Warm Up

• Vocab…– Diploid Cell– Haploid Cell– Homologous Chromosomes– Zygote

– In each of the 4 blocks – look up the terms (above)

I told you before that we categorize human cells in two categories: body cells and sex cells, what do

you THINK makes them so different?

HOW MANY CHROMOSOMES ARE FOUND IN EACH HUMAN BODY CELL?

• In the body cells of animals and most plants, chromosomes occur in pairs

•In each pair, one you inherited from your mom, the other from your dad.

Diploid Cells

From Mom

From Dad

• The two chromosomes of each pair are called homologous chromosomes.

• Each pair of homologous chromosomes has genes for the same traits.

Homologous Chromosomes

• A cell with two of each kind of chromosome is called a diploid cell.

• ALL of your body cells are diploid.

Diploid Cells

• Body cells (diploid) are different from sex cells. • Each person can only pass on ONE of their homologous

chromosomes to their young.

HUMANS = 46 CHROMOSOMES

Each parent passes 23 to their young, so that when the sperm and egg combine- it

will equal 46 Chromosomes.

• So, sex cells CANNOT be diploid.•A cell containing only one of each kind of

chromosome is called a haploid cell.

Haploid Cells

Sexual Reproduction

Zygote

Chromosome Numbers of Common OrganismsOrganism Body Cell

Fruit fly 8

Garden pea 14

Corn 20

Tomato 24

Leopard Frog 26 13

Apple 34

Human 46

Chimpanzee 48

Dog 78

Adder’s tongue fern 1260

Sex Cell

4

7

10

12

17

23

24

39

630

Diploid and Haploid Cells

Since they have different amounts of chromosomes, the way they are made differ.

● Body cells are made by the division process MITOSIS.

● Sex cells are made by the division process MEIOSIS.

Before a cell can divide by either method, it must first prepare for cell division.

The cell cycle (or entire life cycle of the cell) consists of:

INTERPHASE: The longest/busiest stage in a cell’s life

1. Chromosomes (DNA) replicates

2. Cell grows bigger

CELL DIVISION: Mitosis or Meiosis

CYTOKINESIS: When the cytoplasm divides and the cell completely separates.

(RECAP)Mitosis: cell division that makes BODY CELLS.

It starts with a DIPLOID cell 46Chromosomes Replicate

92

4646

It ends with TWO IDENTICAL DIPLOID cells

Meiosis: cell division that makes SEX CELLS.

It starts with a DIPLOID cell 46Chromosomes Replicate

92

46 46

23 23 23 23It ends with FOUR HAPLOID cells (not identical)

Meiosis I

Meiosis II

MeiosisMeiosis I I

MeiosisMeiosisIIII

The kind of cell division that produces sex cells (haploid) containing half the number of chromosomes as a body cell, is called meiosis.

What do you notice about the chromosomes in each cell betweenMeiosis I and Meiosis II?

• These haploid cells are called sex cells— gametes.

• Male gametes are called sperm.

• Female gametes are called eggs.

Meiosis

XY

XX

• When a sperm fertilizes an egg, the resulting zygote once again has the diploid number of chromosomes.

Meiosis

There are 2 stages of MEIOSIS-- Meiosis I and

Meiosis II

PI MI AI TI result in 2 diploid daughter cells

PII MII AII TII result in 4 haploid daughter cells

1st: chromosomes are replicated (interphase)

Prophase I – chromosomes appear

Metaphase I – chromosomes line up in the center in homologous pairs (called tetrads)

Anaphase I – homologous chromosomes are pulled apart (still look like an “X”)

Telophase I – 2 haploid daughter cells

Prophase II – chromosomes still present

Metaphase II – chromosomes line up in the center (no longer in homologous pairs)

Anaphase II – chromosomes are pulled apart (centromeres split)

Telophase II – 4 haploid daughter cells

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Combinations and Crossing Over23 chromosomes can

segregate in 223 ways → More than 8 million possible

combinations

Crossing-Over Crossing-Over

In Meiosis:Before the sex cells are created, Homologous chromosomes actually break and exchange traits- in a process known as crossing over.

Crossing-OverCrossing-Over

Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time.

This causes the traits found in each gamete varied. This is why siblings of the same parents can look so different from

each other.

• The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction.

•This would occur when homologous chromosomes are suppose to separate from each other.

Nondisjunction

• Down syndrome is the only autosomal trisomy in which affected individuals

survive to adulthood.

Down syndrome: Trisomy 21

• This chart of chromosome pairs is called a karyotype, and it is valuable in identifying unusual chromosome

numbers in cells.

A cell with two of each kind of chromosome is __________.

Question 1

D. polyploid

C. biploid

B. haploid

A. diploid

The answer is A. The two chromosomes of each pair in a diploid cell are called homologous chromosomes. Each has genes for the same traits.

Homologous Chromosome 4

a A

Terminal Axial

InflatedD

Constrictedd

Tall

T

Short

t

Why do organisms need meiosis for

reproduction?

Question 2Meiosis

MeiosisSperm Cell

Egg Cell

Haploid gametes(n=23)

Fertilization

Diploid zygote(2n=46)

Mitosis and Development

Multicellulardiploid adults

(2n=46)

Meiosis is cell division that produces haploid gametes. If meiosis did not occur, each generation would have twice as many chromosomes as the preceding generation.

Meiosis

MeiosisSperm Cell

Egg Cell

Haploid gametes(n=23)

Fertilization

Diploid zygote(2n=46)

Mitosis and Development

Multicellulardiploid adults

(2n=46)

Trisomy usually results from _______.(TX Obj 2; 6C)

Question 2

D. twenty-two pairs of chromosomes C. nondisjunction B. incomplete dominanceA. polygenic inheritance

The answer is C.