Aim:Why is meiosis important?

• Do Now: Label each phase of cell division (IPMATC)

So, what is the difference between

mitosis and meiosis?

Mitosis• 2 identically

diploid cells (same as parent)

• Makes cells used for growth, repair, development & asexual development

Meiosis - 4

genetically different haploid cells

- Makes cells for Sexual reproduction

- Genetic info is exchanged/rearranged.

Meiosis I

• Contains all phases of mitosis

• Homologous chromosomes separatedI P M A T CI

PrayMoreAt The Church

Interphase I• This is where

the chromosomes are duplicated, replicated, or copied.

Eukaryotic Cell Cycle

Prophase I• Chromosomes

condense. The nuclear envelope breaks down.

“Crossing Over” occurs.

• “Crossing Over”-Chromatids exchange genetic material

Metaphase I• Pairs of

homologous chromosomes move to the cell’s middle.

Metaphase = Move Middle

Anaphase I• Homologous

chromosomes move to the cell’s opposite poles.

Telophase I• Chromosomes

gather at the poles.

• The cytoplasm divides.

What about cytokinesis?• In cytokinesis, the nuclear membrane is

reforming around each new cell.

Cytokinesis happens after the division of

chromosomes and cells are made.

Meiosis IINO

INTERPHASE and NO

REPLICATION of

chromosomes!!

P M A T

Prophase II• A new

spindle forms around the chromosomes.

Metaphase II• Chromosomes

line up at the middle.

Anaphase II• Centromeres

divide, and chromatids move to opposite poles.

Telophase II• A nuclear

envelope forms around each set of chromosomes. The cells divide.

What do we end up with at the end of meiosis?

• 4 haploid cells

Answer to Aim:Why is meiosis important?

• Allows genetic information from 2 parents to combine to form offspring that are different from both parents.

Aim: Why is genetic variation so important?

Do Now: Copy the chart on the side board. Fill

in the blank areas about Mitosis and Meiosis.

• Mitosis vs Meiosis

What is a homologous chromosome?

Chromosomes that have the same sequence of genes, that have the

same structure, and that pair during meiosis.

Chromatid is a single strand of chromosomes.

Chromosomes are 2 chromatids together, made of condensed chromatin.

Centromere is where the two chromatids meet.

Chromatin are ribbon-liked structures of DNA and histone proteins.

Label the chromosome

• “Crossing Over”-Chromatids exchange genetic material(Occurs during Prophase I in Meiosis.)

Why is crossing over important?

• With out it, every one will look the same!

• New combinations of genetics

• Variety

Crossing Over Video

• Crossing over is one of the several processes that lead to genetic variation. • The

rearranging of genetic info leads to genetic variation in offspring.

Genetic Variability• Natural

Selection• Allows for

adaptation and ultimate survival of certain organisms when the environment changes.

3 Ways to Promote Genetic Variation

• 1. Crossing over• 2. Independent

Assortment• 3. Random Union of

sperm and egg

• Tetrad- a pair of homologous chromosomes.As the tetrads

form, the chromosomes exchange parts of their chromatids during the process of crossing over.

DisjunctionTetrads separate equally as a result of a process known as disjunction.

During what phase does this

occur?

Anaphase

Non-disjunctionTetrads separate UNEQUALLY.

• Uneven # of chromosomes will result in the gametes.

If these gametes are fertilized, the offspring will have either 1 too many chromosomes or 1 too few chromosomes.

Example of Result of non-disjunction: Down’s Syndrome

Down’s Syndrome• The non-

disjunction of the 21st chromosome.• Occurrence: One in every 700 American children.

Answer to Aim:So why is genetic

variability so important???

AlleleDifferent forms of the same gene. For example,

humans have genes for eye color. However, there exists an allele for brown eyes and an allele for blue eyes.

Responsible for Dominant or Recessive

Traits

• Each individual has 2 allele’s, one on each chromatid.

• You can have 2 of the same alleles or 2 different alleles.

• The dominant allele will determine what trait you have.

• Dominant Traits:

The traits that come out in the individual.* Only 1 dominant trait is needed.

Expressed as the capital letter.

• Recessive Traits:The “weaker” trait that we do not see unless there are 2 of them.

Expressed as the lower case letter.AA, Aa, aA aa

What color eyes does each individual have?

Activity in text page 253 on crossing over.