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1. Construct a data table with the headings
Cycle number Stage Chromosome number
Mitosis
Interphase
Fertilization
Mitosis
Interphase
Fertilization
Mitosis
Interphase
Fertilization

1. Model a cell with a pair of chromosomes
2. Demonstrate mitosis
3. Fuse one of your cells with another student’s cell
4. Repeat the above steps two more times and record your chromosome numbers for each stage

• 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?

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

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

Diploid – A cell with two of each kind of chromosome
(1 from each parent)
We say the cell is a diploid cell or 2 n
( 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)

• 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

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.

Complete Problem Solving Lab 10.2
(pg.270)

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?

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

• When the sperm fertilizes the egg, the resulting cell is a Zygote – 2 n 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?

Meiosis I
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II

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)

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

• 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

• Crossing over can occur anywhere, at random, on a chromosome
• Typically, 2-3 crossovers per chromosome occur during meiosis
• The re-assortment of chromosomes and the genetic information they carry, either by crossing over or by independent segregation of homologous chromosomes is called Genetic
Recombination
• How does crossing over increase genetic variability? Read the Inside Story to find out……

• The failure of homologous chromosomes to separate during meiosis I
• Animation #1
• Animation #2

• Monosomy and Trisomy
– Monosomy refers to a condition in which there is one chromosome is missing, monosomy X is a condition in which cells have only one X chromosome.
– A trisomy has one extra chromosome. Trisomy 21 is an example of a trisomy in which cells have an extra chromosome 21.
• They are generally lethal except monosomy X
(female with one X chromosome) and trisomy 21
(Down’s Syndrome).

• AbnormalityKaryotype
• Down SyndromeTrisomy 21
• Turner Syndrome X
• Triple-X SyndromeXXX
• Klinefelter Syndrome XXY
• Jacob SyndromeXYY