Reproduction Ch.7

Asexual Reproduction • One parent • The prokaryotic chromosome is a ring of DNA

• Binary fission-bacteria– Cell splits in two – Produces two genetically identical daughter cells, exactly

like the original– Identical offspring (clones)

• Different kinds– Binary fission - bacteria– Budding – yeast, sponges– Fragmentation – sea stars

Binary Fission

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1. Attachment of chromosome toa special plasma membranesite indicates that thisbacterium is about to divide.

2. The cell is preparing for binaryfission by enlarging its cell wall,plasma membrane, and overallvolume.

3. DNA replication has producedtwo identical chromosomes.Cell wall and plasma membrane begin to grow inward.

4. As the cell elongates, thechromosomes are pulled apart.Cytoplasm is being distributedevenly.

5. New cell wall and plasmamembrane has divided thedaughter cells.

chromosome

cell wall

plasmamembrane

cytoplasm

SEM 2,345X© Dennis Kunkel Microscopy, Inc./Visuals Unlimited

Sexual Reproduction

• Two parents

• Sexual Reproduction– Cell splits twice

– Produces four daughter cells, genetically different from the parent and each other –meiosis

• Increases genetic variation

Haploid vs. Diploid

HAPLOID• 1n

• One copy of each chromosome

DIPLOID• 2n

• Two copies of each chromosome

• Homologous chromosomes-similar in size, shape and genetic content

Homologous Chromosomes

6

a. Sister chromatids

duplicationduplication

chromosomematernal chromosome

b.

paternal chromosomehomologous pair

nonsisterchromatids

centromere

kinetochore

chromosome

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a: © L. Willatt/Photo Researchers, Inc.

• Homologous chromosomes-similar in size, shape and genetic content

How does Meiosis work?

• Two stages, Meiosis I and Meiosis II• Meiosis I: Homologous chromosomes are separated

from each other– Chromosomes replicate first, so each chromosome has a

“twin”– Example: this is where the X and Y chromosomes separate

from each other and go into different cells• X chromosome is a double X, Y chromosome is a double Y

– Two new cells have 23 duplicated chromosomes

• Crossing over-during Prophase I– Homologous chromosomes exchange genes while

they’re lined up next to each other– You don’t inherit entire chromosomes from your

parents—only pieces of them– Creates more variation and diversity within a

species– Important for natural selection and evolution

2. Crossing Over

• Homologous chromosomes swap genes

• Happens during Prophase I• Increases genetic variation

even more

• Meiosis II– Just like mitosis, but with 23 chromosomes instead

of 46• Remember these are 23 duplicated chromosomes

– Starts with 2 cells, which divide into 4– In males: all 4 cells become sperm– In females: one cell becomes an egg, the other 3

die

What’s a gamete?• Female gamete = egg (ovum); produced in

ovaries; oogenesis• Male gamete = sperm; produced in testes;

spermatogenesis

Overview of Meiosis

14

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2n = 4

• Diploid parent cell

Overview of Meiosis

15

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centrioles nucleoluscentromere

MEIOSISIHomologous pairs

synapse and then separate.

2n = 4 2n = 4

chromosomeduplication

Overview of Meiosis

16

centrioles nucleoluscentromere

sister chromatids

MEIOSISIHomologous pairs

synapse and then separate.

MEIOSISIISister chromatids separate,

becoming daughter chromosomes.

n = 2

Four haploiddaughter cells

Second divisionFirst division

synapsis

2n = 4 2n = 4

chromosomeduplication

n = 2

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Comparison of Mitosis & Meiosis

MitosisResults in 2 daughter

cellsDaughter cells are

diploidDaughter cells are

identical to each other and to parent

MeiosisResults in 4

daughter cellsDaughter cells are

haploidDaughter cells are

different from each other and from parent

Mitosis v. Meiosis

Mechanisms for Increasing Genetic Variation

1. Independent AssortmentEach of the 23 pair of chromosomes separates into gametes independently (223different gametes can

form=8 million possible gamete combinations)

2. Crossing Over

• Homologous chromosomes swap genes

• Happens during Prophase I• Increases genetic variation

even more

3. Random Fertilization

For humans - 23 pairs of chromosomes:Fertilization of an egg by a sperm is

random# possible outcomes 223 (egg) X

223(sperm)= 64 trillion

Determination of Sex

• Females: two X chromosomes (XX)• Males: XY

• All offspring will receive an X chromosome from the mother and either an X or a Y from the father• The X chromosome is large and contains over 1,000 genes

• The Y chromosome is tiny and contains less than 200 genes