Chapter 19 Chromosomes and Cell Division

FIGURE 19.1 The human life cycle

Human Chromosomes Homologous chromosomes: one maternal & one paternal Sex chromosomes: X & Y Autosomes: the 22 pairs of chromosomes other than X & Y

Chromosomes Consist of DNA and Protein Genes 1000-5000 genes/chromosome Most genes code for a protein Mutation causes change in nucleotide sequence of DNA Somatic cells (Non-germ cells) have 46 chromosomes (23 pairs)

Most people have 23 chromosomes 46 chromosomes 23 pairs of chromosomes

Most people have 23 chromosomes 46 chromosomes 23 pairs of chromosomes

FIGURE 19.6b Chromosomes in dividing cells can be examined for defects in number or structure. (b) A karyotype constructed by arranging the chromosomes from photographs like the one in part a. The chromosomes have been arranged on the basis of location of centromere and size.

Figure: 19-02 Title: A map of a human chromosome. Caption: Each chromosome contains a specific assortment of genes in a specific location along the chromosome. Genes are often identified through the disease caused when they do not function normally.

FIGURE 19.3 Changes in chromosome structure because of DNA replication during interphase and preparation for nuclear division in mitosis.

FIGURE 19.3 part 1 Changes in chromosome structure because of DNA replication during interphase and preparation for nuclear division in mitosis.

Chromosomes are composed of DNA Genes Chromatin All of the above None of the above

Chromosomes are composed of DNA Genes Chromatin All of the above None of the above

During Cell Division (Mitosis) Cells Replicate Their Chromosomes

DNA Replication (synthesis) Precedes cell division DNA polymerase (enzyme) catalyzes DNA synthesis Necessary to maintain 46 chromosomes/cell

Figure: 19-04 Title: Changes in chromosome structure because of DNA replication during interphase. Caption:

Overview of Mitosis FIGURE 19.4 An overview of mitosis

Cell Division During metaphase the chromosomes form a line at the center of the cell

FIGURE 19.5 part 1 The stages of cell division (mitosis and cytokinesis) captured in light micrographs and depicted in schematic drawings

FIGURE 19.6a Chromosomes in dividing cells can be examined for defects in number or structure. (a) Photomicrograph of metaphase chromosomes from a human white blood cell.

FIGURE 19.5 part 2 The stages of cell division (mitosis and cytokinesis) captured in light micrographs and depicted in schematic drawings

Cytokinesis Occurs Toward the End of Mitosis Phase when Cytoplasm divides Microfilaments act like purse string

FIGURE 19.7 part 1 Cytokinesis is the division of the cytoplasm to form two daughter cells.

FIGURE 19.7 part 2 Cytokinesis is the division of the cytoplasm to form two daughter cells.

Meiosis

Meiosis Forms Haploid Gametes Occurs in gonads (ovary and testes) Two Major Accomplishments of Meiosis Reduce chromosome number of gametes by ½ Shuffle the chromosomes and genes This increases genetic variability in the population

Meiosis Forms Haploid Gametes Keeps the chromosome number constant through generations and Increases genetic variability in the population Meiosis involves two cell divisions

Overview of Meiosis FIGURE 19.8 Overview of meiosis. Meiosis reduces the chromosome number from the diploid number to the haploid number. Meiosis involves two cell divisions.

The following is accomplished during meiosis Two identical cells are produced Four identical cells are produced The chromosome number is doubled The genes are shuffled The number of chromosomes in the resulting cells is halved

The following is accomplished during meiosis Two identical cells are produced Four identical cells are produced The chromosome number is doubled The genes are shuffled The number of chromosomes in the resulting cells is halved

Comparison of Spermatogenesis and Oogenesis

FIGURE 19.9 Comparison of spermatogenesis and oogenesis. Meiosis results in haploid cells that differentiate into mature gametes. Spermatogenesis produces four sperm cells that are specialized to transport the male's genetic information to the egg. Oogenesis produces up to three polar bodies and one ovum that is packed with nutrients to nourish the early embryo.

Mammalian Egg With Polar Bodies

Meiosis and Mitosis Compared

TABLE 19.1 MITOSIS AND MEIOSIS COMPARED

Crossing over and independent assortment cause genetic recombination (gene shuffling) during meiosis

Crossing Over FIGURE 19.13 Crossing over. During synapsis, when the homologous chromosomes of the mother and the father are closely aligned, corresponding segments of nonsister chromatids are exchanged. Each of the affected chromatids has a mixture of maternal and paternal genetic information.

Independent Assortment FIGURE 19.14 Independent assortment. The relative positioning of homologous maternal and paternal chromosomes with respect to the poles of the cell is random. The members of each homologous pair orient independently of the other pairs. Notice that with only two homologous pairs, there are four possible combinations of chromosomes in the resulting gametes.

Number of different gametes produced by humans 23 pairs of chomosomes Each pair line up randomly on the metaphase plate Therefore 223 different combinations = > 8,000,000 different gametes from each person

Gene shuffling during meiosis is caused by: Crossing over of chromosomes Independent assortment of chromosomes Nondisjunction of chromosomes Cytokinesis Condensation of chromosomes

Mistakes Sometime Happen During Meiosis Failure of chromosomes to separate during meiosis creates cells with extra or missing chromosomes Called Nondisjunction of Chromosomes

FIGURE 19.15 Nondisjunction is a mistake that occurs during cell division in which homologous chromosomes or sister chromatids fail to separate during anaphase. One of the resulting daughter cells will have an extra copy of one chromosome, and the other will be missing that type of chromosome.

Examples of Nondisjunction Down syndrome (three # 21 chromosomes) Turner syndrome (one X chromosome) Klinefelter syndrome (XXY)

Missing or extra sex chromosomes cause Turner syndrome and Klinefelter syndrome. (a) A female with Turner syndrome has one rather than the normal two X chromosomes. Because the ovaries are underdeveloped, she is usually infertile. (b) A male with Klinefelter syndrome is XXY. Klinefelter syndrome is characterized by small testes (and sometimes breast enlargement).