Short version:
Please comment your email address, I'll send you my explanation word doc notes regarding meiosis etc then you can edit it/let me know if any of it is incorrect/clarify it!
Long version:
I did biology last year and got a hsc mark of 92. I think understanding meiosis, independant assortment and crossing over is one of, if not the most challenging part of the whole biology course. I now tutor a couple of subjects including biology. This is one of the few topics which i still find considerably challenging to clearly explain.
If there's any biology guns out there I'd love you to read my explanation of the process of meiosis and the occurrence of independent assortment and crossing over and to correct/comment on any errors or incorrect definitions or any areas you can clarify.\
I wrote it as a sort of "note to self" during my hsc study as i found it hard to track down a comprehensive and clear explanation of the process. I believe it to be correct but if there are any bio wonderkids then i'd love to be corrected if need be.
Feel free to use the attached notes yourself but do your own research too as i may be incorrect in parts.
--------------------------
The file was too big to attach to this post due to the images, comment your email address and i'll send it. It is wayyy better and clearer with the images so please do comment with your email address, but anyway... The text only version is:
------------------------
- Consider a diploid cell with two pairs of homologous chromosomes. Y and y are homologous chromosomes in that they both code for hair colour. S and s are homologous as they both code for muscle build.
{The S’s could code for Strong (S) and weak (s) muscle build respectively. The Y’s could code for Yellow (Y) and brown (y) hair colour respectively. Let Y and S be dominant, as in this case they are labelled as capitals, so it makes more sense, not that dominance is essential for this explanation anyway.}
These homologous chromosomes have similar genes but different alleles and hence are usually the same length - the exception being the sex chromosomes, X and Y, which would have different genes: the X is longer, hence the concept regarding sex-linked disease, where the Y provides no “back-up” healthy chromosome to be dominant and make the organism healthy, if still a carrier.
2)
- Before meiosis begins, cells synthesise DNA and thereby replicate each chromosome, creating sister chromatids, bound at the centre by a centromere. Consider this to “pinch” the two sister chromatids together to form a chromosome which can later split into two identical chromosomes. I.e. the two yellow chromatids are pinched together to form the Y chromosome, which can later split to form two Y chromosomes.
3)
Early in meiosis, the homologous pairs (each member of which now consists of two pinched chromatids but is still considered one chromosome) associate. This means that the homologous pairs form/become obvious as the chromosomes that are homologous (code for the same thing but with different alleles from each parent) come close together.
4)
-During metaphase I, the homologous pairs line up at the mid-plane of the cell.
5)
(same photo)
-Having moved to align horizontally across the mid-plane, the pairs do not always line up in this orientation.
-They always line up so that the homologous pairs are together (above and below) as the pair s + S and the pair Y + y are, but for example the s of the muscle build pair could align (assort) so that it is above the S as shown, but equally it could align so it is below the S chromosome. The “top” chromosomes are about to be pulled off to be two daughter cells, and the “bottom” chromosomes are about to be pulled off to be in two others, so this alignment affects the alleles present in each of the four resulting gametes.
- Since the homologous pairs stay associated (together, above and below) as they line up (like s and S), there is hence always one chromosome from each homologous pair pulled in one direction, and one in the other. However the combination could be, from our example, s + y in one direction and S + Y in the other, OR it could be S + y in one direction and s + Y in the other. These alternative alignments are said to be due to independent assortment. This law states that allele pairs (homologous pairs) separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.
[Remember that the two cells below are just showing the two possible outcomes, only one of these would happen in a meiotic division. I.e. if you had to draw it, there would currently only be one cell in the drawing. Also, as a side note, about 50% of a sample space of meiotic divisions takes on alignment one, and about 50% the other alignment, giving a fair opportunity for the various genotypes in gametes and hence offspring). ]
-Another way of putting it is that the s and Y could be assorted in a way that means they end up in the same daughter cells (like alignment one: both to be pulled up), or they could be aligned so that they end up in different daughter cells (like alignment two: one to be pulled up, one down). We can say, “The alleles of the muscle build and the hair genes have been sorted independently in meiosis.”
Also, at this point Crossing over can occur. I.e. the homologous chromosomes for muscle build may end up being two chromosomes with one s and one S chromatid each – see 2012 HSC question.
6)
-The spindle apparatus now pulls each chromosome of the homologous pairs away from each other
-This forms two cells with the half the amount of chromosomes, but of course with each chromosome consisting of two sister chromatids still joined at the centrosome. This is so they are ready to be pulled apart into four gametes in total in the next step.
