1

meiosis• Sexual reproduction

• Creates a variety of offspring

mitosis• A form of asexual reproduction

• Daughter cells are genetic copies of

the parent and of each other

LM 340!

Independent orientation of chromosomes in meiosis and

random fertilization lead to varied offspring

Combination 1 Combination 2 Combination 3 Combination 4

Gametes

Metaphase II

Two equally probable

arrangements of

chromosomes at

metaphase I

Possibility 1 Possibility 2

2

How crossing over leads to genetic variation

Figure 8.18B

Coat-color

genes

Eye-color

genesC E

c e

C E

c e

C E

c e

C E

C e

c E

c e

C E

C e

c E

c e

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

1

2

3

4

Accidents during meiosis can alter chromosome

number

– Abnormal chromosome count is a result of nondisjunction

• The failure of homologous pairs to separate during meiosis

I

• The failure of sister chromatids to separate during meiosis

II

Sperm cell

Egg cell

n (normal)

n + 1

Zygote

2n + 1

3

Nondisjunction

in meiosis I

Normal

meiosis II

Gametes

n + 1 n + 1 n "1 n "1

Number of chromosomes

Nondisjunctio

n in meiosis II

Normal

meiosis I

Gametes

n + 1 n "1 n n

Number of chromosomes

Figure 8.21A

Figure 8.21B

Accidents during meiosis can alter chromosome

number

Down syndrome is caused by trisomy 21

• An extra copy of chromosome 21

5,0

00!

Figure 8.20A Figure 8.20B

4

The chance of having a Down syndrome child

• Goes up with maternal age

Age of mother

45 50353025 4020

90

0

10

20

30

40

50

60

70

80In

fan

ts w

ith

Do

wn

syn

dro

me

(pe

r 1

,00

0 b

irth

s)

Figure 8.20C

Poor beard

growth

Breast

Development

Under-developed

testes

Characteristic facial

features

Web of skin

Constriction of

aortaPoor breast

development

Under developed

ovaries

XXY X

5

Alterations of chromosome structure can cause birth

defects and cancer

– Chromosome breakage can lead to rearrangements that can

produce genetic disorders or, if the changes occur in

somatic cells, cancer

– Deletions, duplications, inversions, and translocations

Deletion

Duplication

Inversion

Homologous

chromosomes

“Philadelphia chromosome”

Chromosome 9

Chromosome 22

Reciprocal

translocation

Activated cancer-causing gene

Genetics

6

Law of Segregation

1. Alternative versions of genes account for variations in inherited characters.

2. For each characteristic, an organism inherits two alleles, one from each parent.

3. If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's

appearance; the other, the recessive allele, has no noticeable effect on the organism's

appearance.

4. The two alleles for each characteristic segregate during gamete production.

Law of Independent Assortment1. The emergence of one trait will not affect the emergence of

another. This is actually only true for genes that are not linked

to each other.

Mendels Laws

Hypothesis: Dependent assortment Hypothesis: Independent assortment

RRYY rryy

Gametes Gametes

RRYY rryy

RrYy RrYy

RY ry ryRY

Sperm Sperm

RY ry

ry

RY

ry

Ry

ry

RY

RRYY

RrYY

RRYy

RrYy

RrYY

rrYY

RrYy

rrYy

RRYy

RrYy

RRyy

Rryy

RrYy

rrYy

Rryy

rryy

RY ry ryRY

Actual results

contradict hypothesis

Actual results

support hypothesis

Yellow

round

Green

round

Yellow

wrinkled

Green

wrinkled

Eggs

P generation

F1 generation

F2 generation

Eggs

1

2

1

2

1

2

1

2

1

4

1

4

1

4

1

4

1

4

1

4

1

4

1

4

9

16

3

16

3

16

1

16

!

