Chapter 6

Meiosis and Mendel

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6.1 Two groups of Cells

1) Somatic Cells – body

cells

Can NOT pass DNA on to

offspring

2) Gametes – sex cells

Develop from germ cells in

reproductive organs

DNA IS passed on to offspring

Somatic Cells

Are diploid

Full set of

chromosomes

Diploid number

=2n

Ex: 2n = 46

(humans)

= 23 pairs

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Sex Cells (Gametes)

Egg (female)

Sperm (male)

Haploid

Contain half the number of chromosomes

NO pairs!

Haploid number = n

Ex: n = 23 in humans

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Fertilization

Fusion of egg and

sperm forms a zygote

Nuclei fuse and form

one

= diploid cell

1/2 DNA from each

parent

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Chromosome Numbers

Each organism has a characteristic #

Not related to complexity

Consistent within a species

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Homologous Chromosomes

= a pair of

chromosomes

One from each

parent

Similar in:

size

shape

genetic information

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Types of Chromosomes

Autosomes

Chromosomes that do

not determine gender

Sex chromosomes

Determine the sex of

individual

Only two sex

chromosomes

X and Y

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Sex Chromosomes

Male

XY

Y chromosome is

smaller

Determines gender of

zygote

Female

XX

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Human Chromosome Number

46

Essential for normal development

and function

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Down Syndrome

Trisomy 21

Extra copy of

chromosome 21

= Down Syndrome

Caused by

nondisjunction

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Risk of Down Syndrome

Increases with the

mother’s age

All eggs present in

ovaries at birth

Can accumulate

damage

Women over 35

Advised to get prenatal

testing

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Nondisjunction

Gametes form but

chromosomes fail to

separate properly

One gamete gets

both chromosomes

Now 2 in one gamete

and 0 in other

6.2 Meiosis

Form of nuclear division that creates 4

haploid cells from 1 diploid cell

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Mitosis (Review)

Type of cell division

Divides the nuclear

material

Maintains

chromosome #

Forms diploid cells

Forms somatic cells

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Meiosis

From Greek meioun = to

make smaller

Type of cell division

Divides nuclear material

Cuts chromosome # in

half

Forms haploid

reproductive cells

Gametes

Spores

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Meiosis – Sex Cells

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Meiosis

DNA replicated

before the two

divisions of

nucleus

Starts w

homologous

chromosomes

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What’s the difference????

Homologous

chromosomes vs. sister

chromatids?

Chromosome = name

given during anaphase of

Mitosis or anaphase of

Meiosis II

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Homologous Chromosomes

Two separate chromosomes

One from mom, one from dad

Very similar to each other

Same features and functions

Some instructions may be slightly

different

Same genes

NOT exact copies!

Divided in Meiosis I

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Sister Chromatids

Duplicated chromosomes that

are attached by a centromere

Exact copies of each other

Separated in anaphase of

Meiosis II

OR

Separated in anaphase of

Mitosis

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Gene for eye color

Gene for hair color

Before Meiosis . . .

Remember – DNA has already been

copied!!!

Just like what happens before Mitosis!

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Meiosis has 2 Stages:

Meiosis 1

Prophase 1

Metaphase 1

Anaphase 1

Telophase1 and cytokinesis

Meiosis 2

Prophase 2

Metaphase 2

Anaphase 2

Telophase 2 and cytokinesis

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Meiosis 1

Prophase 1

Chromosome become visible

Nuclear envelope breaks down

Centrioles move

Spindle fibers appear

Homologous chromosomes pair

Crossing over occurs

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Prophase 1

UNLIKE mitosis, homologous

chromosomes line up next to each other

Called tetrads

Process = synapsing

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Prophase 1

When hanging out so

close, sometimes

parts are swapped

= Crossing Over

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More Crossing Over

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Looking at Real Tetrads:

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Crossing Over

Increases genetic variation (diversity) . . .

Why is this important in the long haul?

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Metaphase 1

Homologous

pairs moved to

equator by

spindles

Random

arrangement

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Anaphase 1

Homologous chromosomes

separate

Move to opposite poles by

spindle fibers

Each chromosome still

composed of 2 chromatids

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Telophase 1 and Cytokinesis

Chromosomes gather at poles

Cytoplasm divides

2 new cells formed

One chromosome from each pair in each

*Chromosomes do NOT replicate bw

meiosis 1 and meiosis 2

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Beneath the microscope:

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Result of Meiosis 1

Homologous

chromosomes

separated

Two NON-identical

daughter cells

formed

Chromosome # cut

in half

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Ponder . . .

Why are chromosomes NOT replicated between

meiosis I and meiosis 2 ?

Think about it . . .

Keep thinking . . .

Got it!

Makes sense, doesn’t it?!

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Meiosis 2

Starts with 2

daughter cells

formed in Meiosis 1

Similar to mitosis

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Meiosis 1

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Prophase 2

Nuclear membrane

breaks down

Centrioles move

Spindles assemble

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Metaphase 2

Chromosomes lined up at equator

Centromeres attached to spindles

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Anaphase 2

Centromeres divide

Sister chromatids move to opposite poles

(Now called chromosomes)

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Telophase 2 and cytokinesis

Nuclear envelope reforms

Spindle breaks down

Cytokinesis occurs

Two daughter cells formed for each cell

Total: 4 haploid cells

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Under the Microscope

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Under the Microscope:

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Thinker. . .

What is the major difference between

metaphase I and metaphase II?

