Division of sex cells
advertisement
Division of sex cells MEIOSIS VOCABULARY: Diploid = a cell containing TWO sets of chromosomes. chromosomes one set inherited from each parent 2n (number of chromosomes) body b d cells ll (somatic ( ti cells) ll ) MEIOSIS VOCABULARY: y Haploid = a cell with only ONE set of chromosomes. ○1n (number of chromosomes) ○sex cells (gametes) MEIOSIS VOCABULARY: Gamete= sex cells ySperm = male gamete yEgg = female gamete MEIOSIS VOCABULARY: o Homologous chromosomes = paired chromosomes h that h have h genes for the same traits arranged in the same order. One homologous chromosome is nh r t from th the inherited organism’s father, the other from the mother. MEIOSIS VOCABULARY: Meiosis =a two stage type of cell division th t results that s lts in i gametes with h lf the half th number of chromosome h number as the b d cells. body ll MEIOSIS VOCABULARY: Crossing over = when nonsister chromatids of homologous chromosomes exchange genetic information, results in a new combination of genes. MEIOSIS VOCABULARY: Fertilization = the process of j i i gametes. joining t Zygote Z t = when h sperm s (haploid) (haplo d) fertilizes fert l zes the egg (haploid), the resulting cell is th zygote the t (diploid). (di l id) I. GENES,, CHROMOSOMES,, AND NUMBERS: In humans, each somatic cell (any cell ll other than a sperm or egg, has 46 chromosomes) 46 chromosomesÆ 23 pairs (humans get 1 pair from f each h parent)) Sex chromosomes- determine th sex of the f an individual i di id l ylast pair of chromosomes—23rd pair for humans XX = female XY = male The number of chromosomes for an organism i iis NOT related l t d tto th the complexity p y of that organism!! g • Ex: A dog has 78 body chromosomes and humans have 46 body chromosomes A thousand or more genes are lined up on a chromosomes at one time Diploid p & Haploid p Numbers Each somatic cell of an organism contains paired i d chromosomes. h Half of each pair came from each h parent. These h cells ll are said d to have 2n chromosomes, or a full set. y They are DIPLOID yEx: Humans have 46 body chromosomes Each sex cell of an organism contains only l half h lf of f a chromosome h set. t ○These cells are HAPLOID and have 1n chromosomes -Ex: Humans have 23 chromosomes in their gametes oSex S cells ll will ill fuse f with ith another sex cell during fertilization to create a 2n organism. So if human sperm and egg both have 23 chromosomes, after fertilization an embryo would have 46 chromosomes! 2 Reasons why Meiosis is significant!! 1. Meiosis is another form of cell 1 division that creates haploid cells to be used for reproduction Female Egg Cell Male Sperm Cells 2 Reasons why Meiosis is significant!! If mitosis was the only form of cell division,, then new offspring would always have 2 times as many chromosomes as their parents. p 2 Reasons why Meiosis is significant!! Eventually, Eventually there would be so many y chromosomes,, the organism would not survive or be severely mutated. mutated 2 Reasons why Meiosis is significant!! 2. Meiosis 2 M i i provides id GENETIC VARIATION- the reshuffling of genes carried i d by b the h iindividual di id l members b of fa population. II. MEIOSIS vs. MITOSIS: Remember: b mitosis = asexual division of diploid body cells II. MEIOSIS vs. MITOSIS: Meiosis Mitosis Cell type of parent diploid diploid Number of daughter cells produced 4 2 Number of cell divisions 2 1 different identical different identical Genetic relationship of daughter cells to parent cellll Genetic relationship of daughter cells to one another Meiosis I Separates homologous pairs of chromosomes, NOT sister chromatids of individual chromosomes. Interphase p I Metabolic activities & replicate chromosomes Prophase I Synapsis occurs- the pairing of homologous chromosomes Each pair of homologous chromosomes come together to f m a tetrad form t t d (4 part p t structure) st t ) Genetic material is exchanged in a process called crossing over (swapping portions of p adjacent DNA) ○ Must be done with great precision so p that neither chromatid gains or loses any genes! Crossing g Over: 3. Metaphase p I Homologous chromosomes line up at the equator in pairs 4. Anaphase p I Homologous g chromosomes separate and move to opposite ends of the cell. cell ○This occurs because the centromeres do NOT split like i mitosis in it sis 4. Anaphase p I This ensures that each new cell will receive only one chromosome h f each for h homologous pair. pair 5. Telophase p I The new cells are diploid p so another division is required t create to t haploid h l id cells ll Meiosis I: Separate the Homologues Meiosis II The mechanisms of Meiosis II is almost the same as mitosis yHowever, However the chromosomes DO NOT replicate between meiosis I and d meiosis II, the h final outcome of meiosis is halving the number of chr m s m s per chromosomes p r cell. c ll 1. Interphase p II No N replication replic ti n Allows ll the h new cells ll to b become h l id haploid 2. Prophase p II Same as Prophase I except NO tetrads are formed 3. Metaphase p II Sister chromatids line up at the equator 4. Anaphase p II ○ Sister chromatids move to the opposite pp ends of the cell 5. Telophase p II Creates 4 haploid cells (g (gametes) ) But wait..there's more! Meiosis II: Separate the Sister Chromatids Meiosis Animation- w/ sound http://www.sumanasinc.com/webcontent/a http://www sumanasinc com/webcontent/a nimations/content/meiosis.html Meiosis provides genetic variation in 2 ways 1. Independent p assortment of homologous g chromosomes during meiosis I y The Th amountt of f diff differentt chromosomes h s s that can be produced increases greatly ass the th number mb of f chromosomes h m s m s an organism has. Meiosis provides genetic variation in 2 ways • A pea plant has 7 pairs of chromosomes. h Each E h pair i can line up 2 different ways. Therefore, h f each h gamete can have 27= 128 possibilities!! • Humans: n=23; so the number of different kinds of eggs or sperms a person can produce is more than 8 million (223) Meiosis provides genetic variation in 2 ways When fertilization occurs, 223 X 223 zygotes are possible or 70 trillion!! y No wonder brothers and sisters can be so different. Meiosis provides genetic variation in 2 ways 2 Crossing over between 2. homologous g chromosomes during prophase of meiosis I ○Increases the number of genetic variations Meiosis is NOT flawless ○ It is estimated that from 10–20% of all ll human fertilized l eggs contain chromosome abnormalities, and these are the m st common most mm n cause s of f pregnancy p n n failure ((35% of the cases). ) These chromosome abnormalities: Arise from errors in meiosis, usually meiosis I Occur more often (90%) during egg formation f ti than th during d i sperm formation Become more frequent as a woman ages. ages Meiosis Animation- no sound http://www.johnkyrk.com/meiosis.html http://www johnkyrk com/meiosis html