Geneticists use test crosses to determine unknown genotypes

7

Mendel’s laws reflect the rules of probability

– Inheritance follows the rules of probability

– The rule of multiplication calculates the probability of two independent events

Law of Independent Assortment1. The emergence of one trait will not affect the emergence of

another. This is actually only true for genes that are not linked

to each other. Thomas Hunt Morgan

8

Law of Independent Assortment1. The emergence of one trait will not affect the emergence of

another. This is actually only true for genes that are not linked

to each other. Thomas Hunt Morgan

Genetic traits in humans can be tracked through family

pedigrees

– The inheritance of many human traits

• Follows Mendel’s laws

Dominant Traits Recessive Traits

Freckles No freckles

Widow’s peak Straight hairline

Free earlobe Attached earlobeFigure 9.8 A

9

Sex-linked genes exhibit a unique pattern of inheritance– All genes on the sex chromosomes

•Are said to be sex-linked

– In many organisms

•The X chromosome carries many genes unrelated to sex

10

Sex-linked disorders affect mostly malesMost sex-linked human disorders

•Are due to recessive alleles

•Are mostly seen in males

Pedigree analysis

•Dominant

•Recessive

•X-linked

11

Nucleic acids

Nucleic acids are information-rich polymers

of nucleotidesDNA and RNA Serve as the blueprints for proteins and thus

control the life of a cell

DNA polynucleotide

A

C

T

G

T

Sugar-phosphate backbone

Phosphate group

Nitrogenous base

SugarA

C

T

G

T

Phosphate

group

O

O–

OO P CH2

H3C C

C

C

CN

C

N

H

H

O

O

C

O

O

H

C H H

H

C

H

Nitrogenous base

(A, G, C, or T)

Thymine (T)

Sugar

(deoxyribose)

DNA nucleotide

DNA

nucleotide

DNA and RNA are polymers of nucleotides

– DNA is a nucleic acid

• Made of long chains of nucleotide monomers

12

DNA has four kinds of nitrogenous bases

• A, T, C, and G

CC

C

CC

C

O

N

C

H

H

ONH

H3C

H H

H

H

N

N

N

H

OC

H HN

H C

N

N N

N

C

CC

C

H

H

N

N

H

C

CN

C HN

CN

H C

O

H

H

Thymine (T) Cytosine (C) Adenine (A) Guanine (G)

PurinesPyrimidines

RNA is also a nucleic acid

• But has a slightly different sugar

• And has U instead of T

Nitrogenous base

(A, G, C, or U)

Phosphate

group

O

O–

OO P CH2

HC

C

C

C

N

C

N

H

H

O

O

C

O

O

H

C H H

OH

C

H

Uracil (U)

Sugar

(ribose)

Key

Hydrogen atom

Carbon atom

Nitrogen atom

Oxygen atom

Phosphorus atom

13

DNA is a double-stranded helix

– James Watson and Francis Crick

• Worked out the three-dimensional structure of DNA, based on

work by Rosalind Franklin

The structure of DNA

• Consists of two polynucleotide strands wrapped around each

other in a double helix

Figure 10.3C

Twist

14

G C

T A

A T

G

G

C

C

A T

GC

T A

T A

A T

A T

G C

A T

O

O

OH–O

P

O O–O

PO

OO

P– O

– O OP

OO

O

OH

H2C

H2C

H2C

H2C

O

O

O

O

O

O

O

O

PO–

O–

O–

O–

OH

HO

O

O

O

P

P

P

O

O

O

O

O

O

O

O

T A

G C

C G

A T

CH2

CH2

CH2

CH2

Hydrogen bond

Base

pair

The structure of DNA

• Consists of two polynucleotide strands wrapped around each

other in a double helix• Hydrogen bonds between base hold the strands together• Each base pairs with a complementary partner, A with T, and G

with C

DNA replication depends on specific base pairing

G C

T A

A T

G

G

C

C

A T

GC

T A

T A

A T

A T

G C

A T

O

O

OH–O

P

O O–O

PO

OO

P– O

– O OP

OO

O

OH

H2C

H2C

H2C

H2C

O

O

O

O

O

O

O

O

PO–

O–

O–

O–

OH

HO

O

O

O

P

P

P

O

O

O

O

O

O

O

O

T A

G C

C G

A T

CH2

CH2

CH2

CH2

15

DNA replication depends on specific base pairingDNA replication

– Starts with the separation of DNA strandsThen enzymes use each strand as a template

