Process of Meiosis
What is Meiosis? Form of nuclear division Creates FOUR haploid cells from ONE diploid cell Diploid=2N=46 chromosomes (somatic cell) Haploid=1N=23 chromosomes (sex cells) Two rounds of cell division=meiosis I & Meiosis II Each round=four phases, similar to phases in Mitosis
What is Meiosis? reduces chromosome number creates genetic diversity Takes place only at certain times in organism s life cycle Involved in sexual reproduction Makes sperm and egg cells Because human cells have 23 chromosomes, meiosis may result in 2 23, or 8,388,608, possible combinations of chromosomes.
Review!! This is a chromosome Each chromosome is a long, continuous thread of DNA DNA stores genetic information in all organisms DNA determines your physical appearance
Review!! DNA plus proteins is called chromatin. One half of a duplicated chromosome is a chromatid. Sister chromatids are held together at the centromere. Telomeres protect DNA and do NOT include genes. telomere
Paired Up Each pair of chromosomes is known as a homologous pair. Homologous means having the same structure
Homologous Chromosomes Two separate chromosomes one from mother, one from father Very similar to each other Same length Carry the same genes Are NOT copies of each other Divided in Meiosis I
Each half of a duplicated chromosome Sister chromatids=two chromatids together Divided in Meiosis II Chromatids
Homologous Chromosomes vs. Sister Chromatids homologous chromosomes sister chromatids sister chromatids
2 Types of Chromosomes Autosomes & Sex Chromosomes
Autosomes Pairs 1-22 are your Autosomes These chromosomes contain genes for characteristics that are NOT related to the sex of an organism
Sex Chromosomes Sex chromosomes make up the 23 rd pair Sex chromosomes control the development of sexual characteristics Determine male or female Humans have two sex chromosomes: X and Y XX=female XY=male NOT Homologous
X and Y Chromosomes X chromosome Larger sex chromosome Carries the most genes Determines characteristics other than sex of the organism Y chromosome Smallest sex chromosome Carries the fewest genes Determines the development of males
Two Types of Cells Your body is made up of Somatic cells and Gametes. These are the two major groups of specialized cells in your body.
Somatic Cells Also known as body cells Make up most of your body tissues and organs For example: your spleen, kidneys, and eyeballs are made entirely of body cells. DNA in body cells are NOT passed on to your children.
Sex cells Gametes Ova, or eggs, in the female Spermatozoa, or sperm cells, in the male DNA in your gametes CAN be passed on to your children Gametes are formed when germ cells (cells found in the ovaries or the testes) undergo meiosis
Somatic, body, cells are diploid Diploid means a cell has two copies of each chromosome: one copy from the mother, and one from the father. Diploid cells can be represented as 2n. In humans, the diploid chromosome number is 46. Diploid Cells
Haploid Cells Gametes are haploid cells Haploid means that a cell has only one copy of each chromosome Haploid cells are represented as n.
Meiosis I Meiosis I occurs after DNA has been replicated. Meiosis I divides homologous chromosomes in four phases. Prophase I Metaphase I Anaphase I Telophase I
Meiosis I-Prophase I Nuclear membrane breaks down Centrosomes and centrioles move to opposite sides if the cell Spindle fibers start to form Homologous chromosomes pair up, including sex chromosomes
Meiosis I-Metaphase I Spindle fibers randomly align the homologous chromosomes along the cell equator. Meta=Middle Each side has chromosomes from both parents This arrangement mixes up the chromosomal combinations Helps create and maintain genetic diversity
Meiosis I=Anaphase I Paired homologous chromosomes separate from each other and move toward opposite sides of the cell. Sister chromatids remain attached.
Meiosis I=Telophase I Nuclear membrane forms again Spindle fibers disassemble Cell undergoes cytokinesis=cell completely divides Results in two cells that are genetically different Each have a unique combination of 23 duplicated chromosomes coming from both parents
Meiosis II DNA is not replicated between meiosis I and meiosis II. Meiosis II divides sister chromatids in four phases. Prophase II Metaphase II Anaphase II Telophase II Process applies to BOTH cells produced in Meiosis I Results in undoubled chromosomes
Meiosis II=Prophase II Nuclear membrane breaks down Centrosomes and centrioles move to opposite sides of the cell Spindle fibers assemble
Meiosis II=Metaphase II Spindle fibers align the 23 chromosomes at the cell equator. Meta=Middle Each chromosome still has two sister chromatids at this stage.
Meiosis II=Anaphase II Spindle fibers pull the sister chromatids apart from one another and move them to opposite sides of the cell.
Meiosis II=Telophase II Nuclear membranes form again around the chromosomes Spindle fibers break apart Cell undergoes cytokinesis Results in FOUR haploid cells Each with a combination of genes from both the mother and the father
Next step for haploid cells Haploid cells are the end result of meiosis. These cells must undergo changes to become mature gametes before they are ready for fertilization.
Gametogenesis Production of gametes Gametes=sex cells ova, or eggs in the female spermatazoa, or sperm cells in males Gametogenesis differs between females and males
Gametogenesis in Males Sperm become streamlined and motile. Sperm primarily contribute DNA to an embryo.
Gametogenesis in Females Eggs contribute DNA, cytoplasm, and organelles to an embryo. During meiosis, the egg gets most of the contents; the other cells form polar bodies=cells with little more than DNA that are eventually broken down.
Meiosis vs Mitosis Meiosis Two cell divisions Homologous chromosomes pair up long the equator In anaphase I, sister chromatids are still together. Results in haploid cells that are genetically different. Mitosis One cell division Homologous chromosomes NEVER pair up In anaphase, sister chromatids separate. Results in diploid cells that are genetically identical.