TBR bio, page 82 (2nd book)

[LoLCareerGoals]

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The mother of Family 2 has a haploid chromosome number of 23. How many chromatids are present in metaphase of the second meiotic division?
A. 23
B. 46
C. 69
D. 92

There is nothing in the passage to suggest abnormal meiosis in the mother.
The chapter describes chromatids as those pointy things you have 4 of in a tetrad or 2 of typical single chromosome.
Answer is not D, but B.
The book justifies B in that basically it implies "we mean per formed secondary gammete not total".
Am I supposed to be a mind reader or are there some supposed between the lines things I need to get used to understanding for the real MCAT?

 

[image187]

hmmm... not sure if there trying to trick you, but here's how I would solve the problem. Obviously haploid of 23 means 46 chromosomes. After meiosis I there will be 23 pairs of sister chromatids in each cell, but genetic material mostly from 1 parent for each pair so it's a haploid celll where n=23. During Meiosis II when these pairs line up in the middle of the cell for division (metaphase), there will still be 23 pairs in the cell, with 2 sister chromatids making up each. There should be a total of 46 chromatids.

Now, during meiosis I n=46 and there will be 23 tetrads with 92 chromatids

 

[LoLCareerGoals]

That's fine. Where in the question do they specify that they want only the local count of chromatids inside each of the post meiosis I cells? There were 92 in the beginning and there 92 at the end. All that's changed is the distribution into 2 and then 4 compartments. If the compartment is not specified is it unreasonable to assume they ask for a total?

 

[image187]

I think you should always assume that the question is about a single cell, unless an question specifies that it wants you to take into account additional cells or chromosomes. If the question is asking about meiosis II, you should take into account ONE cell undergoing meiosis II

 

[LoLCareerGoals]

Will do. Thanks for your input.

 

[RAAS]

You start out with 2 haploid gametes - one from mom, one from dad:
chromosome #: 23
chromatid #: 23

These gametes come together...now cell is diploid
chromosome #: 46
chromatid #: 46

Interphase - replication...still diploid
chromosome #: 46
chromatid #: 92 (tetrad)

Stays at that number until telophase I / cytokinesis when homologous chromosomes separate and 2 new haploid cells are formed. In each haploid cell:
chromosome #: 23
chromatid #: 46

How many chromatids are present in metaphase of the second meiotic division? So, at Metaphase II, chromatid # is 46

Stays at that number until telophase II /cytokinesis when chromatids separate and now 4 haploid gamete cells are formed:
chromosome #: 23
chromatid number: 23