Lesson 13 Outline list Lesson 15
Fundamentals of Biology
Lesson 14
Mitosis and Meiosis
I. Genes.
A. Definition.
1. Mendel: a factor that causes a
particular trait.
2. Current: a section of DNA
that codes for a particular polypeptide chain.
3. Located in the chromosomes.
B. Chromosome structure.
1. DNA double helix.
2. Double helix wrapped around
clusters of 8 proteins called histones, like beads on a string.
3. This bead-like chain is coiled.
4. This coil is coiled again - a
supercoil.
5. Each leg of a chromosome is called
a chromatid.
6. When the chromosome replicates,
the two copies are joined in the middle by a centromere.
II. Mitosis.
A. Purpose: produce two new cells identical to the original.
B. Interphase.
1. When cell is not dividing.
2. Chromosomes are uncoiled,
invisible.
3. Transcription is occurring.
4. DNA replicates and duplicate
chromosomes are formed.
C. Prophase.
1. Centrioles divide and migrate to
opposite sides of the nucleus.
2. Aster becomes visible around each
centriole.
3. Spindle forms between the
centrioles - made of microtubules.
4. Chromosomes coil up, getting short
and thick.
5. Nuclear membrane disintegrates and
nucleolus disappears.
D. Metaphase.
1. Chromosomes line up on the
equatorial plane.
2. Sister chromatids repel each
other.
3. Spindle fibers attach to the
centromeres.
E. Anaphase.
1. Centromeres split.
2. Spindle fibers pull the chromatids
toward either end of the cell.
F. Telophase.
1. Nuclear envelope reforms.
2. Spindle disintegrates.
3. Chromosomes uncoil.
4. Cytoplasm divides.
III. Meiosis.
A. Purpose: produce gametes, with half the number of
chromosomes as the original.
1. Only occurs in diploid cells.
2. Each cell has one set of
chromosomes from the father, and one set from the mother.
3. The new cell must have a complete
set of chromosomes, but only one set.
4. The new set may be composed of
some chromosomes from the father and some from the mother.
B. First Division.
1. At metaphase, homologous
chromosomes pair up on the equatorial plate, forming a tetrad.
2. The centromeres do not divide, and
the chromatids do not separate.
3. Instead, the homologous
chromosomes separate intact, with their sister chromatids still joined at the
centromere.
4. The cell divides, but the
chromosomes do not uncoil.
5. Now each cell is haploid, but with
a copy of the chromosomes.
C. Second Division.
1. Essentially the same as mitosis.
2. Since the cell is now haploid, the
chromosomes do not have homologs.
3. This time the centromeres divide
and the sister chromatids separate.