Meiosis is the process of cell division that produces gametes (sperm and egg cells) for sexual reproduction. It involves two cell divisions that result in four haploid cells, each with half the number of chromosomes as the original cell. This ensures that offspring have a full set of chromosomes when fertilization occurs. The main features of meiosis include homologous chromosome pairing, crossing over to increase genetic variation, and independent assortment of chromosomes into daughter cells through the first and second meiotic divisions. Errors in meiosis can result in gametes with an abnormal number of chromosomes, causing genetic disorders.

1. BIOLOGY
Unit 4 Notes:
Meiosis

2. (1) Reproduction
• Types of Reproduction:
– Asexual
– Sexual
• Sexual Reproduction: Two parent organisms exchange
genetic information to produce genetically different
offspring.
• Asexual Reproduction: A single celled organism which
makes an identical copy of itself (becoming the
offspring).

5. (2) Reproduction & Cell Division
• In multicellular organisms which complete sexual
reproduction, they must make more “sex cells”.
• Meiosis = Cell Division for “sex cells”.
• Sex Cells:
– Also called Gametes.
– Sperm in male organisms.
– Eggs in female organisms.

6. (3) What happens during Meiosis?
• There are 2 divisions.
• Start with 1 cell and end up with 4 genetically
unique cells.
• Start “2n” or “Diploid”  End “1n” or “Haploid”
• 1n = Haploid = ½ Set of Genetic Information
– In Humans that means that each sperm or egg cell
should have 23 chromosomes.

8. (4) Genetic Variation
• The Purpose of Meiosis is to:
– Cut the amount of DNA in ½, so that when a sperm and egg
combine, the resulting baby will have a full set of DNA.
– Increase genetic variation and diversity amongst all sperm and
egg cells.
• You have at least 2 variations for every gene in your DNA:
– Gene = Specific sequence of DNA.
– Allele = Variation of a gene.
• Each sperm/egg cell should have a unique
combination of gene alleles.

10. (5) Meiosis Basics
• Meiosis-1:
– Start with 1 cell and end with 2
– Start with copies of all gene variations (actually “4n”).
– Crossing Over occurs
– Involves Homologous Pairs of Chromosomes
• Meiosis-2:
– Start with 2 cells and end with 4
– Cells are now “2n”.
– Involves Pairs of Sister Chromatids
– The “Law of Independent Assortment” and the “Law of
Segregation” are applied.

12. (6) Meiosis-1 Details
1. Chromosomes form + Nucleus dissolves.
2. Homologous pairs of chromosomes line.
3. Crossing Over occurs.
4. Homologous pairs of chromosomes separate.
5. Cells divide and split, resulting in two cells.

14. (7) Meiosis-1 Vocabulary
• Homologous Pairs of Chromosomes 
– Pairs of sister chromatids with similar segments of DNA
(or genes.)
– Looks like 2 X’s.
• Crossing Over 
– When nearby chunks of DNA are exchanged between
homologous pairs of chromosomes.
– Used to increase genetic diversity.

17. (8) Meiosis-2:
1. Pairs of sister chromatids line up.
2. Pairs of sister chromatids separate using the “Law
of Segregation” and the “Law of Independent
Assortment”.
3. Cells divide, resulting in 4 cells.

19. (9) Meiosis-2 Vocabulary:
• Sister Chromatids:
Identical copies of chromosomes.
• Law of Segregation:
Chromosomes split up so that each sex cell receives
a different variation of each gene.
• Law of Independent Assortment:
Chromosomes split up completely independent of
every other chromosome and gene variation.

21. (10) Meiosis Errors
• When chromosomes do not separate correctly
during meiosis this can cause:
– Sex Cells to have too many chromosomes (more than
23).
– Sex cells to have too few chromosomes (less than 23).
• Individuals born from these sex cells will acquire a
genetic disorder from having the wrong number of
sex chromosomes.