Chapter 4 Introduction

Lesson 1 Sexual Reproductionand Meiosis

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• Sexual reproduction is a type of reproduction in which the genetic materials from two different cells combine, producing an offspring.

• The female sex cell is called an egg

• The male sex cell is called sperm

What is sexual reproduction?

What is sexual reproduction? (cont.)

During a process called fertilization, an egg cell and a sperm cell join together to create a zygote.

• Organisms that reproduce sexually form body cells and sex cells.

• Diploid cells are body cells that have pairs of chromosomes.

Diploid Cells

Diploid Cells (cont.)

Different organisms have different numbers of chromosomes.

MOM DAD

Diploid Cells (cont.)

Pairs of chromosomes that have genes for the same traits arranged in the same order are called homologous chromosomes.

Because one chromosome is inherited from each parent, the chromosomes are not identical.

Egg

46 chromosomes

46

sperm92

chromosomes!

Zygote

• Both sperm and eggs have only ½ the normal number of chromosomes

• Human sperm and eggs have 23 chromosomes

• No matching pairs!!!

Haploid cells are cells that have only one chromosome from each pair of chromosomes.

Haploid Cells

haploid

from Greek haploeides, means “single”

Haploid Cells

In meiosis, one diploid cell divides and makes four haploid sex cells.

• Meiosis occurs during the formation of sex cells only.

• Sex cells (sperm & egg cells)

– Different from ordinary body cells

– Each have 1 chromosome from each homologous pair

• Only 23 chromosomes in human sex cells

Egg

23 chromos

omes

23

sperm46

chromosomes!

Zygote

• Meiosis forms sex cells with the correct haploid number of chromosomes.

• When haploid sex cells join together during fertilization, they make a diploid zygote, or fertilized egg.

• Meiosis also creates genetic variation by producing haploid cells.

Why is meiosis important?

The fertilized egg, formed when sex cells join together, divides by mitosis to create a diploid organism.

Why is meiosis important? (cont.)

• During mitosis and cell division, a body cell and its nucleus divide once and produce 2 identical cells.

• The two daughter cells produced by mitosis and cell division have the same genetic information.

How do mitosis and meiosis differ?

• During meiosis, a reproductive cell and its nucleus divide twice and produce 4 cells—each with half the # of chromosomes as the parent cell.

• Meiosis forms sex cells used for sexual reproduction.

How do mitosis and meiosis differ? (cont.)

Mitosis: The nucleus splits once

resulting in 2 new cells

Meiosis: The nucleus splits twice resulting in 4 new cells

• Sex chromosomes carry genes that determine whether the offspring is male or female

• Humans have 46 chromosomes (23 pairs)

• Pair number 23 determines your sex

• XX= female

• XY= male

• Sexual reproduction produces offspring that have a new combination of DNA.

• Genetic variation occurs in all organisms that reproduce sexually.

• Due to genetic variation, individuals within a population have slight differences that might be an advantage if the environment changes.

Advantages of Sexual Reproduction

Advantages of Sexual Reproduction (cont.)

Selective breeding has been used to develop many types of plants and animals with desirable traits.

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• Organisms have to grow and develop until they are mature enough to produce sex cells.

• Searching for a mate takes time and energy and might expose individuals to predators, disease, or harsh environmental conditions.

Disadvantages of Sexual Reproduction

• Fertilization occurs when an egg cell and a sperm cell join together.

• Organisms produce sex cells through meiosis.

• Sexual reproduction results in genetic variation among individuals.

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