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        • 3.1 Genes
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        • 3.3 Meiosis
        • 3.4 Inheritance
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        • 4.1: Species, Communities and Ecosystems
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3.3 Meiosis

3.3 Syllabus
Picture

U1.One of diploid nucleus divides by meiosis to produce four haploid nuclei.

-Meiosis: One of 2 ways in which the nucleus of eukaryotic cell can divide. Nucleus divides twice. First division produces 2 nuclei each of which divides again to give a total of four nuclei and two divisions are known as meiosis I and meiosis II.
-Nucleus that undergoes the 1st division of meiosis is diploid- has 2 chromosomes of each type.
-Chromosomes of same type are known as homologous chromosomes. Each of 4 nuclei produced by meiosis has just 1 chromosome of each type and the are haploid.

-Meiosis involves halving of the chromosome number. It’s therefore known as a reduction division.
-Cells produced by meiosis I have 1 chromosome of each type, so halving of the chromosome number happens in the first division, not the second division. 2 nuclei produced by meiosis I have the haploid number of chromosomes, but each chromosome still consists of 2 chromatids.
         -> Two nuclei produced by meiosis I have haploid number of chromosomes, but each chromosome still consists of 2 chromatids. These chromatids separate during meiosis II, producing 4 nuclei that have haploid number of chromosomes, with each chromosome consisting of a single chromatid.
​

U2. The halving of the chromosomes number allows a sexual life cycle with fusion of gametes.

  • In asexual life cycle, offspring have the same chromosomes as parent so are genetically identical. In sexual life cycle there are differences between the chromosomes of offspring and the parents, so there is genetic diversity.
  • Sexual reproduction involves process of fertilization (union of sex cells, gametes usually from 2 different parents.) and it doubles the number of chromosomes number every generation. Halving chromosome: meiosis.
  • Meiosis happen at any stage during sexual life cycle but in animals happens during process of creating the gametes. So body cells are therefore diploid and have 2 copies of most genes
  • Meiosis: complex process, its evolution was critical step in origin of eukaryotes. Without meiosis, there cannot be fusion of gametes and sexual life cycle of eukaryotes could not occur.
To compensate for the chromosome doubling during fertilization gametes undergo meiosis, which halves the chromosomes present in gametes compared to the parent.
​

U3. DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.

-During early stages of Meiosis, chromosomes gradually shorten by supercoiling and each chromosome consists of 2 chromatids.
-> This is because all DNA in nucleus is replicated during interphase before meiosis, so each chromosome consists 2 sister chromatids.

-Initially 2 chromatids that make up each chromosome are genetically identical since DNA replication is very accurate and number of mistakes in copying of DNA is extremely small

-We might expect DNA to be replicated again between the 1st and 2nd division of meiosis but it doesn’t happen.
-> Explains how chromosome number is halved during meiosis. 1 diploid nucleus, in which each chromosome consists of 2 chromatids, divides twice to produce 4 haploid nuclei in which each chromosome consists of 1 chromatid.

-In the S-phase of the interphase before meiosis begins, DNA replication takes place. Chromosomes are replicated and these copies are attached to each other at the centromere. The attached chromosome and its copy are known as sister chromatids. Following S-phase, further growth and preparation take place for meiosis.
​

U4. The early stages of meiosis involved pairing of homologous chromosomes and crossing over followed condensation. 

-Most important events of meiosis happen at the start of meiosis I while the chromosomes are still very elongated and cannot be seen with a microscope.

-Firstly, homologous chromosomes pair up with each other because DNA replication has already occurred, each chromosome consists of 2 chromatids and so there’re 4 DNA molecules associated in each pair of homologous chromosomes.
    -> Pair of homologous chromosomes is bivalent and pairing process sometimes called synapsis.

-After synapsis, crossing over takes place. A junction is created where 1 chromatid in each of homologous chromosomes breaks and rejoins with the other chromatid. Crossing over occurs at random positions anywhere along the chromosomes. (At least one crossover occurs in each bivalent and there can be several.)

-Since crossover occurs at precisely the same position on 2 chromatids involved, there’s mutual exchange of gene between chromatids.
         -> As chromatids are homologous but not identical -> some alleles of exchanges genes are likely to be different. Chromatids with new combinations of alleles are therefore produced.
​

U5. Orientation of pairs of homologous chromosomes prior to separation is random.

-While pairs of homologous chromosomes r condensing inside the nucleus cell in early stage of meiosis, spindle microtubules are growing from poles of the cell. After nuclear membrane has broken down, these spindle microtubules attach to the centromeres of the chromosomes.
-Attachment of spindle microtubules are not the same as in mitosis:
         -> Each chromosome is attached to 1 pole only, not to both
         -> 2 homologous chromosomes in a bivalent are attached to different poles.
               -> Pole to which each chromosome is attached depends on which way the pair of chromosomes is facing. = orientation
               -> Orientation of bivalents is random, so each chromosome has an equal chance of attaching to each pole, and eventually of being pulled to it
                -> The orientation of 1 bivalent does not affect other bivalents.
-Random orientation occurs - each bivalent aligns independently and hence the daughter nuclei get a different mix of chromosomes.
​

U6. Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number .

-Telophase I
=The nuclei are now haploid (N) not diploid (2N): they each contain one pair of sister chromatids for each of the species’ chromosomes.

