Biology 067 Section 14 Cell Division A. Definitions: In a human cell, a nucleus holds all the chromatin that condenses to form chromosomes when cells divide every cell in the body has the same set of chromosomes what makes them function differently is what genes on those chromosomes are turned on or off (except red blood cells they don t have chromosomes because they don t have a nucleus) Chromatin located in the nucleus -network of fine threads made of DNA and proteins Chromosome chromatin that is condensed into a compact structure Chromatid ½ of a duplicated chromosome Duplicated chromosome made up of 2 sister chromatids Centromere part of a chromosome that holds sister chromatids together Centrosome central microtubule organizing center contains 2 centrioles in animal cells. Centriole structure that exist in pairs and thought to organize the mitotic spindle during chromosome movement during mitosis and meiosis Mitosis called duplicating cell division cell division in which daughter cells receive the exact chromosomal and genetic makeup of parent cell occurs during growth and repair Meiosis cell division that occurs as part of sexual reproduction in which daughter cells receive the haploid # (half) of chromosomes in varied combinations Karyotype- is a display of all the chromosomes according to shape, #, size etc. B. Overview of cell division Humans have 46 chromosomes that occur in 23 pairs (nb Karyotype diagram) 22 are autosomes (any chromosome that is not a sex chromosome) 1 pair is sex chromosomes these contain genes that control gender Females have 2 x chromosomes, males an x and a y Karyotype shows that a normal cell is diploid meaning it contains 2 sets of chromosomes (46 chromosomes) Mitosis occurs in all the somatic (body) cells in the body - daughter cell receives the exact same# of chromosomes as parent = diploid =2n Meiosis occurs in sex cells (=gametes = egg and sperm) results in 4 daughter cells with a haploid # (half) of chromosomes of parent cell = n=haploid
C. The cell cycle = cell division and time between cell division 1. Interphase: the time between cell division 3 phases of interphase: Interphase 1) G1 organelles double 2) S DNA replication each chromosome gets a duplicate 3) G2 cell makes proteins needed for cell division 2. Cell division: G1 growth S growth and DNA replication M G2 growth and final preparations for division 2 parts of cell division 1) mitosis or meiosis (division of the nucleus) and 2) cytokinesis (division of the cytoplasm) D. Mitosis 1. Intro to mitosis Duplicated chromosomes (=2 sister chromatids) held together at centromere divide Sister chromatids separate One of each kind goes into 1 of 2 daughter cells Each daughter gets same # and type of chromosomes as parent (both 2n = diploid) Happens during growth and repair 2. Stages of Mitosis (PMAT) a) Prophase: a. Centrosomes have duplicated and carry 2 pairs of centrioles to poles of cell b. Spindle fibers (made up microtubules) appear between separating centrosomes and attach to centromere of sister chromatids c. Nuclear envelope fragments d. Nucleolus (special region of DNA) disappears as chromosomes coil and become condensed e. Chromosomes are now visible and contain 2 sister chromatids b) Metaphase a. Chromosomes align at equator of cell on spindle fibers b. Centromeres joining sister chromatids split c) Anaphase a. Sister chromatids separate now each is called a chromosome b. Move to poles of cell because sets of spindle fibers lengthen while others shorten pulling them to the poles d) Telophase a. Chromosomes decondense form chromatin
b. Spindle disappears c. Nuclear envelope reassembles Cytokinesis - The second part of cell division division of cytoplasm and organelles Cleavage furrow forms and split cell into 2 daughter cells An animation going through step by step of mitosis (also look for YouTubes and other animations) this animation divides it into a few more steps than our notes do remember these stages are arbitrary anyway for our course just ignore these extra steps and focus on what happens during PMAT http://www.sumanasinc.com/webcontent/animations/content/mitosis.html E. Meiosis -occurs in sex cells 1. Intro to meiosis Takes place in testes in male and ovaries in females during gamete (gamete=reproductive cell or sex cell) production Produces 4 haploid (n) sex cells Requires 2 cell divisions Each daughter cell has ½ the number of chromosomes as a parent cell and are not genetically identical 2. Overview of Meiosis: 2n n = diploid haploid 2 cell divisions called Meiosis I and Meiosis II Before meiosis I, each chromosome has duplicated (2 sister chromatids per chromosome) during interphase These chromosomes occur in pairs = homologous chromosomes (2 chromatids per chromosome, so the pair is made up of 4 chromatids -they are called homologous because they look alike and carry genes for the same traits) a) Meiosis I During meiosis I homologous (paired) chromosomes come together and line up side by side on the equator forming a tetrad in a process called synapsis. Non sister chromatids can exchange genetic material in a process called crossing over = gene recombination Homologous chromosomes separate so each daughter nucleus will receive one of the pair. Cell division occurs, 2 daughter cells, each with 1 chromosome from paired homologous chromosomes (at this point sister chromatids still together) End up with ½ the number of total chromosomes but each chromosome is still duplicated Daughter cells are not identical because of gene recombination.
