1. Sexual Reproduction and Meiosis

2. What would happen without meiosis?
Construct a data table with the headings
Cycle number
Stage
Chromosome number 1 2 3
Mitosis
Interphase
Fertilization

3. What would happen without meiosis?
Model a cell with a pair of chromosomes
Demonstrate the cell cycle and mitosis
Fuse one of your cells with another student’s cell (representing fertilization)
Repeat the above steps two more times and record your chromosome numbers for each stage

4. What would happen without meiosis?
Summarize – How does the chromosome number in your model change with each cycle of mitosis and fusion?
Infer – What must occur when cells fuse in order for chromosome number to remain constant?

5. Organisms have tens of thousands of genes
- Genes determine individual traits
- Genes are lined up on chromosomes
- One chromosome can contain a thousand or more genes

6. Homologous Chromosome
Paired Chromosomes
One set of chromosomes
from your dad, one
set of chromosomes
from your mom.

7. -The Chromosomes in body cells of most plants and animals occur in pairs
-One chromosome in each pair came from the male parent, one came from the female parent
-Humans have 46 chromosomes 23 paired chromosomes

8. Diploid – A cell with two of each kind of chromosome
(1 from each parent)
We say the cell is a diploid cell or 2n
(This supports Mendel’s conclusion that organisms have two factors – alleles – for each trait)
Haploid – A cell with one of each kind of chromosome (gametes)
We say the cell is a haploid or n
(This supports Mendel’s conclusion that parents give one allele for each trait to their offspring)

9. Each species of organisms contains a different number of chromosomes
The chromosome numbers of a species is not related to the complexity of the organism

10. Homologous Chromosomes – Paired chromosomes
Each pair of homologous chromosomes has genes for the same traits
Ex. Pod shape
On homologous chromosomes, these genes are arranged in the same order
However, because there are different possible alleles for the same gene, the two chromosomes in the homologous pairs are not always identical to each other.

11. Complete Problem Solving Lab 10.2 (pg.270)

12. Why Meiosis?
When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cell.
What would happen if mitosis were the only form of cell division?

13. Meiosis occurs only in the specialized body cells that produce gametes
Meiosis – cell division which produces gametes containing half the number of chromosomes as the parent’s body cells
Meiosis occurs only in the specialized body cells that produce gametes
(Male and Female reproductive organs)
The process of meiosis produces:
male gametes (sperm) - n, haploid
female gametes (eggs) – n, haploid

14. The zygote then can develop by mitosis into a multicellular organism
When the sperm fertilizes the egg, the resulting cell is a Zygote – 2n diploid
The zygote then can develop by mitosis into a multicellular organism
This process is called Sexual Reproduction
Two parents produce an offspring that is not identical to either parent
What are the benefits to sexual reproduction?

15. The Phases of Meiosis Meiosis I Interphase Prophase I Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II

16. Meiosis I
Interphase – just like interphase of Mitosis the cell replicates its chromosomes
(2 identical sister chromatids held together by a centromere)
Prophase I – The chromosomes coil up and the spindles form. Each pair of homologous chromosomes come together, matched gene by gene (forms a four part structure called a tetrad). This is where crossing over can occur.
Metaphase I – The tetrads are pulled to the middle of the cell (homologous chromosomes are lined up side by side)
Anaphase I – Homologous chromosomes separate and move to opposite ends of the cell
Telophase I – The spindle is broken down, the chromosomes uncoil, and the cytoplasm divides (2 new cells), each with only half the genetic information of the original cell. (another cell division is needed because each chromosome is still doubled – two sister chromatids)

17. Meiosis II
Prophase II – Spindle fibers form and attach to the chromosomes
Metaphase II – The chromosomes line up on the equator
Anaphase II – Sister chromatids are pulled apart
Telophase II – Nuclei re-form, the spindles break down, the cytoplasm divides, 4 new cells are formed

18. Meiosis Videos
Animation: How Meiosis Works
Meiosis Square Dance

19. Meiosis Start with 1 diploid cell (2n), end with 4 haploid cells (n)
Each haploid contains one chromosome from each homologous pair
The haploid cells become gametes which will transmit their genes to offspring

20. Why are you not identical to your siblings?
You each have half of your DNA from your mom and half from your dad…. Why are you different from each other?

21. Genetic Recombination
There are two ways that this occurs
Crossing over can occur anywhere, at random, on a chromosome
Typically, 2-3 crossovers per chromosome occur during meiosis
Re-assortment of chromosomes also occurs during meiosis by independent segregation of homologous chromosomes

22. Can mistakes happen in meiosis?
Nondisjunction
The failure of chromosomes to separate during meiosis
Animation #1
Animation #2

23. Results of Nondisjunction
Monosomy and Trisomy
Monosomy refers to a condition in which there is one chromosome is missing.
A trisomy has one extra chromosome.
They are generally lethal, with a few exceptions

24. Common Abnormalities from Nondisjunction
AbnormalityKaryotype
Down SyndromeTrisomy 21
Turner Syndrome X
Triple-X SyndromeXXX
Klinefelter Syndrome   XXY
Jacob SyndromeXYY