Transcript Meiosis
Meiosis Making Gametes Review • Asexual Reproduction is also called _______________ • Mitosis starts with one parent cell and ends up with __________________________________________ • Genetically, the daughter cells are ________________ to the parent cell • Making an identical copy of a cell is also called _______________ • What type of cells in YOUR BODY divide by Mitosis? ________________ • The scientific term for Body cells is ______________ • These cells are considered 2n (diploid). What does that mean? _______________________________________ Review Cont’d • How many Chromosomes should a normal human have? _____ • If a human skin cell must divide to heal a wound, how many chromosomes would each of the daughter cells have? _____ • If a human bone cell must divide because a baby is growing, how many chromosomes would each of the daughter cells have? _________ • What are two reasons for Mitosis? __________ & ___________ • Which organelle is in control of cell division? ___________ • What happens when the nucleus loses control? __________ Mitosis • Asexual Cell Reproduction • Necessary for growth & repair • Chromosomes in Somatic Cells are in pairs • 2N (diploid) • Two new daughter cells are identical copies of original parent cell • Clones Cell Cycle • Sequence of phases in the life cycle of the cell Notice! Mitosis is only a SMALL part of the cell cycle! In what phase do cells spend most of their time? _____________ Mitosis & Cell Cyle Sex Cells are Different • Human beings have 46 chromosomes in every Somatic (Body) Cell • This is a complete set • But, if mom’s egg cell has 46… • And, if dad’s sperm cell has 46… • The Zygote (the fertilized egg cell) would have 92! • Would THAT make a human? Meiosis •Cell division of reproductive cells (sex cells) • Gametes or germ cells (sperm and egg) • Produced in Gonads •Each new cell is not exactly like the parent cell • Allows for variation • Important for the survival of our species! Meiosis • In each new daughter cell, the chromosomes are not in pairs • NOT 2n (not diploid!) • At the end of Meiosis, four daughter cells contain half of the original number of chromosomes (N) • Each is unique! • Not identical to the parent Gametes (aka germ cell) •Mature egg or sperm cell • Produced in the Gonads •Each gamete has half the original number of chromosomes •1N (haploid) •HALF IS IMPORTANT! Why is Half is Important? • Meiosis makes unique sex cells! • Half the number of chromosomes (haploid) is important for sexually reproducing organisms! • N = The number of chromosomes in a gamete • To create a new organism, two parents give half of their own chromosomes to make a baby with a complete set of chromosomes! • Half from mom • Half from dad • N + N = 2N (Zygote!) Sexual Reproduction • Sexual Reproduction requires two parents • Offspring have SOME traits inherited from each parent… but not all! • YOUR DNA is not identical to either parent • It is a mix of genes from both parents Meiosis Sexual Reproduction • Fusion of two (N) gametes to produce a single zygote (2N) • Introduces greater genetic variation, allows genetic recombination • A VERY GOOD THING! Chromosome Pairing •Homologous pair • Homologous means similar • Each chromosome in the pair are Homologous to the other (carry genes for same trait) • Only one pair differs - sex chromosomes X or Y Different! Metaphase I in Meiosis… Chromosomes form TETRADS (four together) Metaphase II in Meiosis… Chromosomes line up in the middle and then a SECOND DIVISION happens! The two daughter cells are divided into four and each has HALF the chromosome number! Independent assortment of homologous chromosomes POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Figure 8.16 Combination 2 Combination 3 Combination 4 Crossing over increases genetic variability • The exchange of genes between two homologous chromosomes happens when in Tetrad formation Tetrad Chaisma Centromere Figure 8.18A SPERMATOGENESIS OOGENESIS spermatogonium oogonium egg spermatids polar bodies (will disintegrate) Review Mitosis & Meiosis •Both are forms of cell division •Both involve replication •Both involve disappearance of the nucleus, and nucleolus, nuclear membrane •Both involve formation of spindle fibers