Cell Cycle, Cell Division and Mitosis

Phases of the Cell Cycle The cell cycle consists of Interphase – normal cell activity The mitotic phase – cell division INTERPHASE Growth G 1 (DNA synthesis) Growth G2 Cell Divsion

Cell Division All cells are derived from pre-existing cells New cells are produced for reproduction, growth and to replace damaged or old cells Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals)

Functions of Cell Division Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Cell Division An integral part of the cell cycle Results in genetically identical daughter cells Cells duplicate their genetic material Before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA

Cell Devision DNA must be copied or replicated before cell division Each new cell will then have an identical copy of the DNA

Identical Daughter Cells Two identical daughter cells Parent Cell

Prokaryotic Chromosome The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane

Eukaryotic Chromosomes All eukaryotic cells store genetic information in chromosomes Each chromosome is composed of a single, tightly coiled DNA molecule Chromosomes can’t be seen when cells aren’t dividing and are called chromatin

Called Sister Chromatids Duplicated chromosomes are called chromatids & are held together by the centromere Called Sister Chromatids

Chromosomes Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere

Homologues Chromosome Look the same Control the same traits May code for different forms of each trait Independent origin - each one was inherited from a different parent

Chromosome Duplication In preparation for cell division, DNA is replicated and the chromosomes condense Each duplicated chromosome has two sister chromatids, which separate during cell division 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centrometers A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells.

Non-sister chromatids Two duplicated chromosomes Chromosome Duplication Because of duplication, each condensed chromosome consists of 2 identical chromatids joined by a centromere. Each duplicated chromosome contains 2 identical DNA molecules (unless a mutation occurred), one in each chromatid: Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two unduplicated chromosomes Centromere Sister chromatids Duplication Non-sister chromatids Two duplicated chromosomes

Structure of Chromosomes The centromere is a constricted region of the chromosome containing a specific DNA sequence, to which is bound 2 discs of protein called kinetochores. Kinetochores serve as points of attachment for microtubules that move the chromosomes during cell division: Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metaphase chromosome Kinetochore microtubules Centromere region of chromosome Sister Chromatids

Structure of Chromosomes Diploid - A cell possessing two copies of each chromosome (human body cells). Homologous chromosomes are made up of sister chromatids joined at the centromere. Haploid - A cell possessing a single copy of each chromosome (human sex cells).

Types of Cell Reproduction Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells Mitosis & binary fission are examples of asexual reproduction Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells Meiosis is an example of sexual reproduction

Cell Division in Prokaryotes Prokaryotes such as bacteria divide into 2 identical cells by the process of binary fission Single chromosome makes a copy of itself Cell wall forms between the chromosomes dividing the cell Parent cell Chromosome doubles Cell splits 2 identical daughter cells

Phases of the Cell Cycle Interphase G1 - primary growth S - genome replicated (DNA replication) G2 - secondary growth M - mitosis C - cytokinesis

Phases of the Cell Cycle

Interphase G1 - Cells undergo majority of growth S - Each chromosome replicates (synthesizes) to produce sister chromatids (DNA replication) Attached at centromere Contains attachment site (kinetochore) G2 - Chromosomes condense - Assemble machinery for division such as centrioles

Interphase - G1 Stage 1st growth stage after cell division Cells mature by making more cytoplasm & organelles Cell carries on its normal metabolic activities

Interphase – S Stage Synthesis stage DNA is copied or replicated Two identical copies of DNA Original DNA

Interphase – G2 Stage 2nd Growth Stage Occurs after DNA has been copied All cell structures needed for division are made (e.g. centrioles) Both organelles & proteins are synthesized

Cell Divides into Identical cells DNA Copied Cells prepare for Division Cells Mature Daughter Cells Cell Divides into Identical cells

DNA duplication during interphase Mitosis Some haploid & diploid cells divide by mitosis. Each new cell receives one copy of every chromosome that was present in the original cell. Produces 2 new cells that are both genetically identical to the original cell. DNA duplication during interphase Mitosis Diploid Cell

Mitosis Division of the nucleus Also called karyokinesis Only occurs in eukaryotes four stages Doesn’t occur in some cells such as brain cells

Four Mitotic Stages Prophase Metaphase Anaphase Telophase

Early Prophase Chromatin in nucleus condenses to form visible chromosomes Mitotic spindle forms from fibers in cytoskeleton or centrioles (animal) Cytoplasm Nucleolus Nuclear Membrane Chromosomes

Late Prophase Nuclear membrane & nucleolus are broken down Chromosomes continue condensing & are clearly visible Spindle fibers called kinetochores attach to the centromere of each chromosome Spindle finishes forming between the poles of the cell