b) Meiosis II Duplicated chromosomes are composed of 2 sister chromatids that separate during cell division. Resulting in 4 daughter cells that are haploid each chromosome consists of 1 chromatid. Therefore chromosomal # stays constant in each generation remember during fertilization -2 sex cells come together each with a haploid number of chromosomes combine to form a diploid cell. Egg (n) + sperm (n) embryo (2n) 3. Stages of Meiosis Meiosis I Remember acronym PMAT =prophase, metaphase, anaphase, telophase a) Prophase I Spindles appear Nuclear envelope fragments Homologous chromosomes undergo synapsis and form tetrads and crossing over can occur b) Metaphase I Tetrads line up at equator of cell c) Anaphase I Homologous chromosomes separate and move to poles d) Telophase I Nuclear envelope reforms Spindles disappear, furrow appears 2 daughter cells formed containing 1 homologous chromosome (paired sister chromatids) also called dyads Interkinesis: period of time between meiotic cell divisions No DNA replication occurs between the 2 phases of meiosis Meiosis II a) Prophase II Spindles appear Nuclear envelope fragments Dyads (sister chromatids) attach to spindle
b) Metaphase II dyads line up at equator c) Anaphase II dyads separate and move to poles each pole has same # of chromosomes d) Telophase II Nuclear envelope reforms Spindles disappear, furrow of plasma membrane to form 2 daughter cells with haploid number of chromosomes (so now have 4 daughter cells from the original 1) animation of meiosis: (also look for others and YouTubes) http://highered.mheducation.com/sites/0072495855/student_view0/chapter28/animation ho w_meiosis_works.html F. Comparison of Meiosis and Mitosis 1. Differences Mitosis and meiosis both nuclear divisions but have several differences: DNA replication occurs only once before both meiosis and mitosis However, meiosis requires 2 nuclear divisions, mitosis requires 1 4 daughter cells at the end of meiosis, 2 daughter cells after mitosis 4 daughter cells after meiosis are n haploid (1/2 the chromosome # of the parent) 2 daughter cells after mitosis are 2n diploid (same # of chromosomes as parent) Daughter cells after meiosis are NOT genetically identical to each other or parent Daughter cells after mitosis are genetically identical to each other and parent 2. When do these types of cell division occur? Meiosis occurs only at certain times in the life cycle of sexually reproducing organisms in humans it only occurs in reproductive organs to produce gametes. Mitosis is more common occurs in all tissues during growth and repair (mitosis is out of control when cancer occurs can result in the proliferation of a body cell) 3. Comparison of Meiosis and Mitosis At beginning of meiosis I and mitosis, chromosomes have duplicated so have 92 chromatids at this point if it s human **Key to remember: meiosis I is reduction division reduce # of chromosomes by half this phase it is this phase that makes mitosis and meiosis different mitosis and meiosis II are the same process except in meiosis you are now dealing with ½ the number of chromosomes. Meiosis I is called the reduction division because it 1/2s the # of chromosomes because instead of 1 pair of chromatids lining up on the equator you have 2 pairs.