1 ---> 2
3 4
5
7)
-Each sister chromatid is now labelled as its own chromosome since it is about to be pulled apart to be a chromosome for the resulting four gametes. (Eight for us in our diagrams, since we are looking at the two possible outcomes from the different alignments in independent assortment.)
1 -> 2
-The chromosomes consisting of the sister chromatids line up vertically down the middle, ready to be pulled sideways to give one chromosome (single chromatid is now called a chromosome) coding for each gene to each gamete. They are then pulled sideways as mentioned.
3 -> 4
-This new arrangement can be drawn so that is clear where the gametes will form and so it can be seen what their complement of chromosomes will be.
5 -> 6
7 -> 8
- The four resulting gametes are formed. (Eight since we are looking at both possible alignments at the assortment stage).
- There are two gametes for each of the possible genotypes, as is shown by the highlighted two S/Y genotype gametes. The two gametes of each other genotype are labelled. In one meiotic division two genotypes would result, with two others possible from different independent assortment (different alignment earlier).
-Instead of going through the whole process, if given a cell that is going to form gametes for reproduction, you can use the foil method to work out the possible combinations of alleles the gametes could have (in this case four combinations as there are two genes. We know there are two genes as there are two letters present. i.e. A and R. The alleles in the original cell are are R r and B b (they are hard to see in the diagram).
-Note that b,B and R,r respectively are not counted in the possible outcomes. There will never be two alleles for one gene/characteristic in one gamete as there needs to be two for the homologous pairs of the zygote/ new organism after fertilisation, not three or more of the same gene! …
…This is the Law of segregation. This principle states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly (in that any gamete could be involved) united at fertilization.
[There is one allele from the mother in the egg and one allele from the father in the sperm that unite as a homologous chromosome in the formation of a zygote after fertilisation. Then dominance becomes significant as the father’s brown haired allele may be dominant over the mother’s blonde allele in the homologous pair of the child’s chromosomes.]
Ipmatc
Pmatc
Interphase- not very "I"nteresting
Prophase- chromosomes first a"P"pear
Metaphase- chromosomes line up in the "M"iddle. Centrosomes at opposite poles
Anaphase- chromosomes move "A"way
Telophase- now you have "T"wo
Cytokinesis – the cells are now completely separate etc.
Please comment your email address, I'll send you my explanation word doc notes regarding meiosis etc then you can edit it/let me know if any of it is incorrect/clarify it!
Long version:
I did biology last year and got a hsc mark of 92. I think understanding meiosis, independant assortment and crossing over is one of, if not the most challenging part of the whole biology course. I now tutor a couple of subjects including biology. This is one of the few topics which i still find considerably challenging to clearly explain.
If there's any biology guns out there I'd love you to read my explanation of the process of meiosis and the occurrence of independent assortment and crossing over and to correct/comment on any errors or incorrect definitions or any areas you can clarify.\
I wrote it as a sort of "note to self" during my hsc study as i found it hard to track down a comprehensive and clear explanation of the process. I believe it to be correct but if there are any bio wonderkids then i'd love to be corrected if need be.
Feel free to use the attached notes yourself but do your own research too as i may be incorrect in parts.
--------------------------
The file was too big to attach to this post due to the images, comment your email address and i'll send it. It is wayyy better and clearer with the images so please do comment with your email address, but anyway... The text only version is:
------------------------
- Consider a diploid cell with two pairs of homologous chromosomes. Y and y are homologous chromosomes in that they both code for hair colour. S and s are homologous as they both code for muscle build.
{The S’s could code for Strong (S) and weak (s) muscle build respectively. The Y’s could code for Yellow (Y) and brown (y) hair colour respectively. Let Y and S be dominant, as in this case they are labelled as capitals, so it makes more sense, not that dominance is essential for this explanation anyway.}
These homologous chromosomes have similar genes but different alleles and hence are usually the same length - the exception being the sex chromosomes, X and Y, which would have different genes: the X is longer, hence the concept regarding sex-linked disease, where the Y provides no “back-up” healthy chromosome to be dominant and make the organism healthy, if still a carrier.
2)
- Before meiosis begins, cells synthesise DNA and thereby replicate each chromosome, creating sister chromatids, bound at the centre by a centromere. Consider this to “pinch” the two sister chromatids together to form a chromosome which can later split into two identical chromosomes. I.e. the two yellow chromatids are pinched together to form the Y chromosome, which can later split to form two Y chromosomes.
