MEIOSIS IB BIO II March 3 2015 Van

MEIOSIS IB BIO II March 3, 2015 Van Roekel

Observing Mitosis Lab • Finish observations by looking at both onion root tip and white fish mitosis cells (10 minutes to finish observations and questions) • We will go over questions then turn them in

Meiosis – A Source of Distinction What are some characteristics you share with your parents? Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis.

Meiosis • Type of cell division called reduction division that cuts the number of chromosomes in half in order to produce sex cells (or gametes)

Why do we need meiosis? • Meiosis is necessary to halve the number of chromosomes going into the sex cells • This type of cell division is referred to as a reduction division, because the number of chromosomes has been reduced Why halve the chromosomes in gametes? • At fertilization the male and female sex cells will combine to provide ½ of the chromosomes each – so the offspring has genes from both parents and has a complete set of chromosomes

Meiosis does two things 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes four cells with a single copy of every chromosome (haploid). In meiosis, one diploid cells produces four haploid cells.

2) Meiosis creates a lot of genetic diversity. This trick is accomplished through independent assortment, crossing-over, and random orientation. Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. Genetic diversity is important for the evolution of populations and species.

The Stages of Meiosis: • Occurs in two separate Divisions: • Meiosis I • Interphase • Prophase I • Metaphase I • Anaphase I • Telophase I • Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II

Stages of Meiosis • Draw, label, and annotate the stages of Meiosis using pages 240 -241 in green Campbell-Reece Biology books

BILL • What is meiosis? What two things does meiosis do? • Meiosis is a reduction division that produces gametes (sex cells). • It cuts the number of chromosomes in half and turns one diploid cell into 4 haploid cells • It creates genetic diversity by independent assortment, random orientation, and crossing over.

Meiosis Parent cell – chromosome pair Chromosome s copied 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – mouse testes Parent cell 1 st division 2 nd division 4 gametes

Meiosis I : Separates Homologous Chromosomes • Homologous Chromosomes: pairs of chromosomes that are similar in size and shape and carry the same genes (AKA tetrads/bivalent) • Interphase • Each of the chromosomes replicate • The result is two genetically identical sister chromatids which remain attached at their centromeres

Prophase I • This is a crucial phase for meiosis. • During this phase each pair of chromatids match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. • Extremely IMPORTANT!!! It is during this phase that crossing over can occur. • Crossing Over is the exchange of segments from homologous chromosomes during synapsis.

Metaphase I • The homologous chromosomes, aka the bivalents, line up at the equator attached by their centromeres to spindle fibers from centrioles.

Anaphase I • The spindle guides the movement of the chromosomes towards the poles • Sister chromatids remain attached • Move as a unit towards the same pole • The homologous chromosome moves toward the opposite pole • Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

Telophase I • This is the end of the first meiotic cell division. • The cytoplasm divides, forming two new daughter cells. • Each of the newly formed cells half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division

Cytokinesis • Occurs simultaneously with telophase I • Forms 2 daughter cells • NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS • Cells are now considered haploid because they contain only one copy of each chromosome

Figure 13. 7 The stages of meiotic cell division: Meiosis I

Meiosis II : Separates sister chromatids • Proceeds similar to mitosis • THERE IS NO INTERPHASE II !

Prophase II • Each of the daughter cells forms a spindle, and the sister chromatids move toward the equator

Metaphase II • The individual chromosomes (sister chromatids) line up on the equator of each cell in random order. This is referred to as random orientation • Spindle fibers from opposite poles attach to each of the sister chromatids at centromeres

Anaphase II • The centromeres of sister chromatids finally separate • The sister chromatids of each pair move toward opposite poles • Now considered individual chromosomes

Telophase II and Cytokinesis • Nuclei form at opposite poles of the cell and cytokinesis occurs • After completion of cytokinesis there are four daughter cells • All daughter cells are haploid (n)

Figure 13. 7 The stages of meiotic cell division: Meiosis I

Figure 13. 7 The stages of meiotic cell division: Meiosis II

Meiosis I & II • Meiosis I takes place in order to produce two cells with a single set of chromosomes. In other words, it separates homologous chromosomes • Crossing over also occurs during meiosis I, which allows for an exchange of genetic material between non-sister chromatids • Meiosis II takes place in order to separate sister chromatids, or copies of an individual chromosome.

Bill • Explain why meiosis, rather than mitosis is necessary for gamete production. What is the difference between meiosis I and meiosis II? • Because the resulting daughter cells only contain half the genetic material from the female parent cells. The number of chromosomes passes from 2 n to n. • Meiosis I is the separation of homologous chromosomes to create haploid cells, meiosis II is the separation of sister chromatids.

