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Lecture

Chap 13 - Meiosis.docx

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Department
Biology
Course
BIOL 1010
Professor
Brent Sellinger
Semester
Fall

Description
Reece et al., 9 Ed Chapter 13 1 Meiosis and Sexual Life Cycles I. Background • The passage of genetic information is critical to the reproduction of cells • The physical mechanism of inheritance depends on the behavior of chromosomes • Genes are segments of DNA representing discrete units of hereditary information • The physical location of a gene on the chromosome is called its locus (pl. - loci). Each chromosome contains a sequence of several hundred to a few thousand genes • The fidelity or the precision of DNA replication is important to heredity. Why? II. Types of Reproduction A. Asexual reproduction • Asingle parent cell or organism gives rise to identical offspring e.g., mitotic division of a mother cell gives rise to two daughter cells or multicellular hydra produces identical offspring by budding Except for mutations, which are rare events, the offspring are identical to the parent. In other words, the offspring of asexual reproduction are clones (identical copies) of their parents. B. Sexual reproduction • Results in greater variation in genetic material ▯ Offspring vary in genetic composition from parents and siblings. Why? • Genetic material comes from two parents and genetic variability is created during meiosis and fertilization There are two types of cells in humans i) Gametes - haploid cells that carry one set of chromosomes are referred to as “1n” e.g., In humans, an egg (1n) and a sperm (1n) are the reproductive cells that fuse during fertilization (syngamy) to produce a diploid (2n) zygote (a cell with 2 chromosome sets) In the “1n” notation, the “n” represents the number of chromosomes in a set of chromosomes. For example Human gametes (1n) have one set of 23 chromosomes, that is 1n = 23 Reece et al., 9 Ed Chapter 13 2 • The gametes are vehicles for carrying and passing on genetic information from parents to their offspring ii) Somatic cells in humans are diploid cells and carry two sets of chromosomes (2n) human ▯ 2n = 46 chromosomes • The chromosomes in a cell vary in size, shape, centromere position and banding patterns • In diploid cells, one can pair chromosomes in relation to size, shape and type (e.g., banding pattern). This is called Karyotyping or the arrangement of chromosomes in relation to the number, size, and type (Refer to figure 13.3) • The similarity between paired chromosomes extends to loci on chromosomes. • The chromosomes of a pair are called homologous chromosomes and will carrying genes controlling the same inherited characters (i.e., each member of the pair will have a version of the same gene specifying a particular inherited trait at equivalent loci) There is one exception for humans • one homologous pair - determines sex specific traits - sex chromosomes e.g., females XX males XY The X and Y chromosomes are very different and only small parts are homologous • The remaining 22 pairs of chromosomes are called autosomes and the members of each pair are homologous What is the significance of the homologous pairs? Their origin!!!!! ▯ one member of each homologous pair comes from each parent! e.g., Human gametes ▯ haploid cells - single set of chromosomes (1n = 23) AND Each parent contributes one set of chromosomes and therefore one member of each homologous pair. Fertilization Egg + sperm ▯ zygote (1n) (1n) (2n) Sexually reproducing species have a characteristic diploid and haploid number Fruit fly gametes (1n) have one set of 4 chromosomes, that is 1n = 4 Fruit fly somatic cells (2n) have two sets of 4 chromosomes, that is 2n = 8 chromosomes Mitosis generates daughter cells identical to the parental cell, but HOWARE GAMETES FORMED? th Reece et al., 9 Ed Chapter 13 3 th Reece et al., 9 Ed Chapter 13 4 III. Meiosis • production of reproductive cells or gametes • During meiosis chromosome number is halved by partitioning one member of each homologous pair into different gametes • Two rounds of cell division produce 4 daughter cells; each with only 1 set of chromosomes (i.e., half the chromosome number) Mitosis conserves chromosome number Meiosis halves the chromosome number Stages of Meiosis (Figure 13.8) 1. Interphase • replication of DNA A. Meiosis I • This meiotic division is referred to as a reduction division because it separates the members of homologous pairs. 1. Prophase I • chromosomes condense • Synapsis = homologous chromosomes pair up along their length - precisely aligned gene by gene - and form tetrads (i.e., four chromatids). • The chromosome pairs are physically connected to each other along their length by a zipper like protein complex known as the synaptonemal complex. • Chiasmata (sing. - chiasma) - crossing over occurs between nonsister chromatids
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