Meiosis - is derived from the Greek word is derived from the Greek word meionmeion, which means to , which means to reduce. reduce.

6.1 - Meiosis

• Meiosis is an important aspect of sexual reproduction (2 parents; offspring not genetically identical)

• The process of meiosis also shuffles genetic information (DNA) and results in variation in genes. Therefore producing a variation in offspring

Diploid, Haploid & Chromosomes Normal body cells have a diploid (2n) chromosome number,

meaning chromosomes occur in pairs (46 chromosomes). In humans, the male and female each contribute half (haploid

(n)), 23 chromosomes.

Gametes & Fertilization Gametes – specialized cells necessary for reproduction

Females – egg, Males – sperm When fertilization takes place:

23 (egg,) + 23 (sperm) = 46 (zygote)

The zygote goes on to develop into an embryo, and on into a complete individual.

Meiosis

46 chromosomes

23 chromosomes

23 chromosome

23 Chromosomes

occurs in reproductive organs and only produce gametes with half the chromosome compared to body cells

23 pairs, 46 chromosomes

23 Chromosomes

Meiosis I - Homologous Pair Separate

Prophase I - Homologous chromosomes pair up, and non-sister chromatids exchange genetic material. This process is called crossing over.Metaphase I - Homologous chromosomes pair up at the equator. (one chromosome from each parent)Anaphase I - Homologous chromosomes separate and are pulled to opposite poles by the spindle fibres. (sister chromatids move to opposite poles )Telophase I - One chromosome from each homologous pair is at each pole of the cell.Interkinesis - results in 2 daughter cells with ½ # of chromosomes. This stage between cell divisions the cell will grow and make proteins as in interphase of mitosis. Unlike interphase in mitosis, there is no replication of DNA during this stage.

Crossing over

Is when sister chromatids exchange DNA during the Prophase

Crossing over creates a number of genetic variations in gametes

Independent Assortment Independent Assortment

A shuffling of the 23 homologous pairs, in which there are two possibilities for how chromosomes will sort themselves into daughter cells.

“independent assortment” leads to variations in species as there are more than 8 million combinations possible for the 23 pairs in any egg or sperm cell.

70 trillion different zygotes are possible from one sperm and one egg cell.

Meiosis II Meiosis II is like mitosis because in both processes the chromatids of each chromosome are pulled

to opposite poles. Each daughter cell inherits one chromatid from each chromosome resulting in four haploid cells

each with half the N of chromosomes.

- Separates sister chromatids

Prophase II - There is one chromosome of the homologous pair in each cell.Metaphase II - The X-shaped chromosomes form a single line across the middle of the cell.Anaphase II - Sister chromatids move to opposite poles of the cell. Once they separate, each sister chromatid is considered to be a chromosome.Telophase II - Spindle fibres begin to disappear, and a nuclear membrane forms around each set of chromosomes.Cytokinesis - In cytokinesis, the two daughter cells are separated.

Male and Female Gametes In males, all 4 cells resulting from meiosis develop into sperm In females, 1 cell gets most of the cytoplasm and becomes the egg

while other 3 die, due to unequal cytoplasm and organelles

Differences in Differences in Mitosis and Meiosis Mitosis and MeiosisMitosisMitosis # daughter chromosomes = # daughter chromosomes =

# parent chromosomes# parent chromosomes Two identical daughter Two identical daughter

cellscells Occurs in all cells except Occurs in all cells except

reproductive cellsreproductive cells Produces diploid cellsProduces diploid cells Diploid cells = 2nDiploid cells = 2n

MeiosisMeiosis # daughter chromosomes = # daughter chromosomes =

half the # parent chromosomeshalf the # parent chromosomes Four non-identical daughter Four non-identical daughter

cellscells Occurs only in reproductive Occurs only in reproductive

cellscells Produces haploid cellsProduces haploid cells Haploid cells = nHaploid cells = n

Review of Mitosis vs.

MeiosisComparison animation

Chromosome Mutation

Chromosome Mutations

Sometimes occur spontaneouslyChromosome changes during meiosis can cause changes in the genetic information.

Occur because of mutagensChromosome changes, sometimes leading to genetic disease or death, can be cause by mutagens such as radiation or chemicals

Can occur if there is failed separation of chromosomes in meiosisFailed separation means that a gamete may end up with no chromosome or too many of a chromosome - Zygotes that result from these gametes rarely survive, and if they do, they will have serious genetic disorders

Types: inverted, deleted, duplicated or moved to another spot

Genetic Disorders

By using a karyotype, geneticists can study an individual’s chromosomes

Chromosomes are identified and paired by size, centromere location and banding patterns.

