CHAPTER 8 The Cellular Basis of Reproduction and Inheritance Modules 8.12 – 8.18

8.12 Chromosomes are matched in homologous pairs MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs Somatic cells of each species contain a specific number of chromosomes Human cells have 46, making up 23 pairs of homologous chromosomes Chromosomes Centromere Sister chromatids Figure 8.12

8.13 Gametes have a single set of chromosomes Cells with two sets of chromosomes are said to be diploid Gametes are haploid, with only one set of chromosomes

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

Multicellular diploid adults (2n = 46) Mitosis and development The human life cycle Haploid gametes (n = 23) Egg cell Sperm cell MEIOSIS FERTILIZATION Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development Figure 8.13

8.14 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: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Microtubules attached to kinetochore Sites of crossing over Metaphase plate Sister chromatids remain attached Spindle Nuclear envelope Chromatin Sister chromatids Tetrad Centromere (with kinetochore) Homologous chromosomes separate Figure 8.14, part 1

Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells

MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Sister chromatids separate Haploid daughter cells forming Figure 8.14, part 2

8.15 Review: A comparison of mitosis and meiosis For both processes, chromosomes replicate only once, during interphase

PARENT CELL (before chromosome replication) Site of crossing over MITOSIS MEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE PROPHASE I Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication 2n = 4 Chromosomes align at the metaphase plate Tetrads align at the metaphase plate METAPHASE METAPHASE I ANAPHASE I TELOPHASE I ANAPHASE TELOPHASE Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together Haploid n = 2 Daughter cells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Figure 8.15

Each chromosome of a homologous pair comes from a different parent 8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Each chromosome of a homologous pair comes from a different parent Each chromosome thus differs at many points from the other member of the pair

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

Two equally probable arrangements of chromosomes at metaphase I POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16

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

C E C E C E c e c e c e Coat-color genes Eye-color genes Brown Black White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes Figure 8.17A, B

8.18 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

Tetrad Chaisma Centromere Figure 8.18A

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE 8.19 A karyotype is a photographic inventory of an individual’s chromosomes 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

Preparation of a karyotype Fixative Packed red And white blood cells Hypotonic solution Blood culture Stain White Blood cells Centrifuge 3 2 1 Fluid Centromere Sister chromatids Pair of homologous chromosomes 4 5 Figure 8.19

8.20 Connection: An extra copy of chromosome 21 causes Down syndrome This karyotype shows three number 21 chromosomes 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 Figure 8.20C

8.21 Accidents during meiosis can alter chromosome number Abnormal chromosome count is a result of nondisjunction Either homologous pairs fail to separate during meiosis I Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n + 1 n – 1 n – 1 Number of chromosomes Figure 8.21A

Or sister chromatids fail to separate during meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n + 1 n – 1 n n Number of chromosomes Figure 8.21B

Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg cell n + 1 Zygote 2n + 1 Sperm cell n (normal) Figure 8.21C

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

Table 8.22

A man with Klinefelter syndrome has an extra X chromosome Poor beard growth Breast development Under- developed testes Figure 8.22A

A woman with Turner syndrome lacks an X chromosome Characteristic facial features Web of skin Constriction of aorta Poor breast development Under- developed ovaries Figure 8.22B

8.23 Connection: 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

Homologous chromosomes Deletion Duplication Homologous chromosomes Inversion Reciprocal translocation Nonhomologous chromosomes Figure 8.23A, B

Chromosomal changes in a somatic cell can cause cancer A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia Chromosome 9 Reciprocal translocation Chromosome 22 “Philadelphia chromosome” Activated cancer-causing gene Figure 8.23C

How crossing over leads to genetic recombination Coat-color genes Eye-color genes How crossing over leads to genetic recombination Tetrad (homologous pair of chromosomes in synapsis) 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I 3 Separation of chromatids at anaphase II and completion of meiosis 4 Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Figure 8.18B Gametes of four genetic types