Download Meiosis

Chapter 10.2
Genes & Traits
Genes:
Organisms have thousands of genes that determine
individual traits
They are lined up on chromosomes
1 chromosome holds 100-1000 genes
Traits:
Hair color, length of leg, size of hand, shape of eyes, etc.
Each trait can have different forms (called alleles)
Examples:
Trait
Possible alleles
Hair color
=
blonde, brunette, red, black
Shape of nose
=
pointy, round, square
Eye Color
=
brown, blue, hazel, green
Chromosomes
Chromosome: organelle that contains DNA
Carries genetic material that is copied and passed
from generation to generation of cells
Darkly colored when stained so it is easy to see under
the microscope
Chromosome Number:
A specific number of chromosomes that an organism has
Examples:
Humans have 46 chromosomes or 23 pairs
Pea Plants have 14 chromosomes or 7 pairs
Dogs have 78 chromosomes or 39 pairs
Diploid vs. Haploid
Body Cells
Chromosomes occur in pairs
Diploid Cell: cell with two of each type of
chromosome
Contains a diploid, or 2n, number of chromosomes
Offspring receive ½ of their parents DNA
One chromosomes from male parent
One chromosome from female parent
Haploid Cells: a cell with one of each kind of
chromosome
Contains a haploid, or n, number of chromosomes
Chromosomes are matched pairs because they
carry the same trait
Not necessarily the same allele!
Example:
Trait
Allele from Mom
Allele from Dad
Chromosome pair #1
Hair color
Brown
Blonde
Chromosome pair #2
Nose Shape
Pointy
Pointy
Chromosome pair #3
Eye Color
Blue
Green
Still matched pairs even because they are for the
same trait, even though they are different
Homologous Chromosomes:
Paired chromosomes of a diploid cell.
Each pair has genes for same traits not necessarily
alleles
How do offspring have the same number of
chromosomes as their parents? Why aren’t
they doubled?
Meiosis: cell division that produces gametes
containing half the number of chromosomes as
a parent’s cell
Occurs in specialized cells that produce gametes
Gamete – male and female sex cells (sperm and eggs)
Phases of Meiosis
Meiosis has two separate divisions
Meiosis I and Meiosis II
Meiosis I
Starts with one diploid (2n) cell
At end of Meiosis II
4 haploid cells (n)
Called sex cells (gametes)
Male gametes – sperm
Female gametes – eggs
Phases of Meiosis
Some is same as Mitosis
Spindle fibers form
Cytoplasm splits
Interphase
Cell replicates its chromosomes
Each chromosome consists of two identical sister
chromatids
Held together by a centomere
Meiosis Animation
Prophase I
Chromosomes coil up and spindle fibers form
Each pair of homologous chromosomes come
together
Matched gene by gene
For a four part structure called a tetrad
Tetrad:
Consists of two homologous chromosomes , each
made of 2 sister chromatids
Pair so tightly some genetic information is actually
exchanged, called Crossing Over
Average of 2-3 crossovers on every homologous
chromosomes
Metaphase I
Centromere is attached to the spindle fiber
Spindle fibers pull the tetrads into the equator of the
cell
Line up as tetrads, not homologous chromosomes
Anaphase I & Telophase I
Anaphase I:
Homologous Chromosomes separate and move to
opposite poles
Telophase I:
Spindle fibers breakdown
Chromosomes uncoil
Cytoplasm divides (cytokinesis)
We still need another cell division because we
have doubled chromosomes
Anaphase I & Telophase I
Anaphase I
Telophase I
Meiosis II
Short interphase period
Then it follows the same steps of mitosis
Prophase II
Metaphase II
Anaphase II
Telophase II
You now have 4 (four) haploid cells have been formed
from 1 (one) diploid cell
Each haploid cell contains one chromosome from each
homologous pair
Become gametes
Meiosis Animation
Meiosis II