Stages of Meiosis • Draw, label, and annotate the stages of Meiosis using pages 240 -241 in green Campbell-Reece Biology books, or 232 -233 in red Campbell-Reece Biology Books (15 minutes)

Genetic Variety in Gametes • Meiosis produces sex cells which result in offspring that show genetic diversity • Due to the following: • Crossing Over • Random orientation of Chromosomes • Law of independent Assortment

Crossing Over • Occurs during Prophase I • Results in the exchange of genetic information between paternal and maternal chromosomes. • Resulting chromosomes contain sections of genetic material which originated in two different people • Sister chromatids are no longer identical, so when they are separated, different alleles will be present in each gamete

Crossing Over • Must occur between homologous chromosomes • Occurs when chromatids from homologous chromosomes intertwine and break at the same positions • Place where chromatids connect to each other is referred to as a chiasma (plural, chiasmata)

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types

Random Orientation • Occurs during Metaphase I & II • Random order of chromosomes as they line up at the equator of each cell • Results in random alleles in each gamete

Independent Assortment • The separation of one pair of alleles is independent of the separation of another pair of alleles. • Meaning that one trait, such as flower color, is passed from parent to offspring does not depend on any other traits, such as see color. • Results because of random orientation of chromosomes

Independent Assortment & Meiosis • The genes and, in turn, alleles that will be passes to daughter cells depends on orientation of chromosomes during metaphase I • Produces 2 n distinct gametes, where n is the number of chromosomes, and 2 represents the number in each homologous pair • In humans, n=23, so 223= 8, 388, 608 possible chromosome orientations per gamete

Boy or Girl? The Y Chromosome “Decides” Y chromosome X chromosome

Boy or Girl? The Y Chromosome “Decides”

Division Error • Occasionally, chromosomes do not separate as expected, resulting in unequal distribution of chromosomes • Non-disjunction occurs when two or more homologous chromosomes stick together instead of separating • Results in gametes with 24 chromosomes

Non-disjunction & Trisomy • Different results based on what chromosome is affected • Trisomy is when a child receives 3 chromosomes instead of 2 • When this happens on the 21 st chromosome results in Down’s syndrome • Risk increase with age of the mother, especially over the age of 35

Karyotypes • Karyotypes is a photograph of chromosomes found in a cell, arranged based on size and shape. • Photo is taken during metaphase of mitosis • Obtain cells by: • Amniocentesis: use a hypodermic needle to extract some amniotic fluid around the developing baby • Chorionic Villus Sampling: obtain tissue sample from the placenta in the uterus wall

Checkup – 20 minutes to work • Describe, with the aid of a diagram, the behavior of chromosomes in the different phases of meiosis. • (Total 5 marks) • Explain how meiosis results in great genetic variety among gametes. • (Total 8 marks) • Why is meiosis referred to as reduction division? • (Total 2 marks) • • Draw and label the stages of meiosis I & II (Total 8 marks) • When completed, answer questions 7 & 9 on pg. 90 and questions 1 -3 on pg. 271

Describe, with the aid of a diagram, the behavior of chromosomes in the different phases of meiosis. (Total 5 marks • Chromosomes condense and become shorter/more visible in prophase I. At this time, homologous chromosomes will pair together and crossing over can occur. The homologous pair moves to the middle of the cell during metaphase I and separate during anaphase I, creating two haploid daughter cells in telophase I. The chromosomes condense in prophase II, and this time individual chromosomes move to the middle of the cell in metaphase II. Sister chromatids separate during anaphase II and decondense/uncoil in telophase II.

Explain how meiosis results in great genetic variety among gametes. (8 marks) • In meiosis, homologous chromosome pair together to form bivalents, which allows for the process of crossing over to occur during prophase I. In crossing over, sister chromatids from opposite parents exchange genetic information when they join together at a site called the chiasma. This results in a new combination of alleles on the sister chromatids. Additionally, the homologues will randomly orient their positions during metaphase I, which results in random distribution of alleles to the gametes, as they separate from each other. Lastly, the law of independent assortment states that alleles will separate independently of each other. This leads to the possibility of 223 different gametes forming. This results in a wide range of possible combinations of alleles in each gamete, based on how the chromosomes are separated during meiosis.

Why is meiosis referred to as reduction division? (Total 2 marks) • Meiosis is referred to as a reduction division because it is a type of cell division that reduces the number of chromosomes in each cell in half. The result of meiosis is that one diploid parent cell will create four genetically unique haploid daughter cells.

Draw and label the stages of meiosis I & II (Total 8 marks)

7 & 9 page 90 • 7. Gamete (sperm/egg cell) • 9 Risk of having a baby with Down’s syndrome increase with age because of the increased possibility of damage to the DNA of egg cell, so mothers over the age of 35 are encouraged to have a karyotype done.

1 -3 pg. 271 • 1 Natural selection must have a variety of offspring to allow for the survival of the fittest. If all organisms had same genetics, all would be susceptible to same diseases/predators, so genetic diversity is required for survival of species. • 2. a. Prophase I b. • 3. a Metaphase I • 3. b