B1. Meiosis and Genetic Variation Flashcards
Diploid body cells
What number of chromosomes do normal body cells have and what does this mean?
Chromosome have the same… but different…?
What are homologous chromosomes?
Normal body cells have the diploid number (2n) of chromosomes - meaning each cell contains two of each chromosome (a pair), one from the mum and one from the dad
The chromosomes that make up each pair are the same size and have the same genes, although they could have different versions of those genes (called alleles). These pairs of matching chromosomes are called homologous pairs. Humans have 23 homologous pairs and so 46 chromosomes in total. Therefore the diploid number for humans is 46.
Gametes and sexual reproduction
What are gametes?
What do they have and what does this mean?
What happens in sexual reproduction?
Gametes are the sperm cells in males and egg cells in females. Gametes have a haploid (n) number of chromosomes-they only contain one copy of each chromosome in a homologous pair. The haploid number for humans is 23. In sexual reproduction two gametes join together at fertilisation to form a zygote, which divides and develops into a new organism.
Fertilisation
What happens at fertilisation? (2 points)
What is random fertilsation and what effect does it have?
Figure 1: Diagram to show fertilisation.
At fertilisation, a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. Half these chromosomes are from the father (the sperm) and half are from the mother (the egg).
During sexual reproduction, any sperm can fertilise any egg-fertilisation is random. Random fertilisation produces zygotes with different combinations of chromosomes to both parents. This mixing of genetic material in sexual reproduction increases genetic diversity within a species
Meiosis
Meiosis is a type of cell division. It takes place in the _____________ _______of multicellular, eukaryotic organisms. Cells that divide by meiosis are ________to start with, but the cells that are formed from meiosis are __________- the chromosome number _______. Meiosis in humans and other mammals produces gametes directly. In other organisms (e.g. some insects and plants) it produces haploid cells which later divide by mitosis to become gametes. Without meiosis, you’d get double the number of chromosomes when the gametes fused. Not good.
Stages of Meiosis (6 steps)
Meiosis
Meiosis is a type of cell division. It takes place in the reproductive organs of multicellular, eukaryotic organisms. Cells that divide by meiosis are diploid to start with, but the cells that are formed from meiosis are haploid -the chromosome number halves. Meiosis in humans and other mammals produces gametes directly. In other organisms (e.g. some insects and plants) it produces haploid cells which later divide by mitosis to become gametes. Without meiosis, you’d get double the number of chromosomes when the gametes fused. Not good.
- Before meiosis starts, the DNA unravels and replicates so there are two copies of each chromosome, called chromatids.
- The DNA condenses to form double-armed chromosomes, each made from two sister chromatids.
The sister chromatids are joined in the middle by a centromere. - Meiosis I (first division) - the chromosomes arrange themselves into homologous pairs.
- These homologous pairs are then separated, halving the chromosome number.
- Meiosis II (second division) - the pairs of sister chromatids that make up each chromosome are separated (the centromere is divided).
- Four haploid cells that are genetically different from each other are produced.
Creating genetic variation in gametes
There are two main events during meiosis that lead to genetic variation: (2 things, 5, 3 points)
- Crossing over of chromatids
- During meiosis I, homologous 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
- The crossing over of chromatids in meiosis I means that each of the four daughter cells formed from meiosis II contain chromatids with different alleles
- Each cell has a different chromatid and therefore a different set of alleles, which increases genetic variation in potential offspring.
- Independent segregation of chromosomes
- When the homologous pairs are separated in meiosis 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 leads to genetic variation in any potential offspring.
Tip: In any diploid species there are __ possible combinations of maternal and paternal chromosomes (where n is the number of homologous pairs). This means that in humans (which have 23 homologous pairs) there are __ or ___________ possible combinations of chromosomes.
Tip: In any diploid species there are 2” possible combinations of maternal and paternal chromosomes (where n is the number of homologous pairs). This means that in humans (which have 23 homologous pairs) there are 2” or 8 388 608 possible combinations of chromosomes.
Life cycles
In the exams, you might need to spot when ________happens in an organism with a life cycle you haven’t seen before, e.g. an insect or plant. Just remember that in any organism, ________is needed for ________ ______________because it produces ____________cells (usually ___________) with _____the number of chromosomes of the _________cell.
You might also be told how many chromosomes are in a parent cell, then asked to complete diagrams showing how many chromosomes will be in the daughter cells after the first and second divisions of meiosis. Remember that the chromosome number is …
Life cycles
In the exams, you might need to spot when meiosis happens in an organism with a life cycle you haven’t seen before, e.g. an insect or plant. Just remember that in any organism, meiosis is needed for sexual reproduction because it produces daughter cells (usually gametes) with half the number of chromosomes of the parent cell.
You might also be told how many chromosomes are in a parent cell, then asked to complete diagrams showing how many chromosomes will be in the daughter cells after the first and second divisions of meiosis. Remember that the chromosome number is halved during the first division.
Tip: The malaria parasite is a single- celled eukaryotic organism called a protist. Prokaryotes don’t divide by ________or ___________ _________.
Tip: The malaria parasite is a single- celled eukaryotic organism called a protist. Prokaryotes don’t divide by meiosis or reproduce sexually.
Life cycles - Example
The stages in the life cycle of the malaria parasite are shown in Figure 9. When and where does meiosis take place in the parasite’s life cycle? (5 things)
- Only the zygote of the malarial parasite is diploid (2n). All the other stages of its life cycle are haploid (n).
- The chromosome number only halves when the diploid zygote divides in the mosquito’s gut to form haploid sporozoites.
- In all the other divisions in the parasite’s life cycle, the chromosome number stays the same a haploid cell forms another haploid cell, e.g. haploid sporozoites divide (by mitosis) to form haploid merozoites.
- So only the zygote divides by meiosis and it only happens in the mosquito’s gut.
- Fertilisation of the gametes (sexual reproduction) also takes place in the mosquito’s gut.
Tip: Compare the number of chromosomes in the parent and daughter cells if they halve, the cell must have undergone ________.
Tip: Compare the number of chromosomes in the parent and daughter cells if they halve, the cell must have undergone meiosis.
Tip: Zygotes in humans divide by ________, not by ________. Don’t let this put you off. The important thing here is what’s happening to the ______________number.
Tip: Zygotes in humans divide by mitosis, not by meiosis. Don’t let this put you off. The important thing here is what’s happening to the chromosome number.
Tip: Meiosis doesn’t produce gametes directly in the mosquito life cycle. It still produces ________cells though.
Tip: Meiosis doesn’t produce gametes directly in the mosquito life cycle. It still produces haploid cells though.
Outcomes of mitosis and meiosis
It’s part of the cell cycle the process multicellular organisms use to grow and divide. Mitosis and meiosis have different outcomes. (3 for each)
Why are mitosis and meiosis different and how so?
Outcomes of mitosis and meiosis
Mitosis
- Produces cells with the same number of chromosomes as the parent cell.
- Daughter cells are genetically identical to each other and to the parent cell.
- Produces two daughter cells.
Meiosis
- Produces cells with half the number of chromosomes as the parent cell.
- Daughter cells are genetically different from one another and the parent cell.
- Produces four daughter cells.
You need to be able to explain the different outcomes of mitosis and meiosis. They’re different because mitosis only involves one division (which separates the sister chromatids) whereas meiosis has two divisions (which separate the homologous pairs and then the sister chromatids).
There’s no pairing or separating of homologous chromosomes in mitosis, and so no crossing over or independent segregation of chromosomes. This produces genetically identical daughter cells-unlike meiosis.
