List of chromosomes » Chromosomes xy

Male or female… all about an angle

XX or XY

As a rule, in mammals, what distinguishes males and females is the existence of two chromosomes: chromosome X and chromosome Y. In humans, women carry the XX pair in each of their cells, while men carry the XY pair.


ZZ or ZW?

In birds, the male or female gender is defined in exactly the opposite way! Females carry the ZW pair while males carry the ZZ pair. Chromosome W is responsible for female differentiation!



A protein which makes a difference

What part of the Y chromosome is able to make a little boy out of a fertilised egg? SRY: an amazing gene. The production of SRY protein during the very early stages of embryogenesis is enough to trigger off the programme which will eventually form the little boy’s testicles – a characteristic male trait.

Confused gender

What does SRY protein do? It binds to DNA and twists it into an angle of 70° to 80°. This angle is important for what follows in the developing embryo. If protein SRY is modified and thus unable to bind to DNA – or unable to twist the DNA into the correct angle – then what should have been a little boy…becomes a little girl. And this particular little girl will indeed have ovaries, despite the fact that each of her cells will carry the XY chromosome pair…

Slide 3 – SF



A molecular set square

Is there any scientific instrument capable of measuring the angle produced by the SRY protein when it binds to DNA? No. In fact, to date, there is no scientific instrument capable of ‘seeing’ the structure of one protein. So, in order to understand the structure of the SRY protein when it binds to DNA and to measure the angle it forms, scientists have to use indirect experimental approaches carried out in the laboratory. These approaches locate the position of every single atom that makes up the protein, of which there are thousands! Then, by way of high-performance computer programs, scientists are able to visualise the 3D structure of a protein.

Slide 2 – SF

Protein SRY (in red) binds to DNA (in grey) and folds it to form a 70° to 80° angle.