Genetics Terminology

• Genome: all the genetic information of an organism

  • contained in the nucleus

  • it’s hereditary material and can be passed on

• Genes: specific portions of the genome that code for proteins

• Chromosome: thread-like structure containing a collection of genes

  • humans have 46 chromosomes in 23 pairs

    • 1 set from mother, 1 set from father

Summary of Mitosis

• Interphase occurs prior to mitosis and has three parts

  • G1 phase: cellular contents are duplicated

  • S phase: each of the 46 chromosomes are duplicated by the cell

  • G2 phase: proofreading duplicated chromosomes for any errors

• Prophase

  • Chromosomes condense

  • Mitotic spindle forms

  • Nuclear envelope breaks

• Metaphase

  • Spindle attached to chromosomes

  • Metaphase plate

• Anaphase

  • Chromosomes separate

  • Cell elongates

• Telophase and cytokinesis

  • The mitotic spindle is broken down

  • Two nuclei form

  • Chromosomes decondense

  • The cell splits into two

• Result: 2 genetically identical diploid daughter cells

Summary of Meiosis

Meiosis I

• Prophase I

  • homologous chromosomes condense and pair up into tetrads

  • crossing over occurs

• Metaphase I

  • tetrads line up on the equator of the cell

• Anaphase I

  • separation of homologous chromosomes

• Telophase I & cytokinesis

  • Same as mitosis

Meiosis II

• Prophase II

  • chromosomes condense

• Metaphase I

  • chromosomes line up on the equator of the cell

• Anaphase I

  • separation of sister chromatids

• Telophase I & cytokinesis

  • Same as mitosis

Result of Meiosis

4 genetically different haploid daughter cells

Spermatogenesis

• #n#c refers to number of chromosomes and number of chromatids in that order

  • 2n is diploid

  • 1n is haploid

1. First cells are Spermatogonia: 2n2c germ cells

2. Spermatogonia divide into Primary Spermatocytes (2n4c)

   1. they become 2n4c via DNA replication

3. Primary spermatocytes divide into Secondary Spermatocytes

   1. they become 1n2c via meiosis I

4. Secondary spermatocytes divide into Spermatids

   1. they become 1n1c via mitosis II

   2. spermatids are immature sperm cells that are nonmotile

5. Spermatids become spermatozoa (mature sperm cells) by spermiogenesis, which includes

   1. acrosome formation

      1. acrosome: cap-like structure in sperm cells that has degradative enzymes to go through the egg’s cell membrane

   2. loss of cytoplasmic content

   3. condensation of genetic material

   4. flagella development

Oogenesis

1. At 4 weeks of gestation, the oogonia (2n2c) start to replicate

2. From 4th week of gestation to 4 weeks after birth, the oogonia have formed into primary oocytes (2n4c)

   1. primary oocytes have started meiosis I

3. Primary oocytes are arrested in prophase I until ovulation happens

4. During ovulation, the primary oocytes finish meiosis I and form secondary oocytes (1n2c)

5. Secondary oocytes are arrested in metaphase II until fertilization occurs

6. If fertilization doesn’t occur, the secondary oocyte degenerates.

7. If fertilization does occur, the secondary oocyte becomes an ootid that then becomes a mature ovum (1n1c)

Comparing and Contrasting Spermatogenesis & Oogenesis

• Oogenesis has 2 stages of arrest

  • Primary oocyte gets arrested at prophase I at birth

  • Secondary oocyte gets arrested at metaphase II until fertilization occurs

Clinical Correlation: Down Syndrome

• Most common liveborn chromosomal abnormality and form of intellectual disability

• Key features of Down Syndrome include:

  • Dysmorphic face

    • flat face

    • flat nasal bridge

    • low set ears

    • short neck

    • Epicanthal folds: skin folds of the upper eyelid covering the inner corner of the eye

    • Upslanting palpebral fissures: the imaginary line that goes through the eye is slanted up

  • Transverse palmar creases: single line going down the hand as opposed to 2

  • Intellectual disability

  • Increased risk of congenital heart disease

Down Syndrome Genetics

• Down syndrome is caused by trisomy 21: 3 copies of chromosome 21

  • Trisomy 21 is caused by meiotic nondisjunction: 2 homologous chromosomes/sister chromatids ending up in the same pole

• The extra chromosome is more likely to come from the mother than the father

  • the egg is stuck in meiosis I until ovulation

    • increased chance that things can go wrong

  • this is why down syndrome risk increases as mother’s age increases

• Down syndrome can also occur because of mitotic errors

  • mosaicism: some cells will have a normal chromosome count while other will have trisomy 21