Chapter 3 Cell Cycle & Cell Division Dr. Gobinath P

Cell Division Ensures the Passage of Genetic Information

Cell division is involved in both asexual and sexual reproduction  Somatic cells - body cells make up most of the organism  Asexual reproduction - increase in number of somatic cells or the number unicellular organisms  Germ cells - found only in testes and ovaries, they produce sperm or eggs  Sexual reproduction – requires the production of eggs and sperm

Prokaryotes reproduce asexually  In bacteria and archaea, reproduction consists of duplicating the single chromosome, located in the nucleoid, and distributing a copy to each daughter cell

Prokaryotes use binary fission to reproduce 8-5

Somatic Cells Have a Cell Cycle and Undergo Mitosis and Cytokinesis 8-6

The eukaryotic cell cycle is a set series of events  Interphase - the time when a cell performs its usual functions. 8-7

M (Mitotic) Stage  Cell division occurs during the M stage and encompasses both division of the nucleus and division of the cytoplasm  Mitosis - nuclear division in the cell cycle  Cytokinesis - division of the cytoplasm 8-8

Cytokinesis is a noticeable part of the cell cycle 8-9

Eukaryotic chromosomes are visible during cell division  When a eukaryotic cell is not undergoing division, the DNA within a chromosome is a mass of thin threads called chromatin  Before nuclear division chromatin condenses,  2 identical chromatids are sister chromatids 8-10

A condensed duplicated chromosome 8-11

Somatic Cells Are Diploid (2n)  Diploid (2n) - includes two chromosomes of each kind  During mitosis, a 2n nucleus divides to produce daughter nuclei that are also 2n  A dividing cell is called the parent cell and the new cells are called the daughter cells 8-12

Gametes Are Haploid (n)  Haploid (n) number of chromosomes, contains only one chromosome of each kind  Half the diploid number Figure 8.4B When sister chromatids separate, each daughter nucleus gets a chromosome 8-13

Mitosis maintains the chromosome number  Before mitosis, DNA has replicated, each double helix is in a chromatid and the chromosomes consist of sister chromatids attached at a centromere  The centrosome - the microtubule-organizing center of the cell divides before mitosis  Spindle fibers separate the sister chromatids of the duplicated chromosomes 8-14

8-15 Phases of mitosis in animal cells and plant cells

8-16

Cytokinesis divides the cytoplasm  Cytokinesis follows mitosis in most cells  Cytokinesis in plant cells occurs by a process different than in animal cells  Cell plate - newly formed plasma membrane that expands outward 8-17

Cytokinesis in an animal cell 8-18

Cytokinesis in plant cells 8-19

Cell cycle checkpoints 8-20

Cell Cycle Inhibition  Contact Inhibition - In a culture, cells divide until they line a container in a sheet and then stop dividing  Cells “remember” number of divisions  Telomere - repeating DNA base sequence  Each time a cell divides some portion of a telomere is lost  When telomeres become too short, chromosomes fuse and do not duplicate 8-21

Meiosis Produces Cells That Become the Gametes in Animals and Spores in Other Organisms 8-22

Homologous chromosomes separate during meiosis  Karyotype - picture of chromosomes in numbered pairs called homologous chromosomes or homologues  X and Y chromosomes are the sex chromosomes because they contain the genes that determine gender  Autosomes - all the pairs of chromosomes except the sex chromosomes 8-23

karyotype shows that the chromosomes occur as pairs 8-24

Meiosis  Occurs during the production of the sperm and egg  Requires two divisions  Meiosis I - the chromosomes of each homologous pair separate  Meiosis II - the sister chromatids of each duplicated chromosome separate 8-25

8-26

Synapsis and crossing-over occur during meiosis I  Synapsis - homologous chromosomes come together and line up side by side forming a tetrad  Crossing-over - during synapsis, nonsister chromatids exchange genetic material 8-27

Synapsis of homologues 8-28

Crossing-over of nonsister chromatids 8-29

Reproduction increases genetic variation  Fertilization, the union of a male and a female gamete, enhances genetic variation  Sexual reproduction brings about genetic variation, and some offspring may have a better chance of survival and reproductive success than others 8-30

Independent assortment increases genetic variation 8-31

Meiosis requires two division cycles  Prophase, metaphase, anaphase, and telophase occur during both meiosis I and meiosis II  No replication of DNA occurs during a period called interkinesis between meiosis I and II 8-32

Phases of meiosis I 8-33

Phases of meiosis I 8-34

Phases of meiosis II 8-35

Phases of meiosis II 8-36

The life cycle of most multicellular organisms includes both mitosis and meiosis  Life cycle - in sexually reproducing organisms, all the reproductive events from one generation to the next  Spermatogenesis in males, occurs in the testes and produces sperm  Oogenesis in females, occurs in the ovaries and produces eggs  Zygote - product of the sperm and egg joining during fertilization, has homologous pairs of chromosomes 8-37

Life cycle of plants Life cycle of algae 8-38

Meiosis can be compared to mitosis 8-39

Chromosomal Abnormalities Can Be Inherited 8-40

An abnormal chromosome number is sometimes traceable to nondisjunction  Polyploidy - a eukaryote has three or more complete sets of chromosomes  Aneuploidy - an organism has more or less than the normal number of chromosomes  Monosomy only one of a type of chromosome  Trisomy three of a type of chromosome  Nondisjunction - in meiosis I homologues do not separate and both go into the same daughter cell, or in meiosis II sister chromatids fail to separate and both daughter chromosomes go to the same gamete 8-41

Nondisjunction of chromosomes during meiosis I of oogenesis, followed by fertilization with normal sperm 8-42

Nondisjunction of chromosomes during meiosis II of oogenesis, followed by fertilization with normal sperm 8-43

Abnormal chromosome numbers cause syndromes  Trisomy 21 (Down Syndrome)  Over 90% of individuals with Down syndrome have three copies of chromosome 21  Abnormal Sex Chromosome Inheritance  Turner syndrome females are born with only a single X chromosome  A male with Klinefelter syndrome has two or more X chromosomes in addition to a Y chromosome 8-44

Abnormal chromosome structure also causes syndromes  Various agents in the environment, such as radiation, certain organic chemicals, or viruses, can cause chromosomes to break  Deletion - an end of a chromosome breaks off or two simultaneous breaks lead to the loss of an internal segment  Duplication - the presence of a particular chromosome segment more than once in the same chromosome  Inversion - a segment of a chromosome is turned 180 degrees  Translocation - the movement of a chromosome segment from one chromosome to another nonhomologous chromosome 8-45

Types of chromosomal mutations 8-46

 All cells receive DNA from preexisting cells through the process of cell division  Mitosis is part of the cell cycle, and there are negative consequences if the cell cycle comes out of synchronization  Meiosis is part of the production of gametes, which have half the number of chromosomes as the parent cell Thank you