3/25/2013

1

MEIOSIS Genetics Unit

LIVING ORGANISMS

2 types of cells:

Autosomal (body) cells have 2 copies of every gene

Gametes (sex) cells have 1 copy of every gene

CHROMOSOME NUMBER

Autosomal (body) cells

Humans 46

Gametes (sex) cells

Sperm & Egg cells

Female XX

Male XY

Willie Shoemaker, Wilt Chamberlain

~taken by Annie Liebowitz

3/25/2013

2

SEXUAL REPRODUCTION

Sexual reproduction provides variation due to the

shuffling and recombination of alleles during

meiosis and fertilization

99.99% of our DNA is the same

MEIOSIS is what makes us different!

WHAT IS INHERITED?

Genes – sequence of DNA, that codes for a protein,

that determines a trait

Genes are located on chromosomes (supercoiled

DNA)

Humans have 46, or 23 “homologous” pairs

Half from mom,

Half from Dad

WHY MEIOSIS?

Cell division to make gametes, or sex cells

Sperm in males

Eggs in females

Why would it be

different than Mitosis?

Diploid (2n) – contains a

pair of each chromosome;

mitosis

Haploid (n) – contains only

1 of each chromosome;

meiosis

3/20/13

Objective

Illustrate the steps to meiosis

Warm-Up

1. Multiply (x + 5)(x – 5)

2. How many copies of a gene do body (autosomal)

cells have? How many do sex (gamete) cells

have?

EX. 2N = 4

MEIOSIS I

Prophase I

Tetrads form – homologous

chromosomes pair

Cross-Over occurs –

homologous chromosomes

exchange part of their

chromatin

Produces new gene

combinations!

All other events same as

mitosis

3/25/2013

3

GENETIC VARIATION: CROSSING OVER

Homologous chromosomes group together and exchange pieces.

http://www.youtube.com/watch?v=D1_mQS_FZ0&feature=related

Meiosis I

Crossing

over

MEIOSIS I

Metaphase I

Tetrads line up randomly

on the metaphase plate

Increases variation!

Anaphase I

Homologous chromosomes

are pulled apart

Telophase I

Same events as in mitosis

EXCEPT… now the

daughter cells are Haploid

Prophase I Metaphase I Anaphase I Telophase I

and

Cytokinesis

Interphase I

Meiosis I

MEIOSIS I

MEIOSIS II Exactly like Mitosis; sister chromatids separate

Telophase II

and

Cytokinesis

Prophase II Metaphase II Anaphase II Telophase I and

Cytokinesis I

Meiosis II

3/25/2013

4

MEIOSIS

Four haploid cells (n) each have one chromosome.

Each cell has a different combination of

alleles.

GENETIC VARIATION: PRINCIPLE OF

INDEPENDENT ASSORTMENT

GENETIC VARIATION: RANDOM

FERTILIZATION

Many possible combinations of male and female

gametes.

HUMANS

There is a 50% chance that a particular gamete

will receive a maternal chromosome and a 50%

chance it will receive a paternal chromosome.

There are 23 pairs of chromosomes in human games

cells.

Number of possible combinations of maternal

and paternal chromosomes= 223

HUMANS

One human ovum (egg) represents approximately

8 million possible chromsome combinations.

Same is true for sperm. Thus, one sperm

fertilizing one egg results in 8 million x 8 million

recombination possibilities

3/25/2013

5

MEIOSIS SQUARE DANCE

http://teachersites.schoolworld.com/webpages/DC

urry/life.cfm?subpage=1151190

MEIOSIS FOLDABLE

MALES VS. FEMALES

Males produce 4 haploid gametes (sperm)

Females produce 1 haploid gamete (egg) + 3

“polar bodies”

3/22/13

Objective

Illustrate meiosis

Warm-Up

Subtract (5p2 – 3) – (2p3 – 3p2)

What happens during prophase I that allows for

genetic variation?

3/25/13

Objectives

Differentiate between mitosis and meiosis

Warm-Up

1. Simplify (2m2)(2m3)

2. What is the goal of meiosis I?

3. What is the goal of meiosis II?

AIMS is just

around the

corner!

