Meiosis

Objectives

• Describe how meiosis creates diversity organisms.

• List three ways sexual reproduction creates new combinations of alleles

• Explain the process of crossing over

Chromosomes

DAD

MOM

What are chromosomes made of?

• A chromosome is a long piece of DNA wound around proteins.

• A gene is a section of DNA that has instructions for the cell. A single chromosome contains thousands of genes.

• Somatic (body) cells have 2 sets of every gene.• Gametes (sex cells) only have 1 set of every

gene.• How does this happen?

Big Idea

• Sexual reproduction creates new combinations of alleles in three ways:

1. Chromosomes with the same genes cross-over2. Chromosomes separate randomly3. Fertilization combines DNA from two parents

Diploid (Body) Cell

• A cell that contains 2 sets of chromosomes, one from each parent.

• Body cells have two copies of each chromosome.

• Examples: ALL body cells- bone cells, skin cells, muscle cells

Diploid Cell

• Diploid cells (2n) have two copies of each chromosome. The two copies are called homologous chromosomes because they contain the same genes.

• Written as N• Human diploid cells (2N)= 46 chromosomes• They can have identical alleles (AA) or

different alleles(Bb).

Haploid (Gametes) Cell

• A cell that contains half the number of chromosomes.

• Written as N• Human haploid cells (N)= 23 chromosomes• Examples: Sex cells: Sperm and egg cells.

Example

• Fruit Fly• Diploid Cell has 8

chromosomes.• 2N= 8• Haploid Cell has half

the # of chromosomes.

• N=4

Practice1. An elephant has 56chromosomes in each

diploid cell.2N=?N=?2. A giraffe has 30 chromosomes in each haploid

cell.2N=?N=?

Practice3. A lion has 38 chromosomes in each diploid

cell.2N=?N=?

4. A zebra has 23chromosomes in each haploid cell.

2N=?N=?

Stand Up-Hand Up-Pair Up

• Write your own diploid/haploid math problem.

• Example: A cat has 25 chromosomes in a haploid cell. How many chromosomes are there in a diploid cell?

• Switch with a friend and solve.

• Diploid and haploid cells divide differently.

Question?

• What is the process that divides cells?

Question?

• What is the process that divides cells?

MITOSIS

QuestionsFind a partner and review…

• THINK-PAIR-SHARE

– What is mitosis?– The steps?– What is created?

• You have 1 minute…..Go!• Okay, time is up Sit down!

Questions

• What is mitosis? Division of somatic (body) cells

• The steps? Prophase, Metaphase, Anaphase, Telophase REMEMBER PMAT

• What is created? 2 INDENTICAL daughter cells, same number of chromosomes in each cell

• Mitosis

46 chromosomes

46 chromosomes 46 chromosomes

Genetically identical cells

Mitosis

• Only occurs in somatic (body) cells…• Are those the only cells we have?• Think about how traits are passed from parent

to offspring….

How are sperm cells made? How are egg cells made?

Meiosis

• It the division of sex cells.• In science, sex cells (sperm and egg) are called

gametes.

• Meiosis is when genes are separated into single sets to make gametes.

Replication

• The cell makes an exact copy of each chromosome. The copies stay attached together.

Meiosis I

• After the first cell division, there are two cells. Each cell contains a different set of chromosomes.

Meiosis II

• Chromosomes separate and cells divide again. Four haploid cells (n) each have a single set of chromosomes. Each cell has a different combination of alleles.

Big Picture:Meiosis DIPLOID

(2N)= 4

HAPLOID (N)= 2

Meiosis

• Type of cell division.

• Meiosis is a process of cutting the number of chromosomes per cell in half.

Meiosis

• Diploid(2N)turn into Haploid Cells (N)• sex cells turn into sex cells

How does Meiosis explain how we are different?

• 99.9% of our DNA is the same. • Meiosis helps make the differences!

How does meiosis make us different?

1. Crossing Over- During meiosis chromosomes line up and exchange pieces. – When they pull apart the pieces get separated. – This process if completely random and creates a

different chromosome every time.

Crossing Over

• Homologous chromosomes group together and exchange pieces.

Video• http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=related

• Please watch the short video…you must be logged into a teacher account

How does meiosis make us different?

2. During sexual reproduction there is no way of controlling what sperm and what egg cell will fertilize, the RANDOMLY SEPERATE.

Independent Segregation Independent Assortment Independent Segregation

AaBb

AB Ab aB ab

How does meiosis make us different?

3. Every combination of sperm and egg cell is different creating differences known as FERTILIZATION! Making babies is random!

EVERY EGG CELL AND SPERMCELL ARE DIFFERENT.

RANDOM FERTILIZATIONAn egg cell and a sperm cell join together, combining DNA to

make a new diploid (2n) cell. Half of a child’s DNA comes from mom, the other half from dad, creating a completely new combination of alleles.

Think:

Review

• What are three ways sexual reproduction creates new groups of alleles?

True or False?• Meiosis produces two genetically identical

daughter cells.• Meiosis creates two daughter cells.• Meiosis takes place in ONLY body cells.• Meiosis occurs in organisms that reproduce

sexually.• Meiosis occurs in ONLY gamete cells.• Mitosis results in two identical daughter cells.• Meiosis creates four different haploid cells.

What step of meiosis is this?

Stop

• You are done with this PowerPoint today!

Meiosis Book Scavenger Hunt

• Open book to page 276-277 (fig. 11-15).• Meiosis Scavenger Hunt

–Answer questions.–Draw and describe each step!