Late Prophase Chromosomes Nucleus & Nucleolus have disintegrated

Spindle Fiber attached to Chromosome Kinetochore Fiber Chromosome

Review of Prophase

Spindle Fibers The mitotic spindle form from the microtubules in plants and centrioles in animal cells Polar fibers extend from one pole of the cell to the opposite pole Kinetochore fibers extend from the pole to the centromere of the chromosome to which they attach Asters are short fibers radiating from centrioles

The Spindle

Metaphase Chromosomes, attached to the kinetochore fibers, move to the center of the cell Chromosomes are now lined up at the equator Equator of Cell Pole of the Cell

Metaphase Asters at the poles Spindle Fibers Chromosomes lined at the Equator

Chromosomes at Equator Metaphase Aster Chromosomes at Equator

Metaphase

Anaphase Occurs rapidly Sister chromatids are pulled apart to opposite poles of the cell by kinetochore fibers

Anaphase Sister Chromatids being separated

Anaphase

Telophase Sister chromatids at opposite poles Spindle disassembles Nuclear envelope forms around each set of sister chromatids Nucleolus reappears CYTOKINESIS occurs Chromosomes reappear as chromatin

Comparison of Anaphase & Telophase

Mitosis in a plant cell Nucleus Chromatine condensing Chromosome 1 Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is staring to from. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelop will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubles shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell. 2 3 4 5 Nucleus Nucleolus Chromosome Chromatine condensing

Cytokinesis Means division of the cytoplasm Division of cell into two, identical halves called daughter cells In plant cells, cell plate forms at the equator to divide cell In animal cells, cleavage furrow forms to split cell

Cytokinesis In Animal And Plant Cells Cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm 1 µm Vesicles forming cell plate Wall of patent cell Cell plate New cell wall (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM) Daughter cells

Cell plate in animal cell Cytokinesis Cleavage furrow in animal cell Cell plate in animal cell

Daughter Cells of Mitosis Have the same number of chromosomes as each other and as the parent cell from which they were formed Identical to each other, but smaller than parent cell Must grow in size to become mature cells (G1 of Interphase)

Identical Daughter Cells 2 Chromosome number the same, but cells smaller than parent cell

Mitotic Division of an Animal Cell G2 OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Nonkinetochore microtubules

Mitotic Division of an Animal Cell METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming Centrosome at one spindle pole Daughter chromosomes

G2 of Interphase A nuclear envelope bounds the nucleus. The nucleus contains one or more nucleoli (singular, nucleolus). Two centrosomes have formed by replication of a single centrosome. In animal cells, each centrosome features two centrioles. Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed. The light micrographs show dividing lung cells from a newt, which has 22 hromosomes in its somatic cells (chromosomes appear blue, microtubules green, intermediate filaments red). For simplicity, the drawings show only four chromosomes. G2 OF INTERPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Nucleolus Nuclear envelope Plasma membrane

Prophase The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope. The nucleoli disappear. Each duplicated chromosome appears as two identical sister chromatids joined together. The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”). The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them. PROPHASE Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids

METAPHASE Spindle Metaphase plate Centrosome at one spindle pole Metaphase Metaphase is the longest stage of mitosis, lasting about 20 minutes. The centrosomes are now at opposite ends of the cell. The chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate. For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. The entire apparatus of microtubules is called the spindle because of its shape.

The Mitotic Spindle The spindle includes the centrosomes, the spindle microtubules, and the asters The apparatus of microtubules controls chromosome movement during mitosis The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell Assembly of spindle microtubules begins in the centrosome, the microtubule organizing center An aster (a radial array of short microtubules) extends from each centrosome

The Mitotic Spindle Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell Microtubules Chromosomes Sister chromatids Aster Centrosome Metaphase plate Kineto- chores Kinetochore microtubules 0.5 µm Overlapping nonkinetochore 1 µm

Anaphase Anaphase is the shortest stage of mitosis, lasting only a few minutes. Anaphase begins when the two sister chromatids of each pair suddenly part. Each chromatid thus becomes a full- fledged chromosome. The two liberated chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 µm/min). The cell elongates as the non kinetochore microtubules lengthen. By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes. ANAPHASE Daughter chromosomes

TELOPHASE AND CYTOKINESIS Two daughter nuclei begin to form in the cell. Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system. The chromosomes become less condensed. Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete. TELOPHASE AND CYTOKINESIS Nucleolus forming Cleavage furrow Nuclear envelope forming

Meiosis Formation of Gametes (Eggs & Sperm)