Certain genetic disorders orsyndromes occur when thereare specific chromosomes extra or missing

Down syndrome usually occurs when there is an extra 21st chromosome

Sexual Sexual Reproduction Reproduction

Chapter 6.2Chapter 6.2(18 Ways to make a baby)

Sexual reproduction brings non-identical gametes together to form a new organism - it occurs in 3 stages:

Mating - the process by which gametes are brought together at same place and same time

Fertilization - process by which egg and sperm join to form a new organism (zygote)

Development - the process by which an organism develops as an embryo

Methods of Fertilization

External or Internal Fertilization In order for either of these methods to produce a successfully

developing embryo, certain conditions must be met:

1. enough nutrients.

2. temperature must not be too cold or too hot.

3. Proper moisture so that embryodoes not dry out.

4. protectedfrom predators and itemsin environment that canpotentially harm it.

External Fertilization In external fertilization, sperm and egg join outside parents

Advantages Very little energy required to mate Large numbers of offspring produced Offspring can be spread widely in

the environment - less competitionbetween each other and parents

Disadvantages Many gametes will not survive Many eggs will not be fertilized Offspring are often not protected

by parents, so many of them die Frog Eggs

External Fertilization Example

Internal Fertilization In internal fertilization, sperm and egg join inside parents,

embryo is nourished inside mother

Advantages Embryo protected from predators Offspring more likely to survive,

as many species will protect theiryoung while they mature

Disadvantages Much more energy required to find mate Fewer zygotes produced, resulting in

fewer offspring More energy required to raise and care for offspring

Pollination Most plants transfer male gametes as pollen. Pollen can

be carried by wind or other organisms.

Human Human Reproductive Reproductive System System

Once ovulated, the egg is only viable for about 24 hours if not fertilized.

Fertilization occurs in the top one third of the Fallopian tubes.

After fertilization, the zygote continues to move down the fallopian tube. As it does, development begins. It takes the embryo 7 - 12 days to get to the uterus and implant in the endometrium.

Embryonic Development Embryonic development is the early development of an

organism - in humans, it is the first three months after fertilization

Stages End of the first week - ball of cells

called morula By end of second week it is a

hollow ball called a blastula Cells at this stage are stem cells,

and have the ability to developinto any kind of cell

3-4 weeks embryo isknown as a gastrula

develops 3layers: ectoderm (skin, nerves),

mesoderm (muscles, bones), andendoderm (lungs, liver, digestivesystem lining)

“more blasting gas”

Gastrula

Embryonic DevelopmentEmbryonic Development

Period of the Embryo For the first three months,

organogenesis occurs and it is termed an embryo.

This is a 6 week embryo.

Can you guess which embryos are: chicken, fish, human, rabbit,

salamander, and tortoise

Fetal Development The cell layers now differentiate into the organs and

tissues of a baby - this is divided into 3 trimesters.

First Trimester (0-12 weeks) Organ systems begin to develop

and form. Bone cells form.

Second Trimester (12-24 weeks) Rapid growth from 12-16 weeks.

Third Trimester (24+ weeks) Continued growth, especially of brain. Fat begins

to deposit at 32 weeks to keep baby warm at birth.

This is a 4 month fetus.

The third trimester, period of the baby, is marked by growth and maturation of all organs and organ systems.

The baby is capable of life outside the womb during this period.

Review: Life cycle of animals

Sexual Reproduction: Advantages and Disadvantages

6.3 Assisted Reproductive Technologies

Infertility is the inability of a couple to have a baby Assisted reproductive technologies involve removing

eggs from the woman, fertilizing them, and returning them to the uterus.

Reproductive technologies help childless couples, but carry a higher risk of birth defects. Also creates the problem of “unwanted” embryos. What should be done with them?

Types of Assisted Reproductive Technologies1. Artificial Insemination - donor sperm is placed in the female.2. In vitro fertilization (IVF) - egg and sperm are collected and

fertilization takes place in a dish. Embryo(s) then placed in female’s uterus.

3. Gamete intrafallopian transfer (GIFT) - eggs and sperm are collected, mixed, then injected into the woman’s fallopian tubes.

4. Intracytoplasmic Sperm Injection (ICSI) - a single sperm is injected directly into an egg.