Goal 3 - Meiosis
Genetics
Chromosome
Genes on chromosomes:
Trait
- The Study of Inherited Characteristics
Chromosome
- Holds the genetic information. Made of DNA
Genes on chromosomes:
- Segments of DNA on a chromosome
- Code for a trait
Trait
- Characteristic
- Feature
- Variations in a population
Chromosome Number
Chromosome Number
Autosomes & Sex Chromosomes
- Different number for different species
- Full set = 2N=Diploid
- N = # pairs
- 1 pair from mother
- 1 pair from father
- Humans = 23 pairs or 46 total
- Sets of each pair are called - Homologous chromosomes
Autosomes & Sex Chromosomes
- Autosomes = #1-22 for all traits except sex
- Sex chromosomes = Pair #23 XX (female) or XY (male)
What is a Gamete?
Why do cells divide by Meiosis?
•To reduce chromosome number
•In sex cells (haploid cells – sperm and egg)
•In single celled organisms: Under changing conditions for Chromosome Variety
Gonads make gametes!
•To reduce chromosome number
•In sex cells (haploid cells – sperm and egg)
•In single celled organisms: Under changing conditions for Chromosome Variety
Gonads make gametes!
- Testes make sperm - sperm development in boys begins at puberty and continues until death.
- Ovaries make eggs - egg development begins prior to birth. One egg is released every month during puberty. If the egg is unfertilized then menstruation occurs. Continues until menopause.
- Immature egg or sperm cell = full set of chromosome (2N)
- Undergoes meiosis with 2 cell divisions
- Develops into a mature egg or sperm (1N)
The Cell Cycle and Meiosis
The Cell Cycle of a Gamete
Interphase
•Full set of chromosomes (2N)
•DNA has doubled (4N)
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Final Telophase II
- Interphase
- Meiosis
Interphase
•Full set of chromosomes (2N)
•DNA has doubled (4N)
Prophase I
- DNA coils up and spindle form
- DNA forms homologous chromosomes that line up with each other and form a 4-part structure called a tetrad.
- A tetrad consists of 2 homologous chromosomes each made up of 2 sister chromatids.
- Chromatids pair so tightly that non-sister chromatids can break off and exchange genetic info in a process called crossing-over.
Metaphase I
- Centromere of each chromosome becomes attached to spindle fibers.
- Spindle fibers pull tetrads into the middle (note they are lined up as tetrads)
Anaphase I
- Homologous chromosomes (each with 2 chromatids), separate and move toward opposite ends of the cell (note centromeres do not spilt)
Telophase I
- Spindle breaks down
- Chromosomes uncoil
- Cytoplasm divides
- Result 2 new 2N cells
- 1st cell division (2N)
Prophase II
- DNA does not double – there is NO INTERPHASE
- Spindle forms in each of the two new cells and attaches to the chromosomes
Metaphase II
- Chromatids are pulled to the center of the cell and line up randomly at the equator (middle).
Anaphase II
- Centromeres spilt
- Sister chromatids separate and move toward opposite ends of the pole.
Final Telophase II
- Nucleus reforms
- Spindle breaks down
- Cytoplasm divides
- 2nd division