Part H: Lecture 19 Flashcards
sex chromosomes discovery
Nettie Stevens while studying beetle chromosomes
sex chromosomes examples
X
Y
Z
W
how did sex chromosomes get their names?
they were unknown (not their shape)
evolution of X and Y system
X and Y chromosome were identical before than Y lost most of its genes while X kept its genes
why do X an Y chromosomes pair during mitosis
they were homologous chromosomes millions of years ago
evolution of Z and W system
Two Z identical chromosomes then one Z became W after millions of years
Z is similar to ___ and W is similar to ___
-X
-Y
in some species(drosophila), length of X/Y chromosomes?
Y is longer than X
sex determination in drosophila system
sis gene on X chromosome
two sis+ genes = female
one sis+ gene = male
drosophila system is dependent on ____
X chromosome
one X = male
two X = female
meiosis in drosophila (female)
normal crossovers in prophase I
meiosis in drosophila (male)
no crossovers at all because no crossover possible between X and Y
Dosage compensation in Drosophila
increase X-linked gene expression in males (1 X) with positive TFs = MSL transcription factors
sex chromosomes in mammals
X- has lots of genes
Y- mostly no genes
X and Y have a pseudo-autosomal region with regular genes (homologous region)
meiosis in mammals (female)
normal crossovers in prophase I
meiosis in mammals (male)
slightly unusual
crossovers on autosomes and at pseudo-autosomal region on sex chromosomes
sex determination in mammals system
SRY gene on Y chromosome
SRY+ gene = male
no SRY+ gene =female
mammals system dependent on ____
Y chromosome
Y = male
no Y = female
mammals sex determination steps
1) gonads in embryo becomes ovaries (TF) or testes (TF & SRY+)
2)ovaries and testes synthesize sex hormones and do meiosis
3) sex hormones travel to other cells
4) sex hormones enter cells and bind to receptors
5) sex hormones activate transcription factors which turn on target gene
SRY gene at ___
Yp11.3
ovary sex hormone
estrogen
testis sex hormone
testosterone
sex hormone receptors in cell (male)
estrogen & testosterone
sex hormone receptors in cell (female)
estrogen & testosterone
sex hormone + receptors =
transcription factor
Dosage compensation in mammals
decrease X-linked gene expression in females
Dosage compensation in mammals steps
1) Count the X chr in human embryo at 2 weeks old (500-1000 cells)
2)Turn off all but one - permanent random decision (paternal or maternal X) - Xi is marked
Xa is ____
active X chromosome
Xi is ____
inactive chromosome
Xa decondenses after mitosis?
yes
Xi decondenses after mitosis?
no
Xa replicated during S phase?
yes
Xi replicated during S phase?
yes
Xa in interphase/metaphase
decondensed
condensed
Xi in interphase/metaphase
condensed
condensed
Xi in an interphase nucleus is called ___
Barr body (small condensed DNA dot)
dosage compensation is _____ from sex determination in mammals
independent of
dosage compensation occurs in which type of cell?
somatic cells
dosage compensation does not occurs in which type of cell? why?
germline cells; so the egg will always get an active X
