Mitosis and Sexual Life Cycles

Mitosis and Sexual Life Cycles

• Interphase - the cell grows; in preparation for cell division, the chromosomes are duplicated, with the genetic material (DNA) copied precisely.

• Mitosis - the chromosomes' copies are separated from each other and moved to the opposite ends of the cell

• Cytokinesis - the cell divides into two daughter cells, genetically identical to each other and to the parent cell

• Cell division- the unique capacity to procreate, like all biological functions, has a cellular basis, and the continuity of life

Functions of cell division

1. Asexual reproduction - an amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism.

2. Growth and development

3. Tissue renewal

Cellular Organization of the Genetic Material

• Genome - a cell’s DNA genetic information

• Chromatin - entire complex of DNA and proteins that is the building material of chromosomes

• Sister chromatids are joined copies of the original chromosome

  • Cohesins - protein complexes

  • Sister chromatid cohesion - the attachment

• Centromere - a region made up of repetitive sequences in the chromosomal DNA where the chromatid is attached most closely to its sister chromatid

• Mitosis - the division of the genetic material in the nucleus

• Cytokinesis - the division of the cytoplasm

• Gamogenesis - occurs in the gonads, resulting in haploid gametes (sex cells)

• Fertilization - activation of egg and sperm, diploid zygote

• Cleavage: mitotic division of zygote → multicellular embryo

• Gastrulation - cell migration that establishes primitive layers

• Formation of the body plan: The body plan of the embryo is formalized

• Organogenesis - organs grow and differentiate

Cell cyle

• Walther Flemming - developed dyes that allowed him to observe the behavior of chromosomes

• Mitosis - the distribution of chromosomes into two daughter nuclei

• Cytokinesis - division of cytoplasm, producing two daughter cells

  • Each daughter cell can start a new cell cycle

• Interphase - perform the roles that are attached to them; the majority of the process is done here.

  • G1 phase = metabolic activity and growth

  • S phase = metabolic activity, growth, and DNA Synthesis, where duplication happens

  • G2 phase = metabolic activity, growth, and preparation for cell division

    • A nuclear envelope encloses the nucleus

    • The nucleus contains one or more nucleoli (singular, nucleolus)

    • chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed

    • Two centrosomes have formed by duplication of a single centrosome

      • Centrosomes are regions in animal cells that organize the microtubules of the spindle

• 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 at their centromeres and often all along their arms by cohesins, resulting in sister chromatid cohesion.

  • The mitotic spindle (named for its shape) 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, propelled partly by the lengthening microtubules between them.

• Prometaphase = the nuclear envelope fragments

  • The microtubules extending from each centrosome can now invade the nuclear area.

  • The chromosomes have become even more condensed.

  • A kinetochore, a specialized protein structure, has now formed at the centromere of each chromatid (thus, two per chromosome)

  • Some of the microtubules attach to the kinetochors, becoming “kinetochore microtubules,” which jerk the chromosomes back and forth.

  • Nonkinetochore microtubules interact with those from the opposite pole of the spindle, lengthening the cell.

• Metaphase = the centrosomes are now at opposite poles of the cell

  • The chromosomes have all arrived at the metaphase plate, a plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie at the metaphase plate

  • For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles

• Anaphase = is the shortest stage of mitosis, often lasting only a few minutes

  • Begins when the cohesin protein is cleaved. This allows the two sister chromatids of each pair to separate suddenly. Each chromatid thus becomes an independent chromosome

  • The two new daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region

  • The cell elongates as the nonkinetochore microtubules lengthen

  • By the end of anaphase, the two ends of the cell have identical and complete collections of chromosomes

• Telophase = two daughter nuclei form within the cell. Nuclear envelopes arise from the fragments of the parent cell's nuclear envelope and other portions of the endomembrane system

