Chapter 11: Introduction to Genetics
11.4 – Meiosis
Mr. M. Varco
Saint Joseph High School
Chromosome Number
•
How do we inherit our genes?
•
Chromosomes, which are composed of strands of DNA and proteins, are the
“carriers” of genes.
•
The genes are located in specific positions on chromosomes.
Chromosome Number
Diploid Cells
•
A body cell in an adult fruit fly has 8 chromosomes, of which 4 are inherited
from each parent.
•
The two sets of chromosomes are homologous when each of the four
chromosomes from one parent has a corresponding chromosome from the
other.
•
A cell that contains both sets of chromosomes
is called diploid.
Chromosome Number
Diploid Cells
•
The diploid cells of adult organisms contain two complete sets of inherited
chromosomes and thus two complete sets of genes.
•
Diploid # is represented by the symbol: 2N
•
Ex: An adult fruit fly has diploid number of 8. (2N=8)
Human Karyotype
Chromosome Number
Haploid Cells
•
Some cells contain only a single set of chromosomes, and therefore a single
set of genes.
•
Such cells are haploid, meaning “one-set”
•
The gametes of sexually reproducing organisms are haploid.
N = 4 (Haploid)
2N = 8 (Diploid)
Phases of Meiosis
•
How are haploid (N) gamete cells produced from diploid (2N) cells?
•
Meiosis is a process in which the number of chromosomes per cell is cut in
half through the separation of homologous chromosomes in a diploid cell.
•
Meiosis usually involves two distinct division: Meiosis I and Meiosis II.
Phases of Meiosis: Meiosis I
Prophase I:
•
In prophase I, each replicated chromosome pairs with its corresponding
homologous chromosome, producing sister chromatids.
•
This pairing forms a structure called a tetrad, which contains four chromatids.
Phases of Meiosis: Meiosis I
Prophase I:
•
As the homologous chromosome form tetrads, they undergo a process called
crossing-over.
•
During crossing-over, sections of each chromatid found in the homologous
chromosomes cross over and exchange alleles.
Phases of Meiosis: Meiosis I
Metaphase I and Anaphase I:
•
During metaphase I, paired homologous chromosomes line up across the
center of the cell.
•
During anaphase I, spindle fibers pull each homologous chromosome pair
towards opposite ends of the cell.
Phases of Meiosis: Meiosis I
Telophase I and Cytokinesis:
•
During telophase I, a nuclear membrane reforms around each cluster of
chromosomes.
•
During cytokinesis, two new cells are formed.
Phases of Meiosis: Meiosis I
Results of Meiosis I:
•
Meiosis I results in two daughter cells.
•
As each pair of homologous chromosomes was separated, neither daughter cell
has two complete sets of chromosomes that is would have as a diploid cell.
•
The two cells produced by meiosis I has sets of chromosomes and alleles that
are different from each other and from the parent diploid cell.
Phases of Meiosis: Meiosis II
Meiosis II:
•
Unlike in meiosis I, neither cell goes through round of chromosome replication.
Phases of Meiosis: Meiosis II
Prophase II:
•
As cells enter prophase II, their chromosomes (each consisting of two chromatids)
become visible.
•
The chromosomes do not pair to form tetrads, because the homologous pairs
were already separated during M1.
Phases of Meiosis: Meiosis II
Metaphase II and Anaphase II:
•
During metaphase II, chromosomes line up across the cell center, followed by
the separation of sister chromatids during anaphase II.
Phases of Meiosis: Meiosis II
Telophase II:
•
The final four phases of meiosis II are similar to those in meiosis I
•
However, the result is four haploid daughter cells, each which received two
chromosomes.
•
These four daughter cells now contain the
haploid number (N)
Phases of Meiosis: Meiosis II
Gametes to Zygotes:
•
The haploid cells produced by meiosis II are the gametes that are critical in
heredity. (Male = Sperm; Female = Egg)
•
After fertilization, the effe is referred to as a Zygote.
•
The zygote undergoes cell division by mitosis and eventually forms a new
organism
Mitosis vs. Meiosis
Mitosis:
- When two sets of genetic material separate, each daughter receives one
complete set of chromosomes.
- The original number of chromosomes remains the same as in the original cell.
- Results in the production of two genetically identical diploid cells
Meiosis:
- Homologous chromosomes line up and then move to separate daughter cells.
- The number of chromosomes in the daughter cell is half
- Results in four genetically different haploid cells
haploid.
Analyzing Data
Organism
Ameoba
Chimpanzee
Earthworm
Hamster
Onion
Haploid (N)
25
24
22
Diploid (2N)
g
f
36
g
f
16
g
Homework
- Chapter 11.4 Quiz (pages 323-328 of your textbook)
- Complete the Chapter 11 Review worksheet, due Wednesday.