3.1.9 Genetic Diversity may Arise by Meiosis Flashcards Preview

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What are the gametes in plants?

Ovum and pollen


Why do diploid cells (ones that divide into gametes) have to half their number of chromosomes?

Gametes have to fuse with another gamete to make a full set of genes to code for all the proteins 


Explain how sexual reproduction increases genetic diversity within a species?

Fertilisation of gametes is random & produces zygotes with different combinations of chromosomes to both parents


Where does meiosis take place in?

Reproductive organs


What happens before meiosis starts?

DNA unravels and replicates, forming 2 copies of each chromosomes = called chromatids


How are sister chromatids joined?

By centromere in the middle


Describe what happens in meiosis 1

  1. Prophase 1  
    • Chromosomes condense and super coil
  2. Metaphase 1
    • Chromosomes line up along the centre of nucleus with their homologous pair (bivalents)
  3. Anaphase 1
    • One of each homologous pair get pulled to either side of nucleus
  4. Telophase 1
    • Nuclear envelope reforms to make 2 separate nuclei


Describe what happens in meiosis 2

  1. Prophase 2
    • Chromosomes condense and super coil
  2. Metaphase 2
    • Chromosomes line up along centre of nucleus still attached at centromere
  3. Anaphase 2
    • Each chromatid gets pulled to either side of nucleus (centromere is divided)
  4. Telophase 2
    • Nuclear envelope reforms to make 4 operate nuclei each with single copy of each chromosome
  5. End up with 4 haploid non-identical daughter cells (n)


What is genetic variation?

Differences in the genes of individual organisms


Describe what is happening in this diagram & what this means

  1. Crossing over
  2. Between chromatids of homologous chromosomes
  3. (Chromatid twist around each other & bits of chromatids swap over)
  4. Chromatids still contain same gene but now have different combination of alleles


Name 2 ways how meiosis produces cells that are genetically different

  1. Crossing over of chromatids
  2. Independent segregation of chromosomes


Describe how crossing over of chromatids increases genetic variation

Each of 4 daughter cells formed from meiosis contains a different chromatid with a different set of alleles


Describe how independent segregation increases genetic variation

  • We don't know which maternal or paternal chromosomes will get pulled to each side
    • i.e. we don't know which chromosome (from each pair) ends up in which daughter cell
  • ∴ 4 daughter cells produced have different combinations of those maternal and paternal chromosomes
  • (Leads to genetic variation in any potential offspring)


What is chromosome mutation?

When cell produced in meiosis contains variation in numbers of whole chromosomes or parts of chromosomes

(caused by errors during meiosis)


Describe and explain the appearance of this chromosome

  • Chromsome is formed of 2 chromatids 
  • ∵ of DNA replication 

  • Sister chromatids are held together by centromere


What is the role of the centromere (2x)?

  • Holds chromatids together
  • Allows chromatids to be separated


This cell produces gametes by meiosis. Draw a diagram to show the chromosomes in one of the gametes.

3 chromosomes - one from each homologous pair


Crossing over of chromatids is ________



Name 3 advantages of variation (caused by meiosis) to species

  • Different adaptations 
  • Passing on alleles/genes
  • Allows for changing environment


Explain why Plant A (has 42 chromosomes) is fertile while Plant B (has 21 chromosome) is not

  1. Plant A's cells are diploid - have 2 copies of each chromosome
  2. Plant A's chromosomes can produce haploid gametes
  3. Plant A's gametes receive a copy of every chromosome


Explain the role of independent segregation in meiosis 

  • To provide genetic variation
  • Allows different combinations of maternal and paternal chromosomes


How do you calculate the number of different types of gametes that can be produced from a diploid cell?

Use this formula:


n = number of gene pairs


Calculate the number of different types of gametes that can be produced in a species with a diploid number of 24

212 = 4096


Assuming random fertilisation, calculate the number of different combinations of maternal and paternal chromosomes in the zygotes of this species (with a diploid number of 24)

4096 x 4096 = 16 777 216


Unit 5

During meiosis, when the X and Y chromosomes pair up, they do not form a typical bivalent as do other chromosomes. Explain why. (2)

  • X and Y chromosomes are different sizes / shapes
  • Chromatids unable to line up and form bivalent / only short pairing region / most of length not homologous

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