Chromosomes and Cell Division

Chromosomes Your somatic cells (all excluding egg and sperm cells) contain 46 chromosomes Chromosomes contain the genetic instructions for the structure & function of your body

Chromosomes Autosomes – these are the 44 chromosomes containing similar types of information in cells of both males & females Sex chromosomes – these are 2 of the 46 chromosomes that look like an XX for females and XY for males

Microtubules Microtubules form the Spindle Apparatus which is the “machinery” responsible for moving chromosomes during cell division

Note the pink spindle apparatus moving the chromosomes

Chromosome numbers Diploid = 2n 23 chromosomes that you received from your mother plus 23 chromosomes you received from your father 2 sets of chromosomes Seen in somatic cells (body cells other than egg and sperm)

Homologous Chromosomes Similar chromosomes Contains the same TYPE of information E.g. chromosome number 1 you received from mom and chromosome number 1 you received from dad.

This photo shows homologous chromosome pairs

MITOSIS Cell division in cells that are NOT your sex cells Allows growth and repair 2n >>>>2n

Interphase – Chromosome replication

Interphase Exact copies are made of the chromosomes Exact copies are termed sister chromatids Sister chromatids are joined in the middle at the centromere

Mitosis - Prophase Nuclear envelope breaks down Spindle apparatus forms Chromosomes thicken

Mitosis - Metaphase The microtubules of the spindle apparatus attach to the centromere of sister chromatids Sister chromatids line up along the cell equator

Mitosis - Anaphase Sister chromatids move to opposite poles of the cell

Mitosis - Telophase Sister chromatids have arrived at opposite poles of the cell Nuclear envelope reforms around each set of chromosomes Cytoplasm cleaves yielding 2 identical “daughter” cells

Meiosis This cell division occurs in the gonads (ovaries and testes) Resulting cells are the egg and sperm each with 23 chromosomes Chromosome numbers are cut in half from 46 to 23 Diploid (2n) to Haploid (n)

Interphase Interphase in meiosis is just as it was in mitosis Chromosomes are copied

Meiosis – Prophase I Nuclear envelope breaks down Chromosomes thicken Spindle apparatus forms Homologous chromosomes pair up (this does NOT happen in mitosis) Crossing over occurs randomly

Meiosis – Metaphase I Homologous pairs line up along the equator Lining up is random resulting in more variation

Meiosis – Anaphase I Homologous pairs split apart

Meiosis – Telophase I Chromosomes have arrive at opposite poles New nuclear envelope reforms 2 new cells are NOT identical and are still diploid

Meiosis - Prophase II Now, onto Prophase II without an interphase stage Chromosomes thicken Nuclear envelope breaks down Spindle apparatus forms

Meiosis – Metaphase II Single chromosomes line up on the equator

Meiosis – Anaphase II Chromosomes split apart But, chromosomes are NOT identical

Meiosis – Telophase II Chromosomes have arrived at opposite poles Nuclear envelope reforms 4 resulting haploid cells Each of the four cells is genetically different

Chromosomal inheritance Sex determination XX is female XY is male

Downs Syndrome Mistake during meiosis Commonly due to an extra chromosome # 21