MITOSIS VS. MEIOSIS

3/25/2013

6

MITOSIS VS. MEIOSIS

Mitosis Meiosis

Type of cell Body Cell Sex cell

Beginning cell Diploid (2n) Diploid (2n)

Ending cell Diploid (2n) Haploid (n)

# of cells produced 2 4

# of chromosomes

Same as parent cell Half as parent cell

Purpose

To make more body

cells

To make egg and

sperm

EX. 2N = 4

Haploid

Diploid

Different

Identical

Haploid

*Cross-Over

*Random

Orientation

MITOSIS VS. MEIOSIS

Meiosis:

Random orientation of chromosomes

Crossing over

Random fertilization

Independent assortment

Occurs in 2 phases

Creates 4 different cells (haploid)

Mitosis:

Creates 2 genetically identical cells (diploid)

MEIOSIS LAB

PRACTICE

A human has 23 pairs of chromosomes.

1. What is the diploid number?

2. What is the haploid number?

3. How many chromosomes are in an egg cell?

4. How many chromosomes are in a brain cell?

5. How many chromosomes are in a skin cell?

6. How many pairs of homologous chromosomes?

7. How many chromosomes in a cell produced by

mitosis?

8. How many chromosomes in a cell produced by

meiosis?

MEIOSIS

OREO COOKIE LAB PICTURES

3/25/2013

7

COOKIES

Blue: 1 Pair of Homologous Chromosomes

Pink: 1 Pair of Homologous Chromosomes

Red: Nuclear Envelope

Green: Meiotic Spindles

Brown: Centrioles

COMPLETED OREO COOKIE LAB Blue: 1 Pair

of Homologous Chromosomes

Pink: 1 Pair of Homologous Chromosomes

Red: Nuclear Envelope

Green: Meiotic Spindles

Brown: Centrioles

PROPHASE I METAPHASE I

Crossover

Random orientation

ANAPHASE I TELOPHASE I

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

3/25/2013

8

PROBLEMS WITH

MEIOSIS

Nondisjunction

THIS IS WHAT YOUR CHROMOSOMES LOOK

LIKE DURING MITOSIS.

Scientists take a

picture of the

chromosomes, like

this one, and cut

then them out…

…AND THEN GROUP THEM TOGETHER BY

SIZE.

Karyotype –

picture of

chromosomes

grouped as pairs

Humans-46

chromosomes;

23 pairs

KARYOTYPES ARE MADE MOST OFTEN

DURING PREGNANCY IN A PROCEDURE

CALLED AN AMNIOCENTESIS.

Amniocentesis – The baby’s amniotic fluid cells are taken from the womb through a needle, and a karyotype is made to look for disorders.

PROBLEMS SHOW UP WHEN THE BABY

HAS THE WRONG NUMBER OF

CHROMOSOMES.

What is wrong with this karyotype?

THIS KARYOTYPE SHOWS THAT THE

PERSON HAS THREE OF CHROMOSOME 21.

THIS CAUSES THE BIRTH DEFECT DOWN

SYNDROME.

3/25/2013

9

DOWN SYNDROME – “TRISOMY 21”

Children with Down Syndrome have a variety of problems, including mild mental retardation, heart

problems, and many other unusual physical characteristics.

Nondisjunction – When chromosomes

fail to separate during meiosis.

Down Syndrome occurs when two sister chromatids fail to separate during anaphase of meiosis II. As a result, some of the eggs or sperm produced have too many chromosomes.

When a cell ends up with three copies of a

chromosome instead of two, this is called

trisomy. If a cell has only one copy, this is

called monosomy.

OTHER NONDISJUNCTION DISORDERS:

Klinefelter’s Syndrome (XXY) Males w/an extra X

Small testicles, reduced fertility

Turner’s Syndrome (X__) Girls w/1 X

Short stature, swelling, sterility

Triple X Syndrome (XXX) Girls w/3 X chromosomes

Tall, but normal physical and sexual characteristics

Increased risk of learning disabilities

Slow speech and motor development

Edward Syndrome (trisomy 18) Only 5-10% of live births survive the

1st year of life

Many abnormalities in internal organs and external structures

NONDISJUNCTION IS MOST LIKELY TO HAPPEN

IN OLDER MOTHERS, BECAUSE THEIR EGGS

HAVE BEEN IN THEIR BODIES SINCE BEFORE

BIRTH, AND ARE MORE LIKELY TO HAVE

PROBLEMS DIVIDING.

3/25/2013

10

KARYOTYPING ACTIVITY

Map each karyotype and

determine if it is male or

female?

What is its genetic disorder?

Options:

Klinfelter’s syndrome (XXY)

Edward’s syndrome (trisomy 18)

Down syndrome (trisomy 21)

Trisomy 13