Meiosis-1.pdf

MEIOSIS Organisms that reproduce Sexually are made up of two different types of cells. 1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. 1. Gametes a...

MEIOSIS Organisms that reproduce Sexually are made up of two different types of cells. 1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. 1. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes. n = number of chromosomes in the set… so….2n means 2 chromosomes in the set…. Polyploid cells have more than two chromosomes per set… example: 3n (3 chromosomes per set)  Gametes ► The Male Gamete is the Sperm and is produced in the male gonad the Testes. ► The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries. During Ovulation the ovum is released from the ovary and transported to an area where fertilization, the joining of the sperm and ovum, can occur…… fertilization, in Humans, occurs in the Fallopian tube. Fertilization results in the formation of the Zygote. (fertilized egg) fertilization Sperm + Ovum (egg) Zygote  Fertilization ► The fusion of a sperm and egg to form a zygote. ►A zygote is a fertilized egg sperm n=23 n=23 egg 2n=46 zygote  Chromosomes ► Ifan organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA). ► Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for humans is 46 (46 chromosomes per cell).  Homologous Chromosomes ► Pair of chromosomes (maternal and paternal) that are similar in shape and size. ► Homologous pairs (tetrads) carry genes controlling the same inherited traits. ► Each locus (position of a gene) is in the same position on homologues. ► Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”) eye color eye color locus locus hair color hair color locus locus Paternal Maternal Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues. Homologue Homologue  Autosomes (The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets 1 - 22  Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male Sex Chromosomes “Sex Chromosomes” …….the 23rd set This person has 2 “X” chromosomes… and is a female. 23  Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n) → Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die. Meiosis Meiosis is Two cell divisions (called meiosis I and meiosis II) with only one duplication of chromosomes.  Meiosis in males is called spermatogenesis and produces sperm. Meiosis in females is called oogenesis and produces ova.  Spermatogenesis Secondary Spermatocyte n=23 human sex cell sperm Primary Spermatocyte n=23 n=23 2n=46 Secondary Spermatocyte haploid (n) n=23 diploid (2n) n=23 4 sperm cells are produced from each n=23 primary spermatocyte. meiosis I meiosis II Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte. Interphase I ► Similar to mitosis interphase. ► Chromosomes replicate (S phase). ► Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. ► Centriole pairs also replicate.  Interphase I ► Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus Meiosis I (four phases) ► Cell division that reduces the chromosome number by one-half. ► four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I  Prophase I ► Longest and most complex phase. ► 90% of the meiotic process is spent in Prophase I ► Chromosomes condense. ► Synapsis occurs: homologous chromosomes come together to form a tetrad. ► Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids). PROPHASE 1- subdivided into 5 substages ► Leptonema- Replicated chromosomes have coiled and are already visible. The Number of chromosomes present is the same as the number in the diploid cell. ► Zygonema – Homologue chromosomes begin to pair and twist around each other in a highly specific manner. The pairing is called synapsis and because the pair consists of four chromatids it is referred to as bivalent tetrad. ► Pachynema- Chromosomes become much shorter and thicker. A form of Physical exchange between homologues takes place at specific regions. The process of physical exchange of a chromosome region is called crossing –over. Through the mechanism of crossing over , the parts of the homologous chromosomes are recombined (genetic recombination) Diplonema- the 2 pairs of sister chromatids begin to separate from each other. It is at this point where crossing over shown to have taken place. The area of contact between two non-sister chromatids, called chiasma, become evident. ► Diakinesis- The four chromatids of each tetrad are even more condensed and the chiasma often terminalize or move down the chromatids to the ends. This delays the separation of homologues chromosomes.  Prophase I - Synapsis Homologous chromosomes sister chromatids sister chromatids Tetrad During Prophase I “Crossing Over” occurs. Crossing Over is one of the Two major occurrences of Meiosis (The other is Non-disjunction) ► During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over. Crossing Over creates variation (diversity) in the offspring’s traits. nonsister chromatids Tetrad chiasmata: site variation of crossing over Question: ►A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: ► 10 chromosomes (haploid) Question: ►A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: ► 10 chromosomes Prophase I spindle fiber centrioles aster fibers  Metaphase I ► Shortest phase ► Tetrads align on the metaphase plate. ► INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations Metaphase I OR metaphase plate metaphase plate Anaphase I ► Homologous chromosomes separate and move towards the poles. ► Sister chromatids remain attached at their centromeres. Anaphase I Telophase I ► Each pole now has haploid set of chromosomes. ► Cytokinesis occurs and two haploid daughter cells are formed. Telophase I Meiosis II ► No interphase II (or very short - no more DNA replication) ► Remember: Meiosis II is similar to mitosis Prophase II ► same as prophase in mitosis Metaphase II ► same as metaphase in mitosis metaphase plate metaphase plate Anaphase II ► same as anaphase in mitosis ► sister chromatids separate  Telophase II ► Same as telophase in mitosis. ► Nuclei form. ► Cytokinesis occurs. ► Remember: four haploid daughter cells produced. gametes = sperm or egg Telophase II Non-disjunction Non-disjunction is one of the Two major occurrences of Meiosis (The other is Crossing Over)HJL;L;’/./ ► Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. ► Non-disjunction results with the production of zygotes with abnormal chromosome numbers…… remember…. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring. Non-disjunctions usually occur in one of two fashions. ► The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an organism has Monosomy 23 it has only one chromosome in the 23rd set. Common Non-disjunction Disorders ►Down’s Syndrome – Trisomy 21 ►Turner’s Syndrome – Monosomy 23 (X) ►Kleinfelter’s Syndrome – Trisomy 23 (XXY) ►Edward’s Syndrome – Trisomy 18 Trisonomy 21- Down Syndrome Amniocentesis ►An Amniocentesis is a procedure a pregnant woman can have in order to detect some genetics disorders…..such as non- disjunction.  Amniocentesis Amniotic fluid withdrawn  Karyotype (picture of an individual’s chromosomes) One of the ways to analyze the amniocentesis is to make a Karyotype What genetic disorder does this karyotype show? Trisomy 21….Down’s Syndrome