– To assemble new nucleotides into complementary strands

The Hershey-Chase experiment

Phage

Bacterium

Radioactive

protein

DNA

Phage

DNA

Empty

protein shellRadioactivity

in liquid

PelletCentrifuge

Batch 1

Radioactive

protein

Batch 2

Radioactive

DNA

Radioactive

DNA

Centrifuge

Pellet

Radioactivity

in pellet

Figure 10.1B

Mix radioactively

labeled phages with

bacteria. The phages

infect the bacterial

cells.

1 Agitate in a blender to

separate phages outside

the bacteria from the cells

and their contents.

2 Centrifuge the mixture

so bacteria form a pellet

at the bottom of the test

tube.

3 Measure the

radioactivity in

the pellet and

the liquid.

4

16

17

18

– To replicate, the DNA helix must untwist

G C

A T

G C

A T

C G

AGA

CG

C

GC

G

T

AG

C

T

AT

AA

TT

A

CG

CG

CG

T

A

G

C

T

A

T

A

A

T

T

A

T

C

T

19

– The DNA of the gene is transcribed into RNA

• Which is translated into the polypeptide

DNA

Transcription

RNA

Protein

Translation

Genetic information written in codons is translated into amino

acid sequences

– The “words” of the DNA “language”

• Are triplets of bases called codons

– The codons in a gene

• Specify the amino acid sequence of a polypeptide

Figure 10.8A

UUC

UGU

UGC

UGA Stop

Met or

start

Phe

Leu

Leu

Ile

Val Ala

Thr

Pro

Ser

Asn

Lys

His

Gln

Asp

Glu

Ser

Arg

Arg

Gly

CysTyr

G

A

C

U

U C A G

Th

ird

ba

se

Second base

Fir

st

ba

se

UUA

UUU

CUC

CUU

CUG

CUA

AUC

AUU

AUG

AUA

GUC

GUU

GUG

GUA

UCC

UCU

UCG

UCA

CCC

CCU

CCG

CCA

ACC

ACU

ACC

ACA

GCC

GCU

GCG

GCA

UAC

UAU

UAG Stop

UAA Stop

CAC

CAU

CAG

CAA

AAC

AAU

AAG

AAA

GAC

GAU

GAG

GAA

UGG Trp

CGC

CGU

CGG

CGA

AGC

AGU

AGG

AGA

GGC

GGU

GGG

GGA

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

20

Figure 10.8B

T A C T T C A A A A T C

A T G A A G T T T T A G

A U G A A G U U U U A G

Transcription

Translation

RNA

DNA

Met Lys PhePolypeptide

Start

condonStop

condon

Strand to be transcribed

Transcription of a gene

Exon Intron Exon Intron Exon

DNA

Cap Transcription

Addition of cap and tail

RNA

transcript

with cap

and tail

Introns removedTail

Exons spliced together

mRNA

Coding sequence Nucleus

Cytoplasm

Figure 10.10

Transcription of a gene

21

DNA strand

Transcription

Translation

Polypeptide

RNA

Amino acid

Codon

A A A C C G G C A A A A

U U U G G C C G U U U U

Gene 1

Gene 2

Gene 3

DNA molecule

Figure 10.7

Mutations can change the meaning of genes

Mutations are changes in the DNA base sequence caused by

errors in DNA replication or recombination, or by mutagens

C T T C A T

Normal hemoglobin

Mutant hemoglobin DNA

G A A G U A

Sickle-cell hemoglobin

Normal hemoglobin DNA

Glu Val

mRNA mRNA

Figure 10.16A