  • Nucleoli reappear

  • The chromosomes become less condensed

  • Any remaining spindle microtubules are depolymerized

  • Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete

• Cytokinesis = the division of the cytoplasm is usually well underway by late telophase, so the two daughter cells appear shortly after the end of mitosis

  • In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two 

• Mitotic phase - the reproductive phase of the cells

• The mitotic spindle, which begins to form in the cytoplasm during prophase

• The assembly of spindle microtubules → centrosome (subcellular region containing material that functions throughout the cell cycle to organize the cell microtubules)

• Aster - a radial array of short microtubules extends from each centrosome

• Kinetochore - a structure made up of proteins that have assembled on specific sections of DNA at each centromere

• Kinetochore microtubules - spindle microtubules attach to the kinetochores

• Metaphase plate - an imaginary plate rather than an actual cellular structure

• Cleavage - the process in cytokinesis

• Cleavage furrow =  a shallow groove in the cell surface near the old metaphase plate (in animal cells)

  • Cytokinesis in plant cells - no cleavage furrow; telophase, vesicles derived from the Golgi apparatus move along mircotubules ot the middle of the cell, where they coalesce, producing a cell plate.

• Binary fission - division in half, refers to this process and to the asexual reproduction of single-celled eukaryotes, such as he amoeba.

• Origin of replication - the process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific point on the chromosome

• Chromosome replication begins at the origin → one copy of the origin is now at each end of the cell, where it elongates → replication finishes, and the plasma membrane is pinched inward by tubulin-like protein, and a new cell wall is deposited → two daughter cells result.t

The Evolution of Mitosis

• Mechanisms of cell division in several groups of organisms. Bacteria - During binary fission in bacteria, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism involves polymerization of actin-like molecules and possibly proteins that may anchor the daughter chromosomes to specific sites on the plasma membrane.

1. Dinoflagellates - in unicellular eukaryotes called dinoflagellates, the chromosomes attach to the nuclear envelope, which remains intact during cell division. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a process reminiscent of bacterial binary fission.

2. Diatoms and some yeasts - in these two other groups of unicellular eukaryotes, the nuclear envelope also remains intact during cell division.n

   1. In these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei. ei

• In most eukaryotes, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis.

  • Microtubules separate the chromosomes, and two nuclear envelopes then form

The Cell Cycle Control System

• Cell cycle control system - a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle

• Checkpoint - is a control point where stop and go-ahead signals can regulate the cycle; three important checkpoints are found in the G1, G2, and M phases.

• Cyclin - a protein that gets its name from its cyclically fluctuating concentration in the cell

  • Kinases called cyclin-dependent kinases or Cdks

• MPF - is part of the G2 phase; it stands for “maturation-promoting factor); it is a checkpoint; allows mitosis to occur; it is the “M-phase-promoting factor” because it triggers the cell’s passage into the M phase

The fluctuation of MPF activity and cyclin concentration during the cell cycle

• Synthesis of cyclin begins in late S phase and continues through G2 → Cyclin combines with Cdk, producing MPF, and the cell passes the G2 checkpoint and begins mitosis → MPF promotes mitosis by phosphorylating proteins; MPF activity peaks during metaphase → during anaphase, the cyclin component of MPF is degraded; the cell enters G1 phase → during G1, the degradation of cyclin continues; MPF is recycled.

• G0 - a nondividing state;  most cells of the human body are actually in

• Growth factor is a protein released by certain cells that stimulates other cells to divide

  • PGDF - platelet-derived growth factor - made up of blood cell fragments called platelets

• Density-dependent inhibition - a phenomenon in which crowded cells stop dividing

Two important checkpoints:

1. G1 checkpoints

   1. In the absence of a go-ahead signal, a cell exits the cell cycle and enters the G0, a nondividing state.

   2. If a cell receives a go-ahead signal,  the cell continues on in the cell cycle.e

2. M checkpoint

   1. A cell in mitosis receives a stop signal when any of its chromosomes are not attached to the spindle fibers.

   2. When all chromosomes are attached to spindle fibers from both poles, a go-ahead signal allows the cell to proceed into anaphase.