Metaphase I = pairs of chromosomes

line up. Metaphase II = chromosomes

are NOT paired

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Gametogenesis

Production of

gametes

2 types:

Spermatogenesis

Oogenesis

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Spermatogenesis

Forms sperm

Cells move quickly

Small

Compact

Flagellum

Main contribution = DNA

Yields 4 haploid sperm

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Oogenesis

Forms Ova (eggs)

Begins before birth

DNA, cytoplasm,

organelles, etc

Yields 4 cells

One egg

3 polar bodies

smaller, break down, die

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Meiosis: The Continuation of Life

Meiosis - The Continuation of Life - 11:48

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Basic Vocabulary

Trait

Inherited characteristics

Heredity

The passing on of traits from parents to

offspring

Genetics

The study of heredity

Cross

Mating of two organisms

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6.3 How it all Began. . .

Gregor Mendel – 1860s

Austrian monk

“Father of Genetics”

Studied pea plants

Pisum sativum

Developed rules to predict

patterns of heredity

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Mendel’s Background

Mathematician

Child of peasants

Knew a lot about agriculture

Studied theology

Became a priest

Went to U of Vienna

Studied science and math

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Mendel’s Background

Mendel repeated

experiments of T. A. Knight

British farmer

Crossed pea plants

Mendel counted offspring

Analyzed data

Mendel’s work rediscovered

in 1900

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Useful Features in Pea Plants

Small

Grows easily

Mature quickly

Lots of offspring

Traits occur in two forms

Enclosed flowers

Tend to self-pollinate

Can control crosses

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Genetic Humor

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Before Mendel

“Blending Hypothesis” Tall X Short = Medium

Did NOT agree with

Mendel’s findings!

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Mendel’s Experiment

Monohybrid cross

Cross one pair of contrasting traits

Ex:

Tall x Short

Purple x White

Round x Wrinkled

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Mendel’s Experiment

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Mendel’s Experiment

Used Purebreds = True

breeding

Homozygous

Always produce offspring

with same characteristics

P (parental) generation

First 2 individuals in a cross

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Mendel’s Experiment

F1 generation

First filial

Offspring of P

generation

Showed just one

form of the trait

Filial = “Son”

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Mendel’s Experiment

F2 generation

Second filial

generation

Offspring of the

F1 generation

Showed both forms

of the trait again

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Mendel’s Law of Segregation

Individuals get two copies of “heritable factors” (genes)

Alternative versions of genes (alleles)

One from each parent

Dominant or recessive

Gametes carry only one allele for each gene

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Gregorian Chant . . .

Mendel song 3:30

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6.4 Alleles

Different versions of gene

Everyone has 2 for each trait

One from each parent

Together they code for expression of gene (trait)

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Dominant Alleles

Expressed

If present, shows up!

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Recessive Alleles

Not expressed if dominant allele is

present

Shows up only if both alleles are

recessive

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Genotype

Genetic makeup of organism

Actual alleles inherited

Letters represent alleles

Capital letters = dominant

Lower case = recessive

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Homozygous Genotype

Two similar alleles

Two dominant = two capital letters

Two recessive = two lower case letters

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Heterozygous Genotype

Two different alleles

One dominant

One recessive

One capital letter &

one lower case letter

Dominant always

written 1st

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Comparing Genotypes

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Phenotype

Physical

appearance

Determined by

alleles

Genotype

determines

Phenotype

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6.5 Punnett Squares

Grid system for

predicting outcome

of a cross

Considers all

possible gamete

combinations

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Punnett Squares

Combine alleles

“Multiply” and

fill boxes in

Shows all

Possible

genotypes of

offspring

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Practice Punnett Squares

Determine genotypes

and phenotypes

Monohybrid cross

One trait crossed

Ex:

Pure short pea plant x

hybrid pea plant

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BABY STEPS:

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Practice

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Cross:

homozygyous

recessive individual

with blue eyes with

heterozygyous

individual with brown

eyes

Probability

Likelihood that an event

will happen

Predicts average

number of occurrences

Practice:

Determine the ratios

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Testcross

• Used to figure out

genotype of an

organism

• Must cross

unknown organism

with homozygous

recessive

individual

• WHY?!!!

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Dihybrid Cross

Crossing two different traits

Each gene still represented by 2 alleles

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Law of Independent Assortment

Mendel’s 2nd law

Inheritance of one trait

does not influence the

inheritance of any other

Alleles of different

genes separate

independently during

meiosis

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6.6 Genetic Variation

Rapid because of

meiosis

Key contributors:

1) independent

assortment

2) random

fertilization

3) crossing-over

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1) Independent assortment

Random

distribution of

homologous

chromosomes

during meiosis

Determined by

chance

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1) Independent Assortment

# of different gametes possible from one original human cell:

◦ 223 (~8 million) !WOWZERS! That is a lot of

possibilities!

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2) Random Fertilization

Zygote formed bw random

gametes

Fertilization by random

sperm

Now outcome: 223 x 223

> 70 trillion

Human couple can produce a

child w 1 of ~70 trillion

possible combos

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3)Crossing Over

Results in Recombination

Mixing of parental alleles

DNA exchanged during prophase 1

# of possibilities is nearly unlimited

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Genetic Linkage

Genes close together tend to be

inherited together

Genes far apart sort independently

Allows scientist to calculate distance

bw genes

Create genetic map

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Interactive Website:

NOVA Online | 18 Ways to Make a

Baby | How Cells Divide: Mitosis vs.

Meiosis (Flash)

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