Meiosis Reduction in amount of genetic material in the daughter cell A type of cell division that results in four daughter cells each with half number of chromosome of the parent cell, as in the production of gametes and plant spores The process by which the nucleus divides in all sexually reproducing organisms during the production of spores and gametes The process by which the cell divides producing daughter cell that have a single set of chromosomes and are called haploid as opposed to diploid cells with two sets of chromosome

Facts About Meiosis Preceded by interphase which includes chromosome replication Two meiotic divisions: 1. Meiosis I 2. Meiosis II (Reduction- division) Original cell is diploid (2n) Four daughter cells produced that are monoploid (1 n)

Facts About Meiosis Only diploid cells can divide by meiosis. Prior to meiosis I, DNA replication occurs. During meiosis, there will be two nuclear divisions, and the result will be four haploid nuclei. No replication of DNA occurs between meiosis I and meiosis II.

Facts About Meiosis Daughter cells contain half the number of chromosomes as the original cell Produces gametes (eggs & sperm) Occurs in the testes in males (Spermatogenesis) Occurs in the ovaries in females (Oogenesis)

Meiosis It is the fundamental basis of sexual reproduction Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote

Fertilization 2n = 6 1n =3 “Putting it all together”

Stages of Meiosis Meiosis II Meiosis I Begins with Interphase Chromosomes replicate Meiosis II Prophase II Metaphase II Anaphase II Telophase II Meiosis I Prophase I Metaphase I Anaphase I Telophase I

Replication of Chromosomes Replication is the process of duplicating a chromosome Occurs prior to division Replicated copies are called sister chromatids Held together at centromere Occurs in Interphase Replication is the process of duplicating chromosome. The new copy of a chromosome is formed by DNA synthesis during S-phase. The chromosome copies are called sister chromatids. Sister chromatids are held together at the centromere.

A Replicated Chromosome Gene X Sister Chromatids (same genes, same alleles) Homologs (same genes, different alleles) Homologs separate in meiosis I and therefore different alleles separate.

Meiosis Forms Haploid Gametes Meiosis must reduce the chromosome number by half Fertilization then restores the 2n number from mom from dad child too much! meiosis reduces genetic content The right number!

Meiosis: Two Part Cell Division Sister chromatids separate Meiosis I Meiosis II Homologs separate Diploid Haploid Haploid

Meiosis I: Reduction Division Nucleus Spindle fibers Nuclear envelope Early Prophase I (Chromosome number doubled) Metaphase I Late Prophase I Telophase I Anaphase I

Prophase I Late prophase Early prophase Chromosomes condense. Spindle forms. Nuclear envelope fragments. Early prophase Homolog’s pair. Crossing over occurs.

Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids)   Join to form a TETRAD Called Synapsis

Crossing-Over Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring

Homologous Chromosomes During Crossing-Over

Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment

Metaphase I Homologous pairs of chromosomes align along the equator of the cell

Anaphase I Homolog’s separate and move to opposite poles. Sister chromatid’s remain attached at their centromers.

Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.

Meiosis II Only one homolog of each chromosome is present in the cell. Gene X Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic information.

Meiosis II 4 Identical haploid cells Prophase II Metaphase II Telophase II 4 Identical haploid cells Anaphase II

Prophase II Nuclear envelope fragments. Spindle forms.

Metaphase II Chromosomes align along equator of cell.

Anaphase II Sister chromatids separate and move to opposite poles. Equator Pole Sister chromatids separate and move to opposite poles.

Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.

Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

Oogenesis or Spermatogenesis Gametogenesis Oogenesis or Spermatogenesis

Spermatogenesis Occurs in the testes Two divisions produce 4 spermatids Spermatids mature into sperm Men produce about 250,000,000 sperm per day

Spermatogenesis in the Testes Spermatid

Oogenesis Occurs in the ovaries Two divisions produce 3 polar bodies that die and 1 egg Polar bodies die because of unequal division of cytoplasm Immature egg called oocyte Starting at puberty, one oocyte matures into an ovum (egg) every 28 days

Oogenesis in the Ovaries

Oogenesis Oogonium (diploid) Mitosis Primary oocyte Meiosis I Secondary (haploid) Meiosis II (if fertilization occurs) First polar body may divide Polar bodies die Ovum (egg) Second polar body a A X Mature egg

Mitosis Meiosis Mitosis & Meiosis Functions Function Asexual reproduction Growth, repair Occurs throughout plant Produces clones Diploid parents and offspring Meiosis Function Sexual reproduction Occurs only in cells that give rise to sperm and eggs Produces variable offspring Diploid parents, haploid offspring

Mitosis & Meiosis Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4 Genetically identical? Yes No Chromosome number Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction

Mitosis and Meiosis Mitosis Two diploid cells produced Each identical to parent Meiosis Four haploid cells produced Differ from parent and one another