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Chapter 6 Part I Meiosis and Sexual Life Cycles http://www.sumanasinc.com/webcontent/animations/content/meiosis.html PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: Variations on a Theme • Living organisms are distinguished by their ability to reproduce their own kind: • GENES are hereditary (inherited) units made up of DNA that code for a particular trait/ characteristic. • HEREDITY is the transmission of traits from one generation to the next. • VARIATION is demonstrated by the differences in appearance that offspring show from parents and siblings. • Genetics is the scientific study of heredity and variation. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Acquiring Genes from Parents • In a literal sense, children do not inherit particular physical traits from their parents…it is genes that are actually inherited. • Genes are the units of heredity, and are made up of segments of DNA • Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs). • Each gene has a specific location called a locus on a certain chromosome. • Most DNA is packaged into chromosomes. • One set of chromosomes is inherited from each parent. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-4 Comparison of Asexual and Sexual Reproduction • In asexual reproduction, one parent produces genetically identical offspring by mitosis. • A clone is a group of genetically identical individuals from the same parent. • In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents. • Advantages greater genetic variation! Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sexual Life Cycles • Fertilization and meiosis alternate in sexual life cycles • A life cycle is the generation-togeneration sequence of stages in the reproductive history of an organism Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sets of Chromosomes in Human Cells • Human somatic cells (body cells/ any cell other than a gamete) have 23 pairs of chromosomes. • A karyotype is an ordered display of the pairs of chromosomes from a cell . • The two chromosomes in each pair are called homologous chromosomes, or homologs. • Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters. One is inherited from the mother, and the other from the father. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome Homologous Chromosomes • HOMOLOGOUS CHROMOSOMES are chromosome pairs of the same length, centromere position, staining pattern, and gene possession for the same characters: – One homologous chromosome is inherited from the organism’s father and the other from the mother. • DIPLOID – means “two sets” – This represents the cells in which the chromosomes are paired up and have a partner in size and shape. – A cell containing TWO sets of chromosomes (2n), one set inherited from each parent is referred to as a DIPLOID CELL. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Human female chromosomes shown by bright field G-banding Where are the homologous chromosomes? Homologous Chromosomes in Human Female Review: DIPLOID Cells • Each pair of homologous chromosomes includes one chromosome from each parent • The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father • A diploid cell (2n) has two sets of chromosomes • For humans, the diploid number is 46 (2n = 46) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Key 2n = 6 Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Centromere Pair of homologous chromosomes (one from each set) Sex Chromosomes v. Autosomes • The sex chromosomes are called X and Y • Human females have a homologous pair of X chromosomes (XX) • Human males have one X and one Y chromosome (XY) • The 22 pairs of chromosomes that do not determine sex are called autosomes or autosomal chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Haploid • A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) • For humans, the haploid number is 23 (n = 23) • Each set of 23 consists of 22 autosomes and a single sex chromosome • In an unfertilized egg (ovum), the sex chromosome is X • In a sperm cell, the sex chromosome may be either X or Y Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Review: HAPLOID Cells • HAPLOID – means “one set”; this describes the sex cells or gametes (EGG AND SPERM) – these contain only a SINGLE set of chromosomes. – So, for a human, somatic cells are diploid and have a chromosome number of 46 (23 pairs). – But, sex cells are haploid, and have a chromosome number of JUST 23. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings So how do we transmit all of this information to our offspring? A process called… • Meiosis is the process by which the number of chromosomes per sex cell is cut in half through the separation of homologous chromosomes in a diploid cell – in other words, meiosis is the division of chromosomes into gametes eggs (in females) and sperm (in males) Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS Ovary FERTILIZATION Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)