Cell Reproduction Part II Meisosis

MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs The somatic (body) cells of each species Contain a specific number of chromosomes For example human cells have 46 Making up 23 pairs of homologous chromosomes

The chromosomes of a homologous pair Carry genes for the same characteristics at the same place, or locus 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, eggs and sperm, are haploid With a single set of chromosomes

Sexual life cycles Involve the alternation of haploid and diploid stages Web activity Mitosis and development Multicellular diploid adults (2n = 46) Diploid zygote (2n = 46) 2n Meiosis Fertilization Egg cell Sperm cell n Haploid gametes (n = 23) Figure 8.13

Quiz 8.11 – 8.13 Name 2 functions of mitosis(why do cells divide?). Two chromosomes composing a pair are called? What is a somatic Cell? What is a gamete? What are you thankful for this Thanksgiving? Chromosomes Centromere Sister chromatids

8.14 Meiosis reduces the chromosome number from diploid to haploid Meiosis, like mitosis Is preceded by chromosome duplication But in meiosis The cell divides twice to form four daughter cells Produces haploid gametes in diploid organism.

The first division, meiosis I Starts with synapsis, the pairing of homologous chromosomes. XX XX Tetrad Tetrad In crossing over Homologous chromosomes exchange corresponding segments

Meiosis I separates each homologous pair And produce two daughter cells, each with one set of chromosomes Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is a total of four haploid cells Web activity

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

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

8.15 Review: A comparison of mitosis and meiosis Parent cell (before chromosome replication) Chromosome replication Chromosomes align at the metaphase plate Tetrads align at the metaphase plate Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together No further chromosomal replication; sister chromatids separate during anaphase II Prophase Metaphase Anaphase Telophase Duplicated chromosome (two sister chromatids) Daughter cells of mitosis 2n Daughter cells of meiosis I n 2n = 4 Tetrad formed by synapsis of homologous chromosomes Meiosis i Meiosis ii Prophase I Metaphase I Anaphase I Telophase I Haploid n = 2 Daughter cells of meiosis II Figure 8.15

8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Each chromosome of a homologous pair Differs at many points from the other member of the pair The arrangement of homologous pairs at metaphase I of meiosis affects the resulting gametes.

Two equally probable arrangements of chromosomes at metaphase I Random arrangements of chromosome pairs at metaphase I of meiosis Lead to many different combinations of chromosomes in eggs and sperm Combination 1 Combination 2 Combination 3 Combination 4 Gametes Metaphase II Two equally probable arrangements of chromosomes at metaphase I Possibility 1 Possibility 2 Figure 8.16

Random fertilization of eggs by sperm Greatly increases this variation

8.18 Crossing over further increases genetic variability Genetic recombination Which results from crossing over during prophase I of meiosis, increases variation still further Web activity Chiasma Tetrad Centromere TEM 2,200 Figure 8.18A

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

8.21 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

Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n 1 Number of chromosomes Nondisjunction in meiosis II Normal meiosis I n -1 n Number of chromosomes Figure 8.21B Figure 8.21A

Fertilization after nondisjunction in the mother Sperm cell Egg cell n (normal) n + 1 Zygote 2n + 1 Figure 8.21C

CONNECTION 8.22 Abnormal numbers of sex chromosomes do not usually affect survival Nondisjunction can also produce gametes with extra or missing sex chromosomes Leading to varying degrees of malfunction in humans but not usually affecting survival Figure 8.22A Poor beard growth Breast Development Under-developed testes Figure 8.22B Characteristic facial features Web of skin Constriction of aorta Poor breast development Under developed ovaries

Human sex chromosome abnormalities

CONNECTION 8.23 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

Chromosomal Abnormalities Deletions, duplications, inversions, and translocations Reciprocal translocation Nonhomologous chromosomes Deletion Duplication Inversion Homologous chromosomes Figure 8.23B “Philadelphia chromosome” Chromosome 9 Chromosome 22 Reciprocal translocation Activated cancer-causing gene Figure 8.23C Figure 8.23A

8.17 Homologous chromosomes carry different versions of genes The differences between homologous chromosomes Are based on the fact that they can bear different versions of a gene at corresponding loci Tetrad in parent cell (homologous pair of duplicated chromosomes) e c E C White Pink Meiosis Black Brown Chromosomes of the four gametes Eye-color genes Coat-color genes Brown coat (C); black eyes (E) White coat (C); pink eyes (e) Figure 8.17B Figure 8.17A

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

Preparation of a karyotype from a blood sample Blood culture Fluid Centrifuge Packed red and white blood cells Hypotonic solution Fixative White blood cells Stain Centromere Pair of homologous chromosomes Sister chromosomes 2,600X A blood culture is centrifuged to separate the blood cells from the culture fluid. 1 The fluid is discarded, and a hypotonic solution is mixed with the cells. This makes the red blood cells burst. The white blood cells swell but do not burst, and their chromosomes spread out. 2 Another centrifugation step separates the swollen white blood cells. The fluid containing the remnants of the red blood cells is poured off. A fixative (preservative) is mixed with the white blood cells. A drop of the cell suspension is spread on a microscope slide, dried, and stained. 3 The slide is viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is entered into a computer, which electronically arranges them by size and shape. 4 The resulting display is the karyotype. The 46 chromosomes here include 22 pair of autosomes and 2 sex chromosomes, X and Y. Although difficult to discern in the karyotype, each of the chromosomes consists of two sister chromatids lying very close together (see diagram). 5 Figure 8.19

CONNECTION 8.20 An extra copy of chromosome 21 causes Down syndrome A person may have an abnormal number of chromosomes Which causes problems

Down syndrome is caused by trisomy 21 An extra copy of chromosome 21 5,000 Figure 8.20A Figure 8.20B

Infants with Down syndrome (per 1,000 births) The chance of having a Down syndrome child Goes up with maternal age Age of mother 45 50 35 30 25 40 20 90 10 60 70 80 Infants with Down syndrome (per 1,000 births) Figure 8.20C