Human cells contain either 46 chromosomes (somatic cells) or 23 chromosomes (gamete cells). Somatic cells are formed through mitosis and contain homologous chromosome pairs, while gamete cells are formed through meiosis which reduces the number of chromosomes by half. Meiosis involves the production of haploid gametes through nuclear division from a parent cell containing 46 chromosomes.

2. 2 Types of Cells
o “Body cells” (somatic) vs. reproductive
cells (gametes).
o Examples of somatic cells: skin cell, liver
cell, brain cell, red blood cell…
o Human somatic cells - 46 chromosomes.
o Examples of gametes: egg cell or sperm
cell
o Human gametes - 23 chromosomes.

3. How much DNA is in our cells?
o DNAchromosomes
o How many chromosomes in a human
somatic cell?
o 46 chromosomes or 23 homologous pairs
o What is the origin of these pairs?
o Homologous pair- chromosomes that
carry the same genes
o These cells are diploid.
o The diploid number in humans is 46.

5. How much DNA is in our cells?
o How many chromosomes are in a gamete
(reproductive cell)?
o 23 chromosomes
o Mom and dad pass on one chromosome
from each homologous pair.
o These cells are haploid.
o The haploid number in humans is 23.

7. How are reproductive cells formed?
o Meiosis is nuclear division that produces
haploid gametes.
o Chromosomes - no longer in pairs.
o Each parent passes on one chromosome from
each homologous pair in the gamete.
o Reproductive cells contain half the number of
chromosomes that a somatic cell contains.
o 4623

9. How is Mitosis different from
Meiosis?
MITOSIS MEIOSIS
Chromosome # Chromosome #
stays same reduced half
2 new cells produced 4 new cells produced
Somatic cells Gametes
Identical Variation

10. What are reproductive cells?
o In males reproductive cells originate
from cells called spermatogonium.
o In females reproductive cells (egg cells)
originate from cells called oogonium.

11. Spermatogonium
46
46 46
Sperm
23 23 23 23

12. 46 Oogonium
Polar Body
46 46
Egg
23 23 23
23

13. Meiosis Errors
o Errors during meiosis cause gametes to be
defective.
o Most common error is called
nondisjunction.
o When nondisjunction occurs, chromosomes
fail to separate correctly.
o Creates gametes (egg/sperm cells) with
too many (trisomy) or too few
chromosomes (monosomy).