any biologist in the house?? help w/ this quesion please

[avian777]

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A cell in G2 before meiosis compared with one of the four cells produced by that meiotic division has
a. twice as much dna and twice as many chromosomes
b. four times as much dna and twice as many chromosomes
c. four times as much dna and four times as many chromosmes
d. half as much dna but the same number of chromosomes
e. half as much dna and half as many chromosomes

with explanation please, thanks

 

[Erhatstil]

I think the answer is B. If we have a diploid cell (2n=46 chromosomes) there are 96 chomatids before the first meiotic division. After the first meiotic division, each cell nucleus now has 23 chromosomes and 46 chromatids. Then after the second division, the chromosome number is conserved, but the chromatids are split again, ending with 23 chromosomes and 23 chromatids in each daughter cell. i believe thats what they're looking for

 

[iwannabadentist]

the correct answer is B. Four times as much DNA, and twice as many chromosomes. To count the number of chromosomes, you must count the number of centromeres. 2 centromeres is 2 chromosomes, 4 centromeres is 4 chromosomes and so on. for the simplicity of the problem, assume that the organism has only 1 chromosome. so a diploid organism of this species will have one copy of the maternal chromosome, and 1 copy of the paternal chromosome. you MUST assume that this is a diploid organism, because haploid organisms do not undergo meiosis.

so, in G2, a cell has a duplicated copy of the maternal chromosome, and a duplicated copy of the paternal chromosome. however, the duplicated copies are joined together with 1 centromere, so it would be considered as 1 chromosome. but since its duplicated, each chromosome has 2 identical sister chromatids. so you have:
1 maternal chromosome with 2 sister chromatids (joined at centromere)
1 paternal chromosome with 2 sister chromatids (joined at centromere)

so at this point you have 2 chromosomes, but 4 chromatids. assume that 1 chromatid is equal to 1x DNA, so you have 4X DNA. after the first division of meiosis, the maternal and paternal chromosomes split, so each intermediate cell has either 1 maternal or 1 paternal chromosome but still 2 sister chromatids joined at the same centromere, its considered 1 chromosome. After the 2nd division of meiosis, the sister chromatids split, making 4 cells total - each cell getting 1 chromatid (2 cells from the cell that previously had the maternal chromosome, and 2 cells from the cell that previously had the paternal chromosome). Each haploid cell now has 1 chromatid (1x DNA) but since each chromatid has its own centromere, it is considered as a chromosome in itself. So the new cell has 1X DNA and 1 chromosome, while the old cell had 4X DNA and 2 chromosomes.

Hope that clears some things up!

 

[razblo]

the correct answer is B. Four times as much DNA, and twice as many chromosomes. To count the number of chromosomes, you must count the number of centromeres. 2 centromeres is 2 chromosomes, 4 centromeres is 4 chromosomes and so on. for the simplicity of the problem, assume that the organism has only 1 chromosome. so a diploid organism of this species will have one copy of the maternal chromosome, and 1 copy of the paternal chromosome. you MUST assume that this is a diploid organism, because haploid organisms do not undergo meiosis.

so, in G2, a cell has a duplicated copy of the maternal chromosome, and a duplicated copy of the paternal chromosome. however, the duplicated copies are joined together with 1 centromere, so it would be considered as 1 chromosome. but since its duplicated, each chromosome has 2 identical sister chromatids. so you have:
1 maternal chromosome with 2 sister chromatids (joined at centromere)
1 paternal chromosome with 2 sister chromatids (joined at centromere)

so at this point you have 2 chromosomes, but 4 chromatids. assume that 1 chromatid is equal to 1x DNA, so you have 4X DNA. after the first division of meiosis, the maternal and paternal chromosomes split, so each intermediate cell has either 1 maternal or 1 paternal chromosome but still 2 sister chromatids joined at the same centromere, its considered 1 chromosome. After the 2nd division of meiosis, the sister chromatids split, making 4 cells total - each cell getting 1 chromatid (2 cells from the cell that previously had the maternal chromosome, and 2 cells from the cell that previously had the paternal chromosome). Each haploid cell now has 1 chromatid (1x DNA) but since each chromatid has its own centromere, it is considered as a chromosome in itself. So the new cell has 1X DNA and 1 chromosome, while the old cell had 4X DNA and 2 chromosomes.

Hope that clears some things up!

Good summary, very helpful!

At first I was confused but then I re-read the question and it made sense 😛

 

[avian777]

thanks guys, feel much better now!

 

[Nasem]

Did you check the answer given to this problem? did it show B?

Cause I am still confused as to why its 4x the DNA and not just 2x as much,

I was thinking that if you initially started off with 2 chromosomes, one from dad other from mom, and after S phase, you will still have 2 chromosomes (cause they are attached to the centromere), but the question is asking in the form of "chromosomes" and not chromatids so I'll go with that.

So initially (before S phase) you had 2 homologous chromosomes, after S phase, the amount of everything duplicated.... So initially each chromosome had a double-helix DNA (total counting to 4 strands of DNA), after S phase, you have 8 strands of DNA where 4 are new and 4 are original.... meaning only 2x as much DNA

 

[UCB05]

Did you check the answer given to this problem? did it show B?

Cause I am still confused as to why its 4x the DNA and not just 2x as much,

I was thinking that if you initially started off with 2 chromosomes, one from dad other from mom, and after S phase, you will still have 2 chromosomes (cause they are attached to the centromere), but the question is asking in the form of "chromosomes" and not chromatids so I'll go with that.

So initially (before S phase) you had 2 homologous chromosomes, after S phase, the amount of everything duplicated.... So initially each chromosome had a double-helix DNA (total counting to 4 strands of DNA), after S phase, you have 8 strands of DNA where 4 are new and 4 are original.... meaning only 2x as much DNA

stem cell: 2n, 46 chromosomes
pre-meiosis 1: 2n, 2 copies of 46 chromosomes
end of meiosis 1: 1n, 2 copies of 23 chromosomes
end of meiosis 2: 1n, 1 copy of 23 chromosomes
so compared to a spermatid/ovum, which has only 23 chromosomes, the cell 2 meiotic divisions ago had twice as many chromosomes and twice again as much DNA per chromosome (4x total DNA).