ANAT2341 Lab 1 - Sex Determination

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ANAT2341 Lab 1: Introduction | Gametogenesis | Oogenesis | Spermatogenesis | Fertilization | Sex Determination | Online Assessment | ANAT2341 Lab 1 - Quiz

Introduction

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Human idiogram

Mammalian sex determination is regulated by chromosomes.

  • Females have two X chromosomes. (XX)
  • Males have a single X and a small Y. (XY)
  • The X and Y chromosome are morphologically and functionally different from each other.
  • Evolutionary studies have shown that the Y was once the homologous pair for X.
  • It is only in the last 5 years that we have some idea about how these two types of chromosomes may be regulated and genes of importance located upon them.

X chromosome

Human idiogram-chromosome X.jpg

In females - the main scientific problem was understanding gene dosage, only one copy of X chromosome is needed to be genetically active the other copy is inactivated (More? X Inactivation.

About the X Chromosome

  • 155 million base pairs
  • In contrast to the Y chromosome, the X chromosome contains about 5% of the haploid genome and encodes house-keeping and specialized functions.
  • Genes such as Wnt-4 and DAX-1 necessary for initiation of female pathway ovary development
  • An early discovery (1961) was that in order to have correct levels of X chromosome gene/protein expression (gene dosage), females must "inactivate" a single copy of the X chromosome in each and every cell. The initiator of the X inactivation process was discovered (1991) to be regulated by a region on the inactivating X chromosome encoding an X inactive specific transcript (XIST), that acts as RNA and does not encode a protein.
  • The genetic content of the X chromosome has been strongly conserved between species because these genes have become adapted to working as a single dose - Ohno's law
  • X inactivation occurs randomly throughout the embryo, generating a mosaic of maternal and paternally derived X chromosome activity in all tissues and organs. This can be seen in the fur colour of tortoiseshell cats.

Y chromosome

Human idiogram-chromosome Y.jpg

In males - the main scientific problem was understanding what on the Y chromosome determined "maleness", and how this is done.

About the Y Chromosome

  • 59 million base pairs, hypervariable in length, mostly non-functional repeats
  • Current known protein-coding genes = 48 including SRY
  • SRY encodes a 204 amino acid protein that is a member of the HMG (High mobility group) box class of DNA-binding proteins. Transcription factors bind to specific sites of DNA and regulates the transcription (expression) of other genes.

Male (XY)

Human idiogram-chromosome Y.jpg

Sry was discovered (1990) by studying a human XY female, resulting from a deletion in the Y chromosome that did not allow testis development. Subsequent mapping of this deletion allowed isolation and characterization of the SRY gene.

There is a suggestion that SRY may allow testes development by acting to inhibit DAX1, which is expressed in the indifferent gonad at the same time. The mechanism of an inhibitor inhibiting and inhibitor is seen in some other developing systems.

  • encodes a 204 amino acid protein (Mr 23884 Da) that is a zinc-finger transcription factor.
  • transcription factors bind to specific sites of DNA and regulates the transcription (expression) of other genes, we still do not know all the genes SRY regulates.
  • expressed when testes begin to form, in gonadal tissue and does not require the presence of germ cells.

Notes: The Y chromosome is much smaller than the X chromosome and by definition, cannot contain important genes for other cellular functions.

Nomenclature, capital letters are used for human genes (SRY) and lower case letters are used for the equivalent genes in other species (sry).

Links: Sex reversal in humans caused by abnormal X-Y exchange | Fig. 2 - image

Female (XX)

Human idiogram-chromosome X.jpg
  • In contrast to the Y chromosome, the X chromosome contains about 5% of the haploid genome and encodes house-keeping and specialized functions.
  • The genetic content of the X chromosome has been strongly conserved between species.
Macaque Xi at interphase 02.jpg

Macaque Xi at interphase

1961 - In order to have correct levels of X chromosome gene/protein expression (gene dosage).
  • females must "inactivate" a single copy of the X chromosome (Xi) in each and every cell.
  • this inactivated chromosome can be identified in female blood cells as the "Barr body".
Model for XIST RNA spread from X inactivation center.jpg

Model for XIST RNA spread from X inactivation center

1991 - The initiator of the X inactivation process was discovered.
  • regulated by a region on the inactivating X chromosome encoding an X inactive specific transcript (XIST).
  • acts as RNA and does not encode a protein.
  • Furthermore X inactivation occurs randomly throughout the embryo.
  • generating a mosaic of maternal and paternally derived X chromosome activity in all tissues and organs.


Links: Signaling in genital development | Fig. 1 - image | X Chromosome Inactivation - Epigenetics 1
ANAT2341 Lab 1: Introduction | Gametogenesis | Oogenesis | Spermatogenesis | Fertilization | Sex Determination | Online Assessment | ANAT2341 Lab 1 - Quiz


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Cite this page: Hill, M.A. (2019, October 23) Embryology ANAT2341 Lab 1 - Sex Determination. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/ANAT2341_Lab_1_-_Sex_Determination

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G