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Topic 3: Voice of the genome›Meiosis and sexual reproduction (3.9, 3.8, 3.6, 3.7)

Meiosis and sexual reproduction (3.9, 3.8, 3.6, 3.7)

An overview of meiosis and sexual reproduction (3.9, 3.8) from Edexcel A level Biology A including: meiosis, gametes and fertilisation
3 min

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Meiosis is a type of cell division that produces four genetically distinct gametes (sperm and egg cells) and generates genetic variation through independent assortment of chromosomes and crossing over of alleles between chromatids.

It reduces the chromosome number from diploid to haploid.

Meiosis involves two sequential divisions:

  1. Meiosis I – separates homologous chromosomes.
  2. Meiosis II – separates sister chromatids.
The diagram illustrates the stages of meiosis with labeled phases. At the top, a circle labeled 'Interphase' contains two pairs of loosely aligned red and blue squiggly lines. Below, an arrow points to a circle labeled 'Meiosis I' with more condensed and crossed red and blue lines. This circle splits into two circles; the left one has a pair of crossed red and blue lines, and the right one has two sets of red and blue lines. Two arrows point downward from each circle. The left-side arrow leads to a circle with separated red and blue lines. The right-side arrow, labeled 'Meiosis II,' leads to two circles, each containing separate red and blue lines. The entire diagram is labeled with the copyright '© Medify.'
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A locus (plural: loci) is the specific position of a gene on a chromosome.

Linked genes are genes located at different loci on the same chromosome. They tend to be inherited together during meiosis because they are physically linked. The closer two genes are to each other, the less likely they are to be separated by crossing over and the more likely they are to be inherited together.

The image shows a diagram of a homologous chromosome pair. Two vertical blue chromosomes are depicted side by side. Each chromosome has a highlighted section with a gene locus labeled 'Flower-colour gene locus.' The left chromosome features a gray section labeled 'Allele for white flowers,' and the right chromosome has a red section labeled 'Allele for red flowers.' At the bottom, there is a bracket encompassing both chromosomes with the label 'Homologous chromosome pair.'
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Sex–linked genes are located on the sex chromosomes (X or Y). In humans, most sex-linked genes are found on the X chromosome, as the Y chromosome carries relatively few genes.

Examples of X-linked traits include red-green colour blindness and haemophilia.

Males (XY) are more likely to express X–linked recessive alleles because they have only one X chromosome and therefore no second allele to mask a recessive allele. Females (XX) may be carriers if they possess one recessive and one dominant allele.

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The function of sperm cells is to deliver male genetic material to the egg for fertilisation.

Sperm cell adaptations include:

  • Acrosome: a vesicle at the tip of the sperm containing enzymes that digest the zona pellucida (glycoproteins surrounding the egg) allowing sperm entry.
  • Flagellum (tail): enables motility, allowing the sperm to swim towards the egg.
  • Mitochondria in the midpiece: provides ATP for movement.
  • Haploid nucleus: contains one set of chromosomes (23 in humans) and fuses with the haploid nucleus of the egg during fertilisation.
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The function of an ovum (egg cell) is to provide female genetic material and support early embryo development.

Ovum adaptations include:

  • Zona pellucida: a glycoprotein layer surrounding the egg that helps prevent polyspermy (fertilisation by more than one sperm).
  • Cytoplasm: contains nutrients and organelles to support early embryonic development.
  • Haploid nucleus: contains one set of chromosomes (23 in humans) that fuses with the haploid nucleus of the sperm during fertilisation.
  • Cell surface membrane: can fuse with the sperm membrane during fertilisation.
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Fertilisation is the process in which a male gamete (sperm) and a female gamete (egg) form a zygote. This can be broken down into the following steps:

  • Acrosome reaction – the sperm reaches the egg and binds to its zona pellucida. The acrosome releases digestive enzymes, breaking down the zona pellucida, allowing the sperm to reach and fuse with the egg cell membrane.
  • Cortical reaction – the egg releases cortical granules into the zona pellucida (the layer that surrounds the membrane), which hardens and prevents polyspermy (entry of multiple sperm).
  • Fusion of nuclei – the sperm’s nucleus enters the egg cytoplasm, and the sperm and egg nuclei fuse, forming a diploid zygote with 46 chromosomes (in humans).
The image shows a sequence of events in fertilization. The first frame depicts a sperm cell approaching an egg cell. The second frame shows the sperm contacting the egg, labeled 'Zona pellucida break down and penetration.' The third frame illustrates the fusion of egg and sperm chromosomes, labeled 'Egg and successful sperm chromosomes.' The final frame shows 'Polyspermy block' with a red no entry symbol, indicating the egg releases cortical granules to harden the zona pellucida, preventing entry of more sperm. Each part is labeled accordingly.
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