EXAM HELP ASAP!!!

[ggxoxo]

Just a couple of qns:

1. Are birds mammals? (more specifically, are 'emu-like birds' (I quote an exam) mammals-they should really tell you though!)

2.The first hominid to leave Africa is thought to have been:

A.Australopithecus africansus 4 mya
B.Homo habilis approx 3.2mya
C.Homo ergater approx 1.8 mya
D.early Homo sapiens approx. 100,000 ya

3.Durning mitosis, the phase in which cells have the greatest number of chromosomes at any one time includes:

A. Early prophase
B. Late prophase
C. Early metaphase
D. Early anaphase

[Reckoner]

1. No. emu-like birds I think would just mean flightless birds perhaps?
2.C
3.D anaphase is when the chromatids separate, which doubles the chromosomes as when the chromatids are together they are seen as one chromosome still. 

[Ravit]

yup ^

[ggxoxo]

thanks guys!

[Zebra]

WHY D for question 3? shouldn't it be prophase??

[xXPersonXx]

WHY D for question 3? shouldn't it be prophase??

At late anaphase, the chromosome split up and the centromere breaks. The fact that we can refer chromatids as chromosomes and that the cell has not fully split means at that moment, there is double the diploid number of chromsomes.
So for humans, at late anaphase, we technically have 92 chromosomes (92 chromatids)

[Reckoner]

Exactly ^

[Zebra]

But the chromosomes are replicated in interphase before prophase I so it would have 92 chromosomes anyway?

[Reckoner]

It will have 92 chromatids. When the new "chromosome" is replicated and still joined to the original, it itself is a chromatid while the chromosome is both the new chromatid and the original.
http://en.wikipedia.org/wiki/Chromatid
The image on the right is 1 chromosome made up of 2 chromatids joined at the centromere.

Hope this helps

[Zebra]

sorry can you try again...

say there are 46 chromosomes initially. during interphase these would be replicated and there wud be 92 chromosomes.
how is different to anaphase 1 when there are 92 chromosomes?

[Reckoner]

So say 2n=46. During the S phase in interphase, the chromosomes each form another strand. This new strand is a chromatid. This means that each chromosome looks something like an 'X' with / and \ being chromatids. During G2, prophase... the cell is still diploid (2n), except each of the chromosomes are double stranded (X). When it reaches anaphase, this is when the chromatids separate (/<---X--->\ ). These now individual chromatids are seen as a new chromosome, meaning that after/during anaphase there are 92 chromosomes.

Put more simply,
G1: 46 chromosomes (each single stranded /)
S: 46 chromosomes that are double stranded (X). Each strand is a chromatid, therefore 92 chromatids.
G2:46 chromosomes that are double stranded (X)
prophase:46 chromosomes that are double stranded (X)
Metaphase:46 chromosomes that are double stranded (X) Still 92 chromatids
Anaphase: 92 chromosomes that are single stranded (/)

I'm not sure if I can explain it any better sorry. 

[Zebra]

wait, i think the confusion comes from the terminologies here..?

chromatids
chromosomes

:S wow...

okay i think i kinda get it now!
but why is it that I always thought we start with 46 chromosomes (each double stranded x)?
Isn't this the case? WE HAVE BOTH PATERNAL AND MATERNAL CHROMOSOMES!!?
SO WE SHOULD HAVE 46 DOUBLE STRANDED CHROMOSOMES TO START WITH...

[Reckoner]

We all have 46 chromosomes, but 23 homologous pairs (1 from each parent). These pairs are not joined. Look up some pictures of karyotypes and you will see that we have 46 chromosomes, but they will be shown in those pairs. Chromosomes are only double stranded after the S phase, which is when they are 2 chromatids. 

And I just realised that I've been saying "single stranded." By this I just mean 1 DNA molecule, which in itself is double stranded. I think that made me seem unclear.

[Zebra]

is a tetrad during prophase 1 seen as a single chromosome?

[Zebra]

http://www.youtube.com/watch?v=Z6DqjSFIO3U

around 4:30 you will see a diagram of meiosis I
but if you count the number of chromosomes.. they are the same in prophase 1 and anaphase 1.. right?