Random fertilisation any sperm can fuse with any egg.

Mutation

• Mutation rare changes in DNA produce new alleles.

Random mating

• Random mating which partners reproduce is also a matter of chance.

Section 15 of 22Number of combinations

Independent assortment

• Independent assortment gives 2ⁿ combinations for n chromosome pairs.

Humans

• Humans with 23 pairs, each person can make about 8.4 million different gametes.

Crossing over

• Crossing over adds further variation.

Section 16 of 22Mitosis versus meiosis

Feature	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Production of gametes
Where	All body tissues	Reproductive organs
Divisions	One	Two
Cells produced	Two	Four
Chromosome number	Stays diploid	Halved (haploid)
Genetic make-up	Identical to parent	Different from parent and each other

Section 17 of 22Importance of variation

Changing environment

• Changing environment some individuals may have alleles that help them survive.

Natural selection

• Natural selection those individuals reproduce and pass on their alleles, so the population adapts.

Sexual reproduction

• Sexual reproduction gives an evolutionary advantage over asexual reproduction in changing conditions, though it needs two parents and is slower.

Section 18 of 22Order of events in a life cycle

1

Meiosis halves the chromosome number to make gametes.

2

Fertilisation restores the diploid number and combines alleles.

3

The zygote grows into an embryo by mitosis.

4

Mitosis copies the new genotype to all body cells.

Section 19 of 22Errors in meiosis

Errors in meiosis

Non-disjunction

• Non-disjunction chromosomes fail to separate, so a gamete has an extra or missing chromosome.

Down's syndrome

• Down's syndrome three copies of chromosome 21, giving 47 chromosomes.

Chance

• Chance such conditions arise by chance and are more likely with increased maternal age.

Karyotypes

• Karyotypes show chromosomes arranged in pairs and can detect these conditions.

Section 20 of 22More from the specification

1

Punnett square monohybrid cross analysis

• A Punnett square is used to predict the phenotypic and genotypic ratios of offspring, helping determine the probability of inheriting tall versus short phenotypes.

2

Dominant and recessive alleles in inheritance

• Dominant alleles express their phenotype in both homozygous and heterozygous conditions,
• whereas recessive alleles only express when homozygous.
  • (e.g. for short stem length)

3

Genes, alleles, and protein synthesis

• Genes are specific sequences of DNA nucleotides that code for the amino acid sequence of a functional protein,
• which directly determines observable inherited characteristics.

4

Meiosis and genetic variation in offspring

• Meiosis separates homologous chromosomes and alleles of genes into haploid gametes, ensuring diverse combinations of maternal and paternal traits in subsequent offspring.

5

Punnett square monohybrid cross analysis

• A Punnett square is used to predict the phenotypic and genotypic ratios of offspring, helping determine the probability of inheriting tall versus short phenotypes.

6

Dominant and recessive alleles in inheritance

• Dominant alleles express their phenotype in both homozygous and heterozygous conditions,
• whereas recessive alleles only express when homozygous.
  • (e.g. for short stem length)

7

Genes, alleles, and protein synthesis

• Genes are specific sequences of DNA nucleotides that code for the amino acid sequence of a functional protein,
• which directly determines observable inherited characteristics.

8

Meiosis and genetic variation in offspring

• Meiosis separates homologous chromosomes and alleles of genes into haploid gametes, ensuring diverse combinations of maternal and paternal traits in subsequent offspring.

9

Monohybrid inheritance

• Inheritance of one gene.

10

Phenotype

• The observable characteristic.

11

Specific

• Able to act on only one type of substrate.

12

Heterozygous

• Having two different alleles.

13

Probability

• The chance that an offspring has a given genotype or phenotype.

14

Punnett square

• A grid predicting offspring genotypes.

15

Daughter cell

• Cell produced by cell division.

16

Blood group

• A phenotype set by alleles I-A, I-B and I-O.

17

Protein synthesis

• Making proteins from amino acids using the DNA code.

18

Inheritance

• Transmission of genetic information from generation to generation.

19

Recessive

• Expressed only when no dominant allele is present.

20

Dominant

• Expressed when present in the genotype.

21

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

22

Homozygous

• Having two identical alleles.

23

Sequence

• The order of bases in DNA or amino acids in a protein.

Section 21 of 22Key terms

Term	Meaning
Meiosis	Reduction division giving four genetically different haploid cells.
Haploid	Having one set of chromosomes.
Reduction division	Division that halves the chromosome number.
Gamete	A sex cell with a haploid nucleus.
Gonad	An organ in which meiosis occurs: testis or ovary.
Independent assortment	Random sorting of chromosome pairs into gametes.

Term	Meaning
Meiosis	Reduction division producing four haploid cells.
Haploid	Having one set of chromosomes.
Diploid	Having two sets of chromosomes.
Crossing over	Exchange of genetic material between homologous chromosomes.
Non-disjunction	Failure of chromosomes to separate in meiosis.
Zygote	The diploid cell formed at fertilisation.

Term	Meaning
Independent assortment	Random lining up and separation of chromosome pairs.
Crossing over	Swapping of pieces between homologous chromosomes.
Random fertilisation	Chance fusion of any sperm with any egg.
Mutation	A random change in DNA that can form a new allele.
Karyotype	A display of chromosomes arranged in pairs.
Variation	Differences between individuals of a species.

Section 22 of 22Exam technique

1

Saying meiosis produces two cells — it produces four.

2

Saying meiosis cells are diploid — they are haploid.

3

Saying meiosis makes identical cells — they are genetically different.

4

Confusing meiosis with mitosis — meiosis makes gametes and mitosis makes body cells.

1

Saying meiosis happens in all body cells — it happens in the reproductive organs.

2

Saying gametes are diploid — they are haploid.

3

Saying fertilisation halves the chromosome number — it restores the diploid number.

4

Saying meiosis makes identical cells — they are genetically different.

1

Saying mutation is the only source of variation — meiosis and random fertilisation also contribute.

2

Calling gametes unique instead of genetically different — and state they are haploid.

3

Saying mitosis halves the chromosome number — meiosis does.

4

Saying sexual reproduction produces identical offspring — it produces varied offspring.