During sexual reproduction, two parents each contribute half of their chromosomes to offspring. Homologous chromosomes, which contain the same genes in the same order but may have different alleles, allow this to happen. Meiosis is a two-step cell division process that reduces the number of chromosomes in a cell from diploid to haploid. This includes a first meiotic division that separates homologous chromosome pairs, and a second division that separates sister chromatids, resulting in four haploid daughter cells each with a unique combination of chromosomes.

1. Lecture 3c:
Sexual Reproduction

2. Basics
During sexual reproduction, two parents
contribute half of their chromosomes to
the offspring
Homologous chromosomes and meiosis
are why this can happen

3. Basics
Homologous chromosomes:
chromosomes occur in pairs
humans have 23 pairs, or 46 total
chromosomes
46 is the diploid (2n) number
23 is the haploid (n) number

4. Basics
Homologous chromosomes have the same
size, shape, and location of centromere
They contain copies of the same genes in
the same order
One copy came from each parent
Alternate versions of a gene are called
alleles

5. Basics
In order for fertilization to occur, so you
get half of your genes from each parent,
the number of chromosomes must be cut
in half ahead of time
This is meiosis

6. Basics
Meiosis is part of either spermatogenesis
(production of sperm) or oogenesis
(production of eggs)
Sperm and eggs are haploid
Fertilization results in the sperm and
egg joining to form a zygote
The zygote is diploid

7. Meiosis
Turns a diploid cell into 4 haploid cells
First, chromosomes are duplicated
resulting in sister chromatids
Like in mitosis
Consists of two divisions: meiosis I and
meiosis II

8. Meiosis I
Synapsis: Homologous chromosomes line
up together side by side
Results in a tetrad - there are 4
chromatids present
Meiosis I separates the homologous
chromosomes into 2 cells that are n,
but each chromosome has 2 chromatids

9. Meiosis II
2 n cells are split into 4 n cells
The sister chromatids separate in this
step
It is important to realize that it is
random which chromosome goes to which
daughter cell

10. Crossing Over
While the chromosomes are in a tetrad,
crossing over can occur
Exchange of genetic material between one
nonsister chromatid and another
Results in chromosomes having different
combinations of alleles than the parents
Increases diversity

11. Meiosis I in depth
4 stages same as mitosis, but the
alignment at metaphase is different
See cartoon
Homologous chromosomes separate, so
daughter cells (2) are haploid
Remember each chromosome still has
2 chromatids

12. Meiosis II in depth
Same as mitosis, except cells are haploid
The resulting cells are NOT identical
though, as they would be after mitosis,
because they each can have different
chromosome combinations

13. Vocabulary
Chromosomes: condensed DNA, forms in
regular pattern. Humans have 46
chromosomes, 23 pairs of
Homologous chromosomes: 2 chromosomes that
contain the same genes but may have different
alleles
Chromatids: copies of chromosomes that are
made during DNA replication, but do not
increase the number of chromosomes present