Module 2 : Section 6 - Sexual Reproduction and Meiosis Flashcards
What does meiosis produce
Gametes for sexual reproduction
What happens in sexual reproduction
In sexual reproduction two gametes join together at fertilisation to form a zygote. The zygote then divides and develops into a new organism
What is meiosis
- A type of cell division that happens in the reproductive organs to produce gametes
- Meiosis involves a reduction division
- cells that have the full number of chromosomes to start to with, but the cells that formed from meiosis have half the number
What are haploid cells
Cells with half the normal number of chromosomes
Cells formed by meiosis are….
Genetically different because each new cell ends up with a different combination of chromosomes
Meiosis involves…
Two divisions:
- Meiosis I
- Meiosis II
Which meiosis division is the reduction division
Meiosis I is the reduction division (halves the chromosomes number)
What stages are meiosis I and meiosis II broken up into
Like mitosis:
- Prophase
- Metaphase
- Anaphase
- Telophase
When does the whole of meiosis begin
With interphase. During interphase, the DNA unravels and replicates to produce double-armed chromosomes called sister chromatids
What happens in prophase I in meiosis I
- Chromosomes condense, getting shorter and fatter
- Chromosomes then arrange themselves into homologous and crossing over occurs
- centrioles start moving opposite ends of the cells forming spindle fibres
- nuclear envelope breaks down
What happens in metaphase I in meiosis I
- homologous pairs line up across the centre of the cell and attach to the spindle fibres by the centromere
What happens in anaphase I in meiosis I
- spindles contract separating the homologous pairs - one chromosomes goes to each end of the cell
What happens in telophase I in meiosis I
- A nuclear envelope forms around each group of chromosomes
What happens in cytokinesis in meiosis I
- Cytokinesis (division of the cytoplasm) occurs and two haploid daughter cells are produced
What happens in meiosis II (second division)
- The two daughter cells undergo prophase II, metaphase II, anaphase II, telophase II, which are a lot like the stages in mitosis
What happens in anaphase II in meiosis II
- the pairs of sister chromatids are separated
- each new daughter cell inherits one chromatid from each chromosome
- four genetically different haploid cells are produced
- these are gametes
What happens in prophase I to the chromatids
- During prophase I of meiosis I, homologous pairs of chromosomes come together and pair up
- the chromatids twist around each other and bits of chromatids swap over
- the chromatids still contain the same genes but now have a different combination of alleles
What two main events that happen during meiosis lead to genetic variation
- Crossing over of chromatids
- independent assortment of chromosomes
What does the crossing of chromatids refer to
The crossing over of chromatids in meiosis I means that each of the four daughter cells formed from meiosis contains chromatids with different alleles
What does independent assortment of chromosomes refer to
- each homologous pair of chromosomes in your cells is made up of one chromosome from your mum and one chromosome from your dad
- when the homologous pairs line up in metaphase I and are separated in anaphase I, it’s completely random which chromosome from each pair ends up in which daughter cell
- so the four daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes
- this is called independent assortment of the chromosomes
- this ‘shuffling’ of chromosomes leads to genetic variation in any potential offspring
