Chapter 9 – Meiosis
What kind of cells must have our parents make to produce us?
SPERM & EGG (gametes)
More importantly, if all of our non-gamete cells (somatic cells) contain 46
chromosomes, how many chromosomes should be in our sperm & egg so that
sexual reproduction can ensure that our cells have 46 chromosomes?
Meiosis – a process that reduces the chromosome number in such a way that the
daughter nuclei only receive one member of each homologous pair of chromosomes
Think of it as a double mitotic division with only a single S phase
Phases of meiosis
1. Prophase I
6. Prophase II
2. Metaphase I
7. Metaphase II
3. Anaphase I
8. Anaphase II
4. Telophase I
9. Telophase II
5. Interkinesis
What do you think happens during each of these stages?
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Prophase I Specifics
Crossing over of non-sister chromatids
During prophase I, non-sister chromatids can undergo synapsis, in which the
chromatids line up side-by-side & exchange genetic information between them
This allows new combination of genetic material which will become part of a new
offspring
This bring us to a new term that will be the focus of the remainder of this discussion &
of chapter 23 & 24 – THE GENE!
A gene is the unit heredity that codes for a polypeptide & is passed on to offspring
Or a sequence of DNA that determines or contributes to some characteristic of the
organism
Eye color, hair color, height, personality
Allele – alternative form of a gene (brown eye gene vs blue eye gene)
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Prophase I & Metaphase I Specifics
Independent assortment
As the chromosomes are pushed around during prophase I, eventually lining up along
the metaphase plate during metaphase I, their orientation is different from that of
mitosis metaphase
Instead of lining one on top of the other, the replicated chromosomes line up side by
side according to their homologous characterstics
XX
XX
The key point is that one set of replicated homologous chromosomes has no influence
on how the other chromosomes are going to align during metaphase
This is best seen in the following figure!
Chapter 9 – Meiosis
Chapter 9 – Meiosis
What is the consequence of independent assortment?
Considering the previous example, the 3 chromosome pairs could orient themselves in
23 different combinations or 8 combinations of maternal & paternal chromosomes
A simpler example is the one below (you’ll draw it) utilizing only 2 chromosome pairs
Thus, when you think about the sperm & egg that contributed to your existence, you are
a result of a combination between 1 of 223 genetically different sperm & 1 of 223
genetically different eggs. 223 = 8,388,608 combinations
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Meiosis I
Mitosis
Prophase I
Prophase
Pairing of chromosomes
No pairing
Metaphase I
Metaphase
Homologous chromosomes at metaphase
plate
Duplicated chromosomes at metaphase
plate
Anaphase I
Homologous chromosomes separate
Anaphase
Sister chromatids separate, becoming
Daughter chromosomes
Telophase I
Telophase
Daughter cells are haploid
Daughter cells are diploid
Chapter 9 – Meiosis
Meiosis II
Mitosis
Prophase II
Prophase
No pairing of chromosomes
No pairing
Metaphase II
Metaphase
Haploid # of chromosomes at metaphase
plate
Diploid # of duplicated chromosomes at
metaphase plate
Anaphase II
Sister chromatids separate, becoming
daughter chromosomes
Anaphase
Sister chromatids separate, becoming
daughter chromosomes
Telophase I
Telophase
4 haploid daughter cells
Daughter cells are diploid
Chapter 9 – Meiosis
Chapter 9 – Meiosis
Spermatogenesis
Production of 4 haploid sperm from a a diploid spermatogonium
Follows normal meiosis
Oogenesis
Production of a single egg form a diploid oogonium
Meiosis occurs but at each telophase there is an inequity in the distribution of the
cytoplasmic contents
One daughter cell gets literally everything & the other daughter cell nothing. This
smaller cell is referred to as a polar body. It still contains the separated chromosomes
but nothing else. It will not proceed into meiosis II
The other daughter cell will proceed into meiosis II & if it is fertilized will complete
meiosis II & produce a single haploid egg & another polar body
Chapter 9 – Meiosis
PRACTICE QUESTIONS
1.
What is the significance of crossing over?
2.
What is an allele?
3.
What is independent assortment?
4.
How does spermatogenesis & oogenesis differ from one another?
5.
What is a polar body?