Chromosomes • Chromosomes are located in the _____ of the cell • Each chromosome is made up of segments called genes • Genes produce proteins that determine characteristics and functions of the cell.
Homologous Chromosomes • Humans have 46 chromosomes • 23 from each parent • These are called homologous chromosomes because: • They are the same length • Have the centromere in the same location • Carry genes for the same trait • Same trait different characteristics
Haploid and diploid cells • Every species has a different number of chromosomes • The letter n is used to represent the number of chromosomes in general terms • A gamete with half the number of chromosomes is called haploid and is represented by n • When two haploid cells come together during fertilization a diploid cell is formed and is represented by 2n
Sex cells • Every cell contains a full set of chromosomes • Only half of these chromosomes are passed on to the offspring • Special cells called gametes (sex cells) are produced containing ½ of the original number of chromosomes. • Example: a human has 46 chromosomes in a normal cell. A gamete or sex cell only has 23 chromosomes
The formation of gametes • Is called meiosis I and II • Meiosis is cell division that reduces the number of chromosomes and occurs only in the reproductive organs of organisms that reproduce sexually
Phases of Meiosis I • Interphase – same as regular cell cycle • Prophase I - 1 cell • Pairing of homologous chromosomes • Crossing over – segments of homologous chromosomes are exchanged to increase genetic variation (not like mitosis) • Nuclear envelope breaks down • Spindles form
Phases of Meiosis I • Metaphase I • Homologous chromosomes line up on the equator Spindle fibers attach to centromere • Anaphase I • Homologous chromosomes separate and move to opposite sides of the cell • Telephase I - 2 cells • Spindle fibers break down • Chromosomes uncoil • Two nuclei form • Cell divides (not like mitosis)
Meiosis I is very much like mitosis • If it were a human the cell would have 46 chromosomes at the end of meiosis I • Cell may go through interphase after meiosis I but the DNA will NOT be replicated
Meiosis II • Prophase II – 2 cells • Chromosomes condense • Spindle fibers form and attach to the centromere • Metaphase II • Chromosomes line up on the equator • Anaphase II • Sister chromatids are separated and moved to opposite sides of the cell
Telephase II – 4 cells • Four nuclei form • Spindle fibers break down • Cells divide • Meiosis II is very much like mitosis • If it were a human the cells would have 23 chromosomes at the end of meiosis II
Complete the following questions on white lined paper – write the question • Define gene • List the stages of meiosis I • Compute the number of chromosomes that the gametes of a cat (2n = 38) will have. Show your work • Compare and contrast anaphase I and anaphase II • Devise a theory that explains why the most complex animals reproduce sexually • Hand in your paper
Diagram of Meiosis Start of Meiosis I 1 cell 2n chromosomes 46 End of Meiosis I 2 cells 2n chromosomes 46 46 Meiosis II 4 cell n chromosomes each 23 23 23 23
3 Ways Genetic Variation Occurs: 1. Independent Assortment– Random distribution of homologous chromosomes during meiosis. 2. The DNA exchange that occurs during crossing-overincreases variation. 3. Random Fertilization-The joining of 2 gametes is produced independently.
Meiosis & Gamete Formation • In males this is called spermatogenesis. See p.148. • In females this is called oogenesis.
Questions 1.What is the significance of meiosis in sexual reproduction? 2. How is gamete formation in male different in gamete formation in females? 3. Devise a theory that explains why the most complex animals reproduce sexually.
Types of Reproduction • Asexual – A single parent passes copies of all of its genes to each of its offspring. The offspring are clones. • Ex. prokaryote –binary fission, like bacteria and amoebas • Many eukaryotes also asexually reproduce by fragmentation- body breaks into pieces. • Hydras and yeast undergo budding, new offspring break off from parent.
Advantages to asexual reproduction: • Can produce many offspring in a short period of time. • Spend no energy on mating • Do not have to waste time looking for a mate.
Sexual – There are two parents each donate half of their chromosomes (haploid) and give rise to a diploid offspring. • Each offspring has half the parents genes. • Ex. Human • Dogs • Protists can reproduce both ways.
Advantages to sexual reproduction: • Can make many variations. • Genetic diversity increases organisms survival. • You can repair chromosomal damage.
Questions • Identify the type of reproduction that results in offspring that are genetically identical to their parents? • Describe two types of eukaryotic reproduction?
Haploid Life Cycle • The entire span in the life of an organism from one generation to the next. • The haploid cell fuses with another gamete and is diploid, it immediately undergoes meiosis and begins its haploid life cycle.
Diploid Life Cycle • Diploid life cycle- The first cell of a gamete is called a germ cell. It is diploid and undergoes meiosis to become haploid. • Also when fertilization occurs the zygote is a diploid which undergoes mitosis.
Alternation of Generation • Plants, algae, and some protists that regularly alternate between haploid and diploid phase. • Gametophyte (in a plant) is the haploid phase that produces gametes by mitosis. Two gametes fuse that give rise to a diploid.
A sporophyte is a spore forming cell. Sporophyte is a Diploid undergoing meiosis to become haploid and capable of becoming an adult without fusing with another cell.