Key Area 3
In this section, we will explore the importance of meiosis. We are used to reciting perfectly that "mitosis produces genetically identically daughter cells". Well, if gametes played this game, would you be a clone of your parents and your siblings? See scary image right if you want reassuring that you need meiosis in your life....like immediately!
We need meiosis for its insane capacity to generate variation. Let's find out more!
Before you start, consider the questions in the Jam below. How much can you retrieve from your memory?
Somatic Cells
All somatic cells are diploid - they have 2 sets of chromosomes (2n) - one from each parent. The diploid chromosome complement of a human is 46.
Germline Cells
Gametes carry only one set of chromosomes (n) and are, therefore, haploid. The chromosome complement of a human egg cell is 23. Germline cells that are precursor cells to gametes are diploid and divide by meiosis to generate haploid gametes.
Homologous Chromosomes
The chromosomes in each set have a matching homologous chromosome that is the same size, has the same centromere position and has the same genes at the same loci (Latin: place).
Though the genes are the same on the homologous chromosomes, the alleles may be different as each chromosome in the pair has been inherited from a different parent.
Task 28
In your notes, draw a pair of homologous chromosomes, emphasising that they are the same size. Label the centromere and two genes at different loci.
Answers are available here.
Fertilisation
Fertilisation occurs when the haploid nuclei of two gametes fuse together to form a new diploid nucleus.
Allele combinations
The combining of haploid genomes from two different individuals produces a new combination of alleles in the offspring, so variation is increased.
Meiosis
The production of haploid gametes starts with a type of cell division called meiosis.
Task 29
Explain what happens during fertilisation, with emphasis on the requirement for haploid gametes.
Answers are available here.
Linked genes
Genes on the same chromosome are said to be linked. Genes that are situated closely on a chromosome are less likely to be separated during crossing over, whereas genes that are further apart are much more likely to be separated (see image opposite).
Separating linked genes through crossing over creates recombinants.
Unlinked Genes
In these unlinked genes (A and B), it is easy to see how A and B can be shuffled into different gametes.
Linked Genes
But, how can we separate and shuffle genes C and D without breaking the chromosome apart? The answer is revealed in the details of meiosis.
Mapping Genes
Scientists use the frequency of recombination to map chromosomes, working out where each gene is in relation to others and pinpointing exact locations on the chromosome.
Consider the following example: In certain organisms, genes A, B, C and D are located on the same chromosome. The percentage recombination between pairs of these genes is shown in the table below. Since genes C and D have a percentage recombination of 25%, they will be quite far apart on the chromosome. Conversely, with a % recombination of just 5%, genes A and C will be closer together.
We can use this information to map the genes on the chromosomes as shown:
Task 30
In Drosophila, the genes for wing length (L), eye colour (C), body colour (B) and the presence of bristles (P) are linked (all on the same chromosome). The following table gives the frequency of recombination obtained in crosses involving different pairs of linked genes.
Use the information to show the positions of these genes in relation to each other on a chromosome diagram. Work out in which order the linked genes L, C, B and P would appear on the chromosome.
Answers are available here.
Meiosis
This key area focuses on a form of cell division that we haven't previously encountered - it is called meiosis.
Watch the video opposite and answer the questions in the task that follows.
Task 31
Now that you have watched the Meiosis video above, answer these questions in your notes.
Answers are available here.
Remember from your Higher studies...
The sperm mother cell will reproduce by mitosis (for standard growth and repair) but also by meiosis to produce gametes.
The processes of mitosis AND meiosis also take place in the ovary.
The Process of Meiosis
Before working through the nitty gritty of meiosis, I can't recommend the video opposite (by Hank Green and the Khan Academy) for setting the scene and taking you through the process of Meiosis fully. It is excellent and will make the rest of the information in this page a doddle!
Let's reflect back on what happens during mitosis (nuclear division).
As we discussed back in Topic 1, Key Area 5, mitosis occurs within the Mitotic phase of the cell cycle following interphase and successful completion of all checkpoints.
During mitosis, the nuclear division proceeds through Prophase, Metaphase, Anaphase and Telophase. Eventually the cytoplasm splits during a process called cytokinesis.
The individual stages that take place during meiosis are very similar to what you have already learned - but with some crucial differences, particularly in the way the chromosomes align along the metaphase plate. This is instrumental to the production of GENETICALLY DIFFERENT HAPLOID GAMETES.
Generating variation during meiosis
Step 1: Meiosis I
The steps of Meiosis I are shown in the following sequence. The opportunities for generating variation during Meiosis I are:
- pairing of homologous chromosomes
- random crossing over at chiasmata resulting in exchange of DNA between homologous pairs and recombination of alleles of linked genes.
- Independent assortment and separation of parental chromosomes irrespective of their material or paternal origin.
Homologous chromosomes are separated by spindle fibres and are pulled to opposite ends of the cell (note that CHROMATIDS are not separated at this point). The cell then divides to form 2 haploid daughter cells.
Step 2: Meiosis II
The key events that occur during Meiosis II include:
- Separation of sister chromatids/ chromosomes
- Gametes formation
Task 32
Having worked through the information above, now watch the animation by clicking the pink button below. I have also included a link to an excellent Nature Scitable article to support your understanding of this complex process. Note down that key opportunities for increasing genetic variation in gametes.
Answers are available here.
In the diagram below, you can compare and contrast mitosis and meiosis.
Notice pairing of homologous chromosomes and crossing-over at chiasmata at linked genes in the first box.
Then, note the difference in alignment of chromosomes along the metaphase plate and how this influences the separation of the chromosomes during anaphase.
Task 33
The number of possible combinations of gametes is worked out by calculating 2 (since there are 2 of each chromosome inherited - one from each parent) to the power of however many chromosome pairs there are.
Determine the number of possible combinations in human gametes.
Answer is available here.
Task 34
Meiosis forms variable gametes. A mosquito has six chromosomes.
What is its diploid number?
What is its haploid number?
How many different combinations are possible in the gametes of a mosquito?
Answers are available here.
Task 35: SQA Past Paper Question 1
Discuss the formation of variable gametes during meiosis under the following headings:
The activity of homologous chromosomes (7 marks)
Meiosis II (3 marks)
Marking instructions available here.
Test your understanding of mitosis and meiosis by reading the statements in the boxes below and deciding whether each applies to "mitosis" or "meiosis". Click the box to see if you were correct!
Click here to access a Quizlet on Meiosis.
You are now ready to move onto Topic 2, Key Area 3c on Sex Determination
