Sister chromatids, Komodo dragons and Mouse lifestyle

First up, I think the spermatogenesis figure confusion was caused by counting sister chromatids rather than chromosomes. I think the figure to the left is similar to the figure on the spermatogenesis slide.

In this figure the parent cell has two homologous pairs of chromosomes (2n=4). This is shown in the diagram with two short lines and two longer lines. Each homologous pair is not identical because one was obtained from the mother and one from the father.

During the very first stage of meiosis the chromosomes replicate - each producing genetically identical sister chromatids that remain attached together at the centromere. These are not counted as separate chromosomes. So the chromosome number of the cell as it enters meiosis (second line) is still 2n=4. It is only during the second meiotic division, the last line on the figure, that they finally are separated and distributed into separate cells. However as soon as the joined chromatids are separated they are no longer called sisters because they are no longer connected to each other. Instead they are now called unreplicated chromosomes.

Next, virgin birth in Komodo dragons. Here's an interesting paper from Nature that answers most scientific questions on the phenomenon, and here are some press reports.

Speaking of hormones, which I think we were, Friday's Psychology Neuroscience & Behavior colloquium series talk will be by Dr. Laurence Tecott (UCSF) entitled
"BRAIN SEROTONIN SYSTEM REGULATION OF ENERGY BALANCE AND "LIFESTYLE" IN THE MOUSE"
The seminar will be held in Psychology 1523 on Friday April 17th at 4pm.