Cellular reproduction part 2. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Somatic cells of each species contain a specific

Cellular reproduction part 2

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Somatic cells of each species contain a specific number of chromosomes

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

MEIOSIS AND CROSSING OVER

Chromosomes are matched in homologous pairs

Chromosomes

Centromere

Sister chromatids Figure 8.12


Homologous Chromosomes

• Humans have 23 pairs of homologous chromosomes

– 22 pairs – autosomes – found in both males and females

– 1 pair – sex chromosomes, XX = female,

XY= male


Homologous Chromosomes

• Matched pairs of chromosomes

• Similar in size, shape, and banding pattern

• Both carry genes controlling the same inherited characteristics (the version of the gene may be different)

• The genes are located at the same locus

• One chromosome of each pair is inherited from the mother, the other from the father


• The human life cycle

Figure 8.13

MEIOSIS FERTILIZATION

Haploid gametes (n = 23)

Egg cell

Sperm cell

Diploidzygote

(2n = 46)Multicellular

diploid adults (2n = 46)

Mitosis anddevelopment


Human Life Cycle

Diploid cells (2n) – cells that contain both homologous chromosomes. In humans the diploid number is 46.

Haploid cells (n) – cells with one copy of each homologous chromosome. The gametes (egg and sperm) are haploid. In humans the haploid number is 23.


Meiosis

Meiosis

• The division that reduces the number of chromosomes by half.

• In animals, meiosis results in the formation of haploid egg and sperm cells.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.15

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.16.2


Meiosis

Two nuclear divisions occur:

1. Meiosis I

a. During prophase I homologous chromosomes pair – synapsis

b. During prophase I the paired chromosomes exchange chromosome parts – crossing over

c. Homologous chromosomes are separated

d. 2 cells produced each containing one copy of each homologous chromosome

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.16.3


Meiosis

2. Meiosis II

a. Not preceded by the replication of DNA

b. Sister chromatids of each chromosome are separated

c. Produces 4 haploid cells


Meiosis

Meiosis produces 4 cells that

• Are haploid

• Chromosome makeup of each is different from each other and the parent cell




Meiosis

Spermatogenesis

• Formation of sperm by meiosis

• Occurs in special cells (spermatogonia) in the testes

• All 4 haploid cells become sperm



Meiosis

Oogenesis

• Formation of an egg by meiosis

• Occurs in special cells (oogonia) in the ovaries

• Unequal divisions of the cytoplasm during meiosis I and meiosis II result in the formation of 1 haploid egg and 3 haploid polar bodies

• Only the egg can be fertilized



Genetic Recombination

• Genetic Recombination – the production of gene combinations different from those carried by the parent

• There are 4 processes that contribute to genetic recombination.



Independent Assortment of Chromosomes

• The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes

– This results in 2n possible combinations of gametes

– For humans 2n = 223 = 8 million possible combinations

– http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__random_orientation_of_chromosomes_during_meiosis.html

• Random fertilization also increases variation in offspring


Figure 8.17A, B

Coat-color genes Eye-color genes

Brown Black

C E

c e

White Pink

C E

c e

C E

c e

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

Chromosomes ofthe four gametes


• The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

Homologous chromosomes carry different versions of genes


Figure 8.18A

TetradChaisma

Centromere


• How crossing over leads to genetic recombination

Figure 8.18B

Tetrad(homologous pair ofchromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

Chiasma

Separation of homologouschromosomes at anaphase I

Separation of chromatids atanaphase II and completion of meiosis

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

Gametes of four genetic types

1

2

3

4

Coat-colorgenes

Eye-colorgenes


Animation(must insert Miller and Levine lecture cd ch11)

•Meiosis 1•Crossing over a closer look•Meiosis II•http://bcs.whfreeman.com/thelifewire/content/chp09/0902002.html


Genetic Recombination

Crossing over

• The exchange of genetic information between 2 homologous chromosomes.

• Occurs during prophase I.

Random fertilization

• Depends on which sperm cell and its chromosome combinations fertilizes which egg


• Preparation of a karyotype

Figure 8.19

Blood culture

1

Centrifuge

Packed redAnd white blood cells

Fluid

2

Hypotonic solution

3

Fixative

WhiteBloodcells

Stain

4 5

Centromere

Sisterchromatids

Pair of homologouschromosomes


• To study human chromosomes microscopically, researchers stain and display them as a karyotype

– A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE

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


• Abnormal chromosome count is a result of nondisjunction

– Either homologous pairs fail to separate during meiosis I

– http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.swf

8.21 Accidents during meiosis can alter chromosome number

Figure 8.21A

Nondisjunctionin meiosis I

Normalmeiosis II

Gametes

n + 1 n + 1 n – 1 n – 1

Number of chromosomes


– Or sister chromatids fail to separate during meiosis II

Figure 8.21B

Normalmeiosis I

Nondisjunctionin meiosis II

Gametes

n + 1 n – 1 n n

Number of chromosomes


• Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome

Figure 8.21C

Eggcell

Spermcell

n + 1

n (normal)

Zygote2n + 1


• This karyotype shows three number 21 chromosomes

• An extra copy of chromosome 21 causes Down syndrome

Connection: An extra copy of chromosome 21 causes Down syndrome

Figure 8.20A, B


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


Alterations of Chromosomes

• In most cases abnormal chromosome number results in spontaneous abortion long before birth.

• Nondisjunction in the sex chromosomes has less of an affect on survival


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

– Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

Connection: Abnormal numbers of sex chromosomes do not usually affect survival

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Table 8.1



• Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer

– Four types of rearrangement are deletion, duplication, inversion, and translocation

Connection: Alterations of chromosome structure can cause birth defects and cancer


Figure 8.23A, B

Deletion

Duplication

Inversion

Homologouschromosomes

Reciprocaltranslocatio

n

Nonhomologouschromosomes

Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect.

Duplication – the fragment joins to a homologous chromosome.

Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects.

Translocation– attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.



Abnormalities in the structure of the chromosome may cause disorders (Figure 8.23A)

1. Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect.

2. Duplication – the fragment joins to a homologous chromosome.



3. Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects.

4. Translocation (Figure 8.23B) – attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.

Documents

Cellular reproduction part 2. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Somatic cells of each species contain a specific