3Figure :Cell division cycle. A dividing cell with 46 chromosomes passesthrough the G1, DNA synthesis (S), andG2 phases before entering mitosis, whichconsists, in succession, of prophase (P),metaphase (M), anaphase (A), andtelophase (T). The end result of thecell division cycle is the formation oftwo daughter cells, each with 46chromosomes.
4Figure : Essential features of the phases of the cell division cycle of a cellwith two pairs of chromosomes.
5Figure : Metaphase chromosome. The centromere (primary constriction) ismarked by the downward diagonalpattern. The kinetochore is the region ofattachment of spindle fibers that forms onthe outward portion of the centromere. Atmetaphase the chromosome is duplicated,and each complete chromosome is achromatid.
8Figure : Meiotic process. Meiosis I consists of G1, DNA synthesis (S), and G2phases; prophase I (PI), which includesleptotene (L), zygotene (Z), pachytene(Pa), diplotene (D), and diakinesis (Di);metaphase I (MI), anaphase I (AI), andtelophase I (TI). The products of meiosisI carry 23 duplicated chromosomes (23d)from cells that entered meiosis I with 46chromosomes. Meiosis II consists of acombined G1/G2 phase followed byprophase II (PII), metaphase II (MII),and telophase II (TII). The final productsof the meiotic process are cells thatcontain 23 chromosomes that compriseone of each of the original set of 23 pairsof chromosomes. In human females, eachmeiotic process produces one functionalgamete and three nonfunctional cellscalled polar bodies. In human males, allmeiotic products are functional.
15Figure : Overview of meiosis Figure : Overview of meiosis. Meiosis is a form of cell division in which certain cells are set aside and give rise to haploid gametes.This simplified illustration follows the fate of two chromosome pairs rather than the true 23 pairs. In actuality, the first meiotic divisionreduces the number of chromosomes to 23, all in the replicated form. In the second meiotic division, the cells essentially undergo mitosis.The result of the two meiotic divisions (in this illustration and in reality) is four haploid cells. In this illustration, homologous pairs ofchromosomes are indicated by size, and parental origin of chromosomes by color.
16Meiosis – A Source of Distinction Why do you share some but not all characters of each parent?What are the rules of this sharing game?At one level, the answers lie in meiosis.
17Meiosis does two things - 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number.In meiosis, one diploid cells produces four haploid cells.
18Why do we need meiosis?Meiosis is necessary to halve the number of chromosomes going into the sex cellsWhy halve the chromosomes in gametes?At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents
19This makes for a lot of genetic diversity This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.Genetic diversity is important for the evolution of populations and species.
20Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split2nd division – produces 4 gamete cells with ½ the original no. of chromosomes
21Meiosis I : Separates Homologous Chromosomes InterphaseEach of the chromosomes replicateThe result is two genetically identical sister chromatids which remain attached at their centromeres
22Prophase I This is a crucial phase for mitosis. During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad.Extremely IMPORTANT!!! It is during this phase that crossing over can occur.Crossing Over is the exchange of segments during synapsis.
23Metaphase IThe chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles.Still in homologous pairs
24Anaphase IThe spindle guides the movement of the chromosomes toward the polesSister chromatids remain attachedMove as a unit towards the same poleThe homologous chromosome moves toward the opposite poleContrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)
25Telophase I This is the end of the first meiotic cell division. The cytoplasm divides, forming two new daughter cells.Each of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division
26Cytokinesis Occurs simultaneously with telophase I Forms 2 daughter cellsPlant cells – cell plateAnimal cells – cleavage furrowsNO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS
27Figure :The stages of meiotic cell division: Meiosis I
28Meiosis II : Separates sister chromatids Proceeds similar to mitosisTHERE IS NO INTERPHASE II !
29Prophase IIEach of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator
30Metaphase IIThe chromosomes are positioned on the metaphase plate in a mitosis-like fashion
31Anaphase II The centromeres of sister chromatids finally separate The sister chromatids of each pair move toward opposite polesNow individual chromosomes
32Telophase II and Cytokinesis Nuclei form at opposite poles of the cell and cytokinesis occursAfter completion of cytokinesis there are four daughter cellsAll are haploid (n)
33Figure 13.7 The stages of meiotic cell division: Meiosis II
34One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.In humans, n = 23 and 223 = ???!That’s a lot of diversity by this mechanism alone.
36Figure : Overview of meiosis Figure : Overview of meiosis. Meiosis is a form of cell division in which certain cells are set aside and give rise to haploid gametes.This simplified illustration follows the fate of two chromosome pairs rather than the true 23 pairs. In actuality, the first meiotic divisionreduces the number of chromosomes to 23, all in the replicated form. In the second meiotic division, the cells essentially undergo mitosis.The result of the two meiotic divisions (in this illustration and in reality) is four haploid cells. In this illustration, homologous pairs ofchromosomes are indicated by size, and parental origin of chromosomes by color.
37Figure :Crossing over recombines genes. Crossing over generates genetic diversity by recombining genes and therebymixing parental traits. The capital and lowercase forms of the sameletter represent different variants (alleles) of the same gene. Achromosome actually has hundreds to thousands of genes.
38Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.
40- Genes close on a chromosome are packaged into the same gametes and are said to be “linked” - Linkage has this very precise meaning in genetics..- Linkage refers to the transmission of genes on the same chromosome. Linked genes do not assort independently .
47Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’.In humans this often occurs with the 21st pair – producing a child with Downs Syndrome