Transcript Meiosis - Grant County Schools

Sexual Reproduction and
Genetics
What would happen without
meiosis?
1. Construct a data table with the headings
Cycle number
Stage
1
1
1
2
2
2
3
3
3
Mitosis
Interphase
Fertilization
Mitosis
Interphase
Fertilization
Mitosis
Interphase
Fertilization
Chromosome
number
What would happen without
meiosis?
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
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?
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
Haploid – A cell with
one of each kind of
chromosome (gametes)
(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)
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 – 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?
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
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)
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
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
Genetic Recombination
• 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……
Nondisjunction
• The failure of homologous chromosomes
to separate during meiosis I
• Animation #1
• Animation #2
Results of Nondisjunction
• 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).
Common Abnormalities from
Nondisjunction
•
•
•
•
•
•
AbnormalityKaryotype
Down SyndromeTrisomy 21
Turner Syndrome X
Triple-X SyndromeXXX
Klinefelter Syndrome XXY
Jacob SyndromeXYY