MEIOSIS AND CROSSING OVER

Chromosomes are matched in homologous pairs

Homologous chromosomes: the 2 members of a pair of chromosomes—contain genes for the same traits

Somatic cells of each species contain a specific number of chromosomes

Human cells have 46, making up 23 pairs of homologous chromosomes

Chromosomes

Sister chromatids

Paired chromosomes

Homologous chromosomes both chromosomes of a pair carry “matching” genes

control same inherited characters homologous = same information

diploid2n2n = 4

homologouschromosomes

double strandedhomologous chromosomes

eye color(brown?)

eye color(blue?)

Gametes have a single set of chromosomes

Gametes~ egg or sperm Cells with two sets of

chromosomes are said to be diploid (2n) somatic cells(46 in humans)

Gametes are haploid, with only one set of chromosomes, (1n)(23 in humans)

Human female karyotype

46 chromosomes23 pairs

XX

diploid = 2 copies 2n

46 chromosomes23 pairs

XY

Human male karyotypediploid = 2

copies 2n

Life Cycle At fertilization, a sperm

fuses with an egg, forming a diploid zygote Repeated mitotic

divisions lead to the development of a mature adult

The adult makes haploid gametes by meiosis

All of these processes make up the sexual life cycle of organisms

Why meiosis?

When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells.

Imagine if mitosis were the only means of cell division.

IF the parent organism has 14 chromosomes, it would produce gametes that contained a complete set of 14 chromosomes

The offspring would have cell nuclei with 28 chromosomes, and the next generation

would have cell nuclei with 56 chromosomes

Meiosis reduces the chromosome number from diploid to haploid

Meiosis, like mitosis, is preceded by chromosome duplication However, in meiosis the cell divides twice to

form four daughter cells In the first division, meiosis I,

homologous chromosomes are paired While they are paired, they cross over and

exchange genetic information The homologous pairs are then separated,

and two daughter cells are produced

Meiosis I

In the first division, meiosis I, homologous chromosomes are paired As the chromosomes coil, homologous chromosomes line up with

each other gene by gene along their length, to form a four-part structure called a tetrad.Here synaspsis occurs: the meeting of two homologous pairs

While they are paired, they cross over and exchange genetic information

The homologous pairs are then separated, and two daughter cells are produced

Division in meiosis I occurs in four phases: prophase, metaphase, anaphase, and telophase

Meiosis 1 overview 1st division of meiosis

4 chromosomes

diploid 2n

2 chromosomes

haploid 1n

doublestranded

Copy DNA before meiosis

Line Up 1

Divide 1

gamete

prophase 1 metaphase 1

telophase 1

Meiosis II Meiosis II is essentially the same as mitosis

The sister chromatids of each chromosome separate

The result is four haploid daughter cells

2nd division of meiosis looks like mitosis

Meiosis 2 overview

2 chromosomes haploid 1n

4

gametes

Line Up 2

Bye Bye 2

telophase 1

metaphase 2

telophase 2

Review: A comparison of mitosis and meiosis For both processes, chromosomes replicate

only once, during interphase

Review: A comparison of mitosis and meiosis

Genetic variation Each chromosome of a homologous pair

comes from a different parent The large number of possible

arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes

Random fertilization also increases variation in offspring

Crossing over further increases genetic variability Crossing over is the exchange of corresponding

segments between two homologous chromosomes

Genetic recombination results from crossing over during prophase I of meiosis

This increases variation further

2006-2007

Errors of MeiosisChromosomal Abnormalities

Chromosomal abnormalities

Incorrect number of chromosomes nondisjunction

chromosomes don’t separate properly during meiosis

breakage of chromosomes deletion duplication inversion translocation

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE

• A karyotype is a photographic inventory of an individual’s chromosomes

• Human female karyotype

An extra copy of chromosome 21 causes

Down syndrome• This karyotype shows

three number 21 chromosomes: trisomy 21

• An extra copy of chromosome 21 causes Down syndrome

• The chance of having a Down syndrome child goes up with maternal age

Down syndrome & age of motherMother’s age

Incidence of Down Syndrome

Under 30 <1 in 1000

30 1 in 900

35 1 in 400

36 1 in 300

37 1 in 230

38 1 in 180

39 1 in 135

40 1 in 105

42 1 in 60

44 1 in 35

46 1 in 20

48 1 in 16

49 1 in 12

Rate of miscarriage due to amniocentesis: 1970s data

0.5%, or 1 in 200 pregnancies

2006 data<0.1%, or 1 in 1600 pregnancies

Accidents during meiosis can alter chromosome number

• Nondisjunction~ The failure of homologous chromosomes to separate properly during meiosis

• Abnormal chromosome count will result.

Nondisjunction • Problems in meiosis cause errors in daughter cells– chromosome pairs do not separate properly during Meiosis 1– sister chromatids fail to separate during Meiosis 2– too many or too few chromosomes

2n n

n

n-1

n+1

Abnormal numbers of sex chromosomes do not usually affect

survival

• Nondisjunction can also produce gametes with extra or missing sex chromosomes

• A man with Klinefelter syndrome has an extra X chromosome

• A woman with Turner syndrome lacks an X chromosome

• XXY male– one in every 2000 live

births– have male sex organs, but

are sterile– feminine characteristics

• some breast development• lack of facial hair

– tall– normal intelligence

Klinefelter’s syndrome

Klinefelter’s syndrome

Turner syndrome• Monosomy X or X0

– 1 in every 5000 births– varied degree of effects – webbed neck– short stature– sterile

Nondisjunction

• When a gamete with an extra set of chromosomes is fertilized by a normal

haploid gamete, the offspring has three sets of chromosomes and is triploid.(3n)

• The fusion of two gametes, each with an extra set of chromosomes, produces

offspring with four sets of chromosomes—a tetraploid. (4n)

• This is polyploidy.

Alterations of chromosome structure can cause birth defects

and cancer

• Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer– Four types of

rearrangement are deletion, duplication, inversion, and translocation

Deletion

Duplication

Inversion

Reciprocaltranslocation

Nonhomologouschromosomes

Changes in chromosome structure• deletion– loss of a chromosomal segment

• duplication– repeat a segment

• inversion– reverses a segment

• translocation– move segment from one chromosome to

another