SC435 Genetics Seminar

• Welcome to our Unit 3 Seminar

• We will continue our discussion of heredity with a focus on the chromosomes

• The seminar will begin at 9:00PM ET

Unit 3

• Discussion board• Unit 3 Quiz• Unit 3 Exam• Final Project Topic Submission

• Looking Ahead– Unit 4

• Assignment (2-3 page paper)

UNIT 3 KEY CONCEPTS• Chromosomes in eukaryotic cells are usually present in pairs.• The chromosomes of each pair separate in meiosis, one going

to each gamete.• In meiosis, the chromosomes of different pairs undergo

independent assortment.• Chromosomes consist largely of DNA combined with histone

proteins.• In many animals, sex is determined by a special pair of

chromosomes, the X and Y.• Irregularities in the inheritance of an X-linked gene in Drosophila

gave experimental proof of the chromosomal theory of heredity.

Chromatin Structure

• Each core particle consists of an octamere of pairs each of histone H2A, H2B, H3, and H4; a

segment of DNA containing about 145 base pairs

Fig. 3.15a

Fig. 3.19

6

• The chromosomes in the nuclei of somatic cells are usually present in pairs. For example, humans have 23 pairs of chromosomes

• Cells with nuclei of this sort, containing two similar sets of chromosomes, are called diploid

Chromosomes

Chromosomes

• The chromosome complement = the complete set of chromosomes of plants and animals

• The nucleus of each somatic cell contains a fixed number of chromosomes typical of the particular species

• The number of chromosomes vary tremendously among species and have little relationship to the complexity of the organism

Fig. 3.2

Fig. 3.3 Mitosis: Interphase, Late prophase, Metaphase

Fig. 3.3 Mitosis: Anaphase and Telophase

Fig. 3.3 Mitosis

Meiosis

• Meiosis is a mode of cell division in which cells are created that contain only one member of each pair of

chromosomes• Meiosis consists of two successive nuclear divisions• Meiosis results in four daughter cells, each genetically

different and each containing one haploid set of chromosomes

• Meiosis is a more complex and considerably longer process than mitosis and usually requires days or even weeks

• The oocytes form egg cells and the spermatocytes form sperm cells

• In the females of both animals and plants, only one of the four products develops into a functional cell (the other three disintegrate)

Meiosis

Fig. 3.5

Outline of Meiosis

• Prior to the first nuclear division, the members of each pair of chromosomes become closely associated along their length

• The chromosomes that pair with each other are said to be homologous chromosomes

• Each member of a pair of homologs consists of a duplex of two sister chromatids joined at the centromere. The pairing of the homologous chromosomes therefore produces a four-stranded structure

• At the time of pairing, the homologs can exchange genes which results in chromosomes that consist of segments from one homolog intermixed with segments from the other

• In the first nuclear division, the homologous chromosomes are separated from each other, one member of each pair going to opposite poles of the spindle

• Two nuclei are formed, each containing a haploid set of duplex chromosomes

Outline of Meiosis

• The second nuclear division resembles a mitotic division, but there is no DNA replication.

• At metaphase, the chromosomes align on the metaphase plate, and at anaphase, the chromatids are separated into opposite daughter nuclei

• The net effect of the two divisions is the creation of four haploid nuclei, each containing the equivalent of a single sister chromatid from each pair of homologous chromosomes

Outline of Meiosis

Mitosis vs. Meiosis

• Meiosis produces four cells: each contains one copy of each pair of homologous chromosomes = genetically haploid

• Mitosis produces two cells which contain both members of each pair of homologous chromosomes = genetically diploid

Meiosis: Prophase I

• Diplotene - chromosome repulsion, however, they remain held together by cross-connections resulting from crossing-over. Each cross-connection, called a chiasma is formed by a breakage and rejoining between nonsister chromatids

Fig. 3.9b

• Leptotene - the chromosomes first become visible as long, thread-like structures

• Zygotene - synapsis of homologous chromosomes = bivalent

• Pachytene - crossing-over between homologs

• The second meiotic division (meiosis II) is called the equational division because the chromosome number remains the same in each cell before and after the second division

• In some species, the chromosomes pass directly from telophase I to prophase II without loss of condensation

• After a short prophase II and the formation of second-division spindles, the centromeres of the chromosomes in each nucleus become aligned on the central plane of the spindle at metaphase II

Meiosis II

• In anaphase II, the centromeres divide and the chromatids of each chromosome move to opposite poles of the spindle

• Once the centromere has split at anaphase II, each

chromatid is considered a separate chromosome

• Telophase II is a transition to the interphase condition of the chromosomes in the four haploid nuclei, accompanied by division of the cytoplasm.

Meiosis II

• The chromatids of a chromosome are usually not genetically identical because of crossing-over associated with the formation of chiasmata during prophase of the first division

Meiosis

Chromatin Structure

• Nucleosomes coil to form higher order DNA structure called the 30-nm chromatin fiber

• In the nucleus of a nondividing cell, chromatin fibers form discrete chromosome territories

• Chromosome territories are correlated with gene densities

• Territories of chromosome domains that are relatively gene rich tend to be located toward the interior of the nucleus

Fig. 3.21

(Micrograph courtesy of T.C. Hsu and Sen Pathak)

Nondisjunction

• Experimental proof of the chromosome theory of heredity came from nondisjunction

• Nondisjunction = chromosomes fail to separate (disjoin) and move to opposite poles of the division spindle, results in loss or gain of a chromosome

• Calvin Bridges demonstrated that exceptional behavior of chromosomes is precisely paralleled by exceptional inheritance of their genes

• Special chromosomes determine sex in many organisms

• X and Y chromosomes = sex chromosomes which are non-identical but share some genes

• In most organisms, the Y chromosome carries few genes other than those related to male determination

• X-linked genes are inherited according to sex• Hemophilia is a classic example of human X-

linked inheritance

X-Linked Inheritance