[4.2] MEIOSIS



Meiosis is a reduction division. One dipoloid (2n) cell divides to form four haploid (1n) cell. These cells are called gametes. They can be sperm (male) or ova (female, animal and plant) or pollen (male, plant). Note that ova is plural and ovum is singular. They are commonly referred to as eggs.

external image meio1.gif

Fertilization : Two haploid cells form one diploid cell.


external image fert1.gif


Meiosis viewed under a light microscope.

external image meiosis.pics.jpg



The stages of meiosis can be broken down into two main stages, Meiosis I and Meiosis II
  • Meiosis I can be broken down into four substages: Prophase I, Metaphase I, Anaphase I and Telophase I
  • Meiosis II can be broken down into four substages: Prophase II, Metaphase II, Anaphase II and Telophase II

MEIOSIS I



Prophase 1
Prophase 1

Prophase I - most of the significant processes of Meiosis occur during Prophase I * The chromosomes condense and become visible
  • The centrioles form and move toward the poles
  • The nuclear membrane begins to dissolve
  • The homologs pair up, forming a tetrad
    • Each tetrad is comprised of four chromotids - the two homologs, each with their sister chromatid
  • Homologous chromosomes will swap genetic material in a process known as crossing over (abbreviated as XO)
    • Crossing over serves to increase genetic diversity by creating four unique chromatids


Metaphase I* Microtubules grow from the centrioles and attach to the centromeres
  • The tetrads line up along the cell equator
Compare Metaphase I to Metaphase II and to the Metaphase stage of mitosis.
Metaphase 1
Metaphase 1


Note that the description below is not accurate. The centromeres are pulled toward the opposite poles due to contracting spindle fibers. However, they do not bral. The homologous synapisis break and the chromosomes, not the chromatids, are separated.

Anaphase 1
Anaphase 1

Anaphase I* The centromeres break and homologous chromosomes separate (note that the sister chromatids are still attached)
  • Cytokinesis begins
Compare Anaphase I to Anaphase II and to the Anaphase stage of mitosis.


Telophase I
  • The chromosomes may decondense (depends on species)
  • Cytokinesis reaches completion, creating two haploid daughter cells
Compare Telophase I to Telophase II and to the Telophase stage of mitosis.
Telophase 1
Telophase 1

MEIOSIS II

Anaphase 2
Anaphase 2

Prophase II* Centrioles form and move toward the poles
  • The nuclear membrane dissolves



Metaphase II
  • Microtubules grow from the centrioles and attach to the centromeres
  • The sister chromatids line up along the cell equator
Compare Metaphase II to Metaphase I and to the Metaphase stage of mitosis.
Metaphase 2
Metaphase 2



Anaphase 2
Anaphase 2

Anaphase II* The centromeres break and sister chromatids separate
  • Cytokinesis begins


Telophase II
  • The chromosomes may decondense (depends on species)
  • Cytokinesis reaches completion, creating four haploid daughter cells
Compare Telophase II to Telophase I and to the Telophase stage of mitosis.
Telophase 2
Telophase 2






Mitosis and Meiosis compared.

external image c13x8meiosis-comparison.jpg

Crossing over between homologous chromosome pair.


external image crossovr.gif



Cross-over animation
Cross-over animation

Variation and Meiosis http://waynesword.palomar.edu/lmexer2a.htm

7. Number Of Chromosome Combinations During Meiosis

No. of homologous
chromosome pairs
(heterozygous genes)