3)
Early in meiosis, the homologous pairs (each member of which now consists of two pinched chromatids but is still considered one chromosome) associate. This means that the homologous pairs form/become obvious as the chromosomes that are homologous (code for the same thing but with different alleles from each parent) come close together.
4)
-During metaphase I, the homologous pairs line up at the mid-plane of the cell.
5)
(same photo)
-Having moved to align horizontally across the mid-plane, the pairs do not always line up in this orientation.
-They always line up so that the homologous pairs are together (above and below) as the pair s + S and the pair Y + y are, but for example the s of the muscle build pair could align (assort) so that it is above the S as shown, but equally it could align so it is below the S chromosome. The “top” chromosomes are about to be pulled off to be two daughter cells, and the “bottom” chromosomes are about to be pulled off to be in two others, so this alignment affects the alleles present in each of the four resulting gametes.
- Since the homologous pairs stay associated (together, above and below) as they line up (like s and S), there is hence always one chromosome from each homologous pair pulled in one direction, and one in the other. However the combination could be, from our example, s + y in one direction and S + Y in the other, OR it could be S + y in one direction and s + Y in the other. These alternative alignments are said to be due to independent assortment. This law states that allele pairs (homologous pairs) separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.
[Remember that the two cells below are just showing the two possible outcomes, only one of these would happen in a meiotic division. I.e. if you had to draw it, there would currently only be one cell in the drawing. Also, as a side note, about 50% of a sample space of meiotic divisions takes on alignment one, and about 50% the other alignment, giving a fair opportunity for the various genotypes in gametes and hence offspring). ]
-Another way of putting it is that the s and Y could be assorted in a way that means they end up in the same daughter cells (like alignment one: both to be pulled up), or they could be aligned so that they end up in different daughter cells (like alignment two: one to be pulled up, one down). We can say, “The alleles of the muscle build and the hair genes have been sorted independently in meiosis.”
Also, at this point Crossing over can occur. I.e. the homologous chromosomes for muscle build may end up being two chromosomes with one s and one S chromatid each – see 2012 HSC question.
6)
-The spindle apparatus now pulls each chromosome of the homologous pairs away from each other
-This forms two cells with the half the amount of chromosomes, but of course with each chromosome consisting of two sister chromatids still joined at the centrosome. This is so they are ready to be pulled apart into four gametes in total in the next step.
1 ---> 2
3 4
5
7)
-Each sister chromatid is now labelled as its own chromosome since it is about to be pulled apart to be a chromosome for the resulting four gametes. (Eight for us in our diagrams, since we are looking at the two possible outcomes from the different alignments in independent assortment.)
1 -> 2
-The chromosomes consisting of the sister chromatids line up vertically down the middle, ready to be pulled sideways to give one chromosome (single chromatid is now called a chromosome) coding for each gene to each gamete. They are then pulled sideways as mentioned.
3 -> 4
-This new arrangement can be drawn so that is clear where the gametes will form and so it can be seen what their complement of chromosomes will be.
5 -> 6
7 -> 8
- The four resulting gametes are formed. (Eight since we are looking at both possible alignments at the assortment stage).
- There are two gametes for each of the possible genotypes, as is shown by the highlighted two S/Y genotype gametes. The two gametes of each other genotype are labelled. In one meiotic division two genotypes would result, with two others possible from different independent assortment (different alignment earlier).
-Instead of going through the whole process, if given a cell that is going to form gametes for reproduction, you can use the foil method to work out the possible combinations of alleles the gametes could have (in this case four combinations as there are two genes. We know there are two genes as there are two letters present. i.e. A and R. The alleles in the original cell are are R r and B b (they are hard to see in the diagram).
-Note that b,B and R,r respectively are not counted in the possible outcomes. There will never be two alleles for one gene/characteristic in one gamete as there needs to be two for the homologous pairs of the zygote/ new organism after fertilisation, not three or more of the same gene! …
…This is the Law of segregation. This principle states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly (in that any gamete could be involved) united at fertilization.
[There is one allele from the mother in the egg and one allele from the father in the sperm that unite as a homologous chromosome in the formation of a zygote after fertilisation. Then dominance becomes significant as the father’s brown haired allele may be dominant over the mother’s blonde allele in the homologous pair of the child’s chromosomes.]
Ipmatc
Pmatc
Interphase- not very "I"nteresting
Prophase- chromosomes first a"P"pear
Metaphase- chromosomes line up in the "M"iddle. Centrosomes at opposite poles
Anaphase- chromosomes move "A"way
Telophase- now you have "T"wo
Cytokinesis – the cells are now completely separate etc.