In meiosis: homologous chromosomes pair up and undergo crossing over during prophase I -but not during mitosis (mitosis they also don t pair up) In meiosis -Paired homologous chromosomes line up on the equator during meiosis I paired chromosomes = 4 chromatids altogether -2 chromatids/chromosome In mitosis individual chromosomes line up on the equator -each have 2 chromatids (in humans 46 chromosomes are lined up on the equator = 92 chromatids) In Meiosis II individual chromosomes line up on the equator each with 2 chromatids but now have only 1/2 the number of chromosomes ( in humans, 23 chromosomes are lined up on the equator = 46 chromatids) In meiosis I homologous chromosomes (with centromeres intact) separate and move to opposite poles in anaphase I In mitosis and meiosis II, sister chromatids (=chromosomes) split at the centromere and move to opposite poles during anaphase. In this way, just by looking, you can tell whether it is meiosis I or II or mitosis: Only meiosis 1 has paired chromosomes on the equator If a cell has a total of 46 chromosomes (92 chromatids) lined up on the equator it is mitosis If a cell has a total of 23 chromosomes (46 chromatids) on the equator then it is meiosis II. G. Spermatogenesis and Oogenesis (meiosis) 1. Spermatogenesis Occurs in testes in male Sperm production through meiosis Meiosis I Meiosis II 1 o spermatocyte2 o spermatocyte 4 spermatids 1 cell 2cells mature to form 4 sperm 2n n n Reduction division 2. Oogenesis Produces egg in ovary of female Meiosis I Meiosis II Fertilization 1 o oocyte 1 cell 2 o oocyte 1cell (egg) -n plus stops at metaphase II unless fertilized 1 egg + 1 sperm + 2 or 3 polar bodies a polar body -n 2n n n + n Stops at prophase I till puberty, then each month one 1 o oocyte will go as far as metaphase II A human female typically has about 400,000 follicles/potential eggs by puberty, all formed before birth (born with up to 2 million). Only several hundred (about 480) of these eggs will actually ever be released during her reproductive years. Normally, in humans, after the onset of puberty, one egg per cycle matures and is released from its ovary. One month the left ovary will release a potential egg and the next month the right ovary will release a potential egg.
Ovulation is the release of a mature egg due to the stimulation of leutenizing hormone (LH), which then stimulates the remaining follicle cells to turn into a corpus luteum (temporary secretory structure which forms from remains of ovarian follicle), which then secretes progesterone to prepare the uterus for possible implantation. If an egg is not fertilized by a sperm and does not implant, the corpus luteum disintegrates and when it stops producing progesterone, the lining of the uterus breaks down and is shed. Each month after puberty one of these 1 o oocytes divide into a 2 o oocyte and a polar body the 2 o oocyte receives almost all of the cytoplasm the polar body acts like a garbage can to hold discarded chromosomes the polar body stays with 2 o oocyte The 2 o oocyte begins meiosis II but stops at metaphase II The 2 o oocyte leaves the ovary during ovulation and enters an oviduct, where it may be fertilized by a sperm an egg lives ~6 to 24 hours unless fertilization occurs. The stalled 2 o oocyte begins meiosis II but stops at metaphase II until it is fertilized by a sperm when a sperm enters, fertilization occurs, and meiosis II completes with the formation of at least 1 other polar body. Therefore at this point there is one egg and 2 or 3 polar bodies Polar bodies hold excess chromosomes and will disintegrate after fertilization. The mature egg has 23 chromosomes this will combine with the fertilizing sperm (23) to make 46 full complement of chromosomes Number of eggs For the most part, A woman has her maximum number of potential eggs (primary oocytes) while still a fetus, more than 7 million. By birth the number has fallen to 1 or 2 million, and by puberty to about 300,000-400,000. Only 300 to 400 reach maturity. For many years biologists assumed that at birth female mammals of most species had all the eggs they would ever have. 2 Then in 2004 researchers reported 3 discovering germline stem cells in mouse ovaries. These cells enable mice to continue producing new oocytes and follicles after birth. In 2012, researchers reported that the ovaries of reproductive-age women also possess small numbers of germline stem cells similar to those of the mouse. Thus, women may be able to produce new eggs after they are born.
H. Comparison of phases of cell division Meiosis vs Mitosis: Phase ( PMAT ) Mitosis Meiosis I Meiosis II Prophase no pairing Pairing of homologous No pairing chromosomes and crossing over Metaphase Duplicated chromosomes at equator = 2 chromatids Homologous chromosomes at equator = 4 chromatids Haploid # (1/2) of duplicated chromosomes @ equator = 2 Anaphase Sister chromatids separate Telophase 2 daughter cells - diploid Homologous chromosomes separate 2 daughter cells - haploid chromatids Sister chromatids separate 4 daughter cells - haploid