No. of different gametes
from each parent

Total number of zygotic
combinations or squares
in genetic checkerboard

1 (Aa X Aa)
2 (21)
4 (21)2
2 (AaBb X AaBb)
4 (22)
16 (22)2
3 (AaBbCc X AaBbCc)
8 (23)
64 (23)2
4 (AaBbCcDd X AaBbCcDd)
16 (24)
256 (24)2
20 pairs of chromosomes
1,048,576 (220)
1,099,511,627,776 (220)2
23 pairs of chromosomes
8,388,608 (223)
70,368,744,000,000 (223)2
(n) pairs of chromosomes
(2n) n = haploid number
(2n)2
Including Crossover Factor (23) During Meiosis
23 pairs of chromosomes
67,108,864 (226)
4,503,599,600,000,000 (226)2
            • NOTE: the power digits show up as normal numbers. You need to translate (21) as 2 to the power of 1 and notice that during fertilization the gamete number is then raised to the power of 2
            • Considering independent assortment of 23 pairs of chromosomes (n = 23) and crossing over during meiosis, the total number of different ways that human gametes can combine exceeds four quadrillion (252). This number is greater than all the atoms in our solar system. This vast array of different genetic combinations also explains the infinite variation in human faces (with the exception of identical twins). To really appreciate the magnitude of the 16 digit number four quadrillion, consider the 10 digit number 2.5 billion (2,500,000,000). If you counted one number every second (day and night), it would take about 79 years to reach this number. If you slept eight hours each night, it would take about 119 years. Also ponder the number 263. This number exceeds all the grains of rice ever produced in the history of the world! In fact, in some models of the visible universe, the total number of electrons does not exceed 1087.




Non-disjunction of chromosomes can occur during Anaphase I or during Anaphase II

Results in abnormal numbers of chromosomes.
Trisomy means a person has an extra copy of a chromosome.
Monosomy means a person is missing a copy of a chromosome


external image image006.jpg



What is a mutation?
Changes in the genetic material (DNA).

Mutations can occur in two different types of cells:

1. Somatic (body) cells
2. Gamete (sex) cells

Mutations that occur in somatic (body) cells usually result in killing that body cell only.

An exception to this is if the mutation occurs to the DNA that controls regulation of the cell cycle. This can result in cancer.
Somatic mutations usually do not affect the individual.
Somatic mutations can NOT be passed on to the next generation.

Mutations that occur in gamete (sex) cells means that every cell of the developing fetus/baby will have that mutation.
Gamete mutations usually do affect the individual with some genetic disorder.

Gamete mutations can be passed on to the next generation.

There are two types of mutations that can occur in gamete cells:

1. Gene Mutations
2. Chromosomal Mutations

2 types of Single Gene Mutations:
1. Point Mutation: a single point in the DNA sequence is affected.

Can be a substitution in which one base is changed into another base.

DNA: TAC GCA TGG AAT
mRNA: AUG CGU ACC UUA
Amino Acid: Met - Arg - Thr - Leu

DNA: TAC GTA TGG AAT
mRNA: AUG CAU ACC UUA
Amino Acid: Met - His - Thr - Leu

Point mutations usually affect no more than a single amino acid. The protein may be slightly affected or not affected at all.

THE FAT CAT ATE THE RAT

THE FAT CAT ATE THE HAT

2. Frameshift Mutation: a single gene or nitrogen base is deleted or added from the mRNA sequence causing a shift in the “reading frame” of the genetic message.
Can be an insertion in which one base is inserted in the DNA sequence.
Can be a deletion in which one base is deleted in the DNA sequence.
DNA: TAC GCA TGG AAT
mRNA: AUG CGU ACC UUA
Amino Acid: Met - Arg - Thr - Leu

DNA: TAT CGC ATG GAA T
mRNA: AUA GCG UAC CUU A
Amino Acid: Iso - Ala - Tyr - Leu

The affect of frameshift mutations is usually more dramatic. Frameshift mutations may change every amino acid that follows the point of the mutation. Frameshift mutations can alter a protein so much that it is unable to perform its normal functions.

THE FAT CAT ATE THE RAT

THE FAT CAT ATE THE RAT
TEF ATC ATA TET HER AT

5 types of Chromosomal Mutations:
1. Deletion
2. Duplication
3. Inversion
4. Translocation
5. Non-disjunction

Deletion: Involves the loss of all or part of a chromosome.
Duplication: Involves the production of extra copies of parts of the chromosome.
Inversion: Reverses the direction of parts of a chromosome.
Translocation: When one part of a chromosome breaks off and attaches to another chromosome.
Non-disjunction: Means “not coming apart”. When homologous chromosomes fail to separate properly during meiosis.
AMNIOCENTESIS
external image si55550976_ma.jpg

KARYOTYPE
http://trc.ucdavis.edu/biosci10v/bis10v/media/ch08/karyotype.swf




http://trc.ucdavis.edu/biosci10v/bis10v/media/ch08/karyotype.swf