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Chapter 9 – MeiosisWhat 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 – MeiosisProphase I Specifics
Crossing over of non-sister chromatids
During prophase I, non-sister chromatids can undergo synapsis, in which thechromatids 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 – MeiosisProphase 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
XXThe key point is that one set of replicated homologous chromosomes has no influenceon how the other chromosomes are going to align during metaphase
This is best seen in the following figure!
Chapter 9 – Meiosis
Chapter 9 – MeiosisWhat 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 – MeiosisMeiosis I Mitosis
Prophase I Prophase
Pairing of chromosomes No pairing
Metaphase I MetaphaseHomologous chromosomes at metaphase Duplicated chromosomes at metaphaseplate plate
Anaphase I AnaphaseHomologous chromosomes separate Sister chromatids separate, becoming
Daughter chromosomes
Telophase I Telophase
Daughter cells are haploid Daughter cells are diploid
Chapter 9 – MeiosisMeiosis II Mitosis
Prophase II Prophase
No pairing of chromosomes No pairing
Metaphase II Metaphase
Haploid # of chromosomes at metaphase Diploid # of duplicated chromosomes at plate metaphase plate
Anaphase II AnaphaseSister chromatids separate, becoming Sister chromatids separate, becomingdaughter chromosomes daughter chromosomes
Telophase I Telophase
4 haploid daughter cells Daughter cells are diploid
Chapter 9 – Meiosis
Chapter 9 – MeiosisSpermatogenesis
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 – MeiosisPRACTICE 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?