-The cytoplasm begins to divide by cytokinesis, New nuclei form, Chromosomes decondense

-Movement of chromosomes is not the same in first division of meiosis as in mitosis.
         -> Mitosis: centromere divides and two chromatids that make up chromosome move to opposite poles
         -> Centromere does not divide and whole chromosomes move to the poles

-Initially, 2 chromosomes in each bivalent r held tgth by chimasta (The exchange of genetic material occurs between non-sister chromatids at points) but these slide to the ned of chromosomes and the chromosomes can separate.
         -> This separation of homologous chromosomes; disjunction
         -> 1 chromosome from each bivalent moves to 1 of the poles and the other chromosome to the other pole

-Separation of pairs of homologous chromosomes to opposite poles of cell halves the chromosome number of cell.
=Therefore first division of meiosis that’s the reduction division. Because 1 chromosome of each type moves to each pole., both of 2 nuclei formed in first division of meiosis contain one of each type of chromosome = they are both haploid
​

U7. Crossing over and random orientation promotes genetic variation.

-Unpredictable inherit: due to Meiosis
-Every gamete produced by parent has new combination of alleles- meiosis is a source of endless genetic variation
-Apart from genes on X and Y chromosomes, humans have 2 copies of each gene.
     -> Some case 2 copies are same allele and there will be 1 copy of that allele in every gamete produced by the parent
     -> Two alleles different. Each of 2 alleles has an equal chance of being passed on in a gamete. Ex) gen with the allels A and a.              Half of gametes produced by parent will contain A and half will contain a

Ex) Another gene with alleles B and b. Again half of gametes will contain B and half b. However, meiosis can result in gametes with different combinations of these genes: AB, Ab, aB , ab.

  1. Random orientation of bivalents
    • In metaphase I, the orientation of bivalents= random and one bivalent does not influence the orientation of any of the other.
    • Process that generates genetic variation among genes that are on different chromosome types.
    • Every additional bivalent, number of possible chromosome combinations in cell produced by meiosis doubles. For haploid number of n, number of possible combinations is 2^n. Human= 2^23
​
  1. Crossing over
    • Without crossing over in prophase I, combinations of alleles on chromosomes would be forever linked together. Ex) one chromosome carried combination CD and another carried cd = only these combinations could occur in gametes.
    • Crossing over allows linked genes to be reshuffled to produce new combinations such as Cd, cD. It increases the number of allele combination that can be generated by meiosis so much that it is effectively infinite.
​

U8. Fusion of gametes from different parents promotes genetic variation.

-Fusion of gametes to produce a zygote: highly significant event both for individuals and for species.
   -> It’s start of the life of new individual
   -> It allows alleles from 2 different individuals to be combined in 1 new individual
   -> Combinations of alleles unlikely ever to have existed before
   -> Fusion of gametes therefore promote genetic variation in a species
   -> Genetic variation is essential for evolution.

-The fusion of two haploid gametes results in the formation of a diploid zygote
    ->This zygote can then divide by mitosis and differentiate to form a developing embryo

-As meiosis results in genetically distinct gametes, random fertilization by egg and sperm will always generate different zygotes
->Identical twins are formed after fertilization, by the complete fission of the zygote into two separate cell masses
​
Picture
Picture
Picture

A1.Non- disjunction can cause Down syndrome and other chromosome abnormalities.

-Meiosis sometimes subject to errors. Ex) homologous chromosomes fail to separate at anaphase.
=non-disjunction. -> can happen with any of the pairs of homologous chromosomes. Both of chromosomes. Both of chromosomes move to 1 pole and neither to the other pole.
=Result: gamete that either has extra chromosome or deficient in chromosome.
=Human: 45. 47 chromosomes

-Abnormal num of chromosomes often lead to person possessing syndrome.
Ex) possessing syndrome -> down syndrome which leaves the individual with three of chromosome number 21 instead of 2. (Hearing loss, hear and vision disorders, mental growth retardation).

-Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)
->One of the parental gametes had two copies of chromosome 21 as a result of non- disjunction
->The other parental gamete was normal and had a single copy of chromosome 21
->When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21
​

A2. Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

-Karyotyping is the process by which chromosomes are organised and visualised for inspection
   ->Karyotyping is typically used to determine the gender of an unborn child and test for chromosomal abnormalities
​
-Cells are harvested from the foetus before being chemically induced to undertake cell division (so chromosomes are visible)
    ->The stage during which mitosis is arrested will determine whether chromosomes appear with sister chromatids
​
-Finally, chromosomes are stained and photographed, before being organised according to structure
    ->The visual profile generated is called a karyogram
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        • 3.1 Genes
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        • 3.3 Meiosis
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        • 4.1: Species, Communities and Ecosystems
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      • Topic 6: Human Physiology >
        • 6.1 Digestive System
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      • Option D: Astrophysics
    • Geography SL/HL >
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      • Unit 1: Changing population
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      • Unit 5: Human Development and Diversity
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      • Unit 1: Business Organization and Movement >
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      • Unit 2: Human Resource Management >
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        • 2.2 Organizational structure
        • 2.3 Leadership and management
        • 2.4 Motivation
      • Unit 3: Finance and Accounts >
        • 3.1 Sources of Finance
        • 3.2 Costs and Revenues
        • 3.3 Break-even analysis
        • 3.4 Final accounts
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        • 3.7 cash flow
      • Unit 4: Marketing >
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        • 4.4 Market Research
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      • Unit 5: Operations Management >
        • 5.1 The role of operations Management
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    • Economics SL/HL (pending) >
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      • MICROECONOMICS >
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      • macroeconomics
    • Chinese B SL >
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    • Internal Assessment (IA) >
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    • Other notes >
      • Topic 1: Measurements and Uncertainties
      • Topic 2: Mechanics
      • Topic 3: Thermal Physics
      • Topic 4: Waves
      • Topic 5: Electricity and Magnetism
      • Topic 6: Circular Motion and Gravitation
      • Topic 7: Atomic, Nuclear and Particle Physics
      • Topic 8: Energy Production
      • Option D: Astrophysics
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    • YOLO?
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    • Danger of Quotes
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  • About us