• Anchorage dependence - to divide, they must be attached to something, such as the inside of a culture flask or the extracellular matrix of tissue

Loss of Cell Cycle  Controls in Cancer Cells

• Cancer cells do not heed the normal signals that regulate the cell cycle

• If and when they stop dividing, cancer cells do so at random checkpoints; they can divide indefinitely in culture if they are given a continual supply of nutrients

• Transformation - cells in culture that acquire the ability to divide indefinitely

  • Benign tumor - the abnormal cells may remain at the original site if their genetic and cellular changes don’t allow them to move to or survive at another site

  • Malignant tumor - includes cells whose genetic and cellular changes enable them to spread to new tissues and impair the functions of one or more organs; these cells are also sometimes called transformed cells

    • An individual with a malignant tumor is said to have cancer

Meiosis

• Meiosis - a special type of cell division that produces cells with half the chromosomes of the parent cells

  • It occurs only in specialized cells, such as the cells of the testes and ovaries in humans.

• Fertilization - unites a sperm and egg, re-establishing pairs of homologous chromosomes, with both paternal and maternal genes.

• Heredity - from the Latin heres, heir) - the transmission of traits from one generation to the next

• Genetics - the study of both heredity and inherited variation

• Genes - parents endow their offspring with coded information in the form of hereditary units

• The genetic program is written in the language of DNA, the polymer of four different nucleotides

• Gametes are the vehicles that transmit genes from one generation to the next

  • During fertilization, male and female gametes (sperm and eggs) unite, passing on genes of both parents to their offspring

• Somatic cells - humans have 46 chromosomes in them - all cells of the body except the gametes and their precursors

• Gene’s locus - a gene’s specific location along the length of a chromosome

• Asexual reproduction - a single individual, like a yeast cell or an amoeba, is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes

• Eukaryotic organisms can reproduce asexually by mitotic cell division, in which DNA is copied and allocated equally to two daughter cells

• Clone - an individual or group of individuals that are genetically identical to its parent

• Sexual reproduction - two parents give rise to offspring that have unique combinations of genes inherited from the two parents

• Genetic variation is an important consequence of sexual reproduction

Fertilization and Meiosis in Sexual Life Cycles

• Life cycle - the generation-to-generation sequence of stages of the reproductive history of an organism, from conception to production of its own offspring

• Karyotype - images of the chromosomes are arranged in pairs, starting with the longest chromosomes

  • Homologous chromosomes (homologs) - the two chromosomes of a pair have the same length, centromere position, and staining pattern

  • The two chromosomes, X and Y, are an important exception to the general pattern of homologous chromosomes in human somatic cells.

  • Human females have a homologous pair of X chromosomes (XX), while males have one X and one Y (XY)

• Sex chromosomes - the X and Y chromosomes

  • The other chromosomes are called autosomes

• Diploid cell - any cell with two sets of chromosomes

• Haploid cells - gametes contain a single set of chromosomes

  • The haploid number of chromosomes for humans is 23, 22 autosomes, and one sex chromosome

• Fertilization - the union of gametes in fusion of their nuclei

• Zygote (fertilized egg) - diploid because it contains two haploid sets of chromosomes

• Meiosis - this type of cell division reduces the number of sets of chromosomes from two in the parent cell to one in each gamete

• Alternation of generation - includes both diploid and haploid stages that are multicellular

  • Sporophyte - multicellular diploid stage

  • Spores - meiosis in the sporophyte produces it

Stages of Meiosis

• Meiosis - like mitosis, is preceded by interphase, which includes S phase (the duplication of chromosomes)

  • This is followed by not one but two consecutive cell divisions: meiosis I and meiosis II.

• Sister chromatid cohesion - sister chromatids are two copies of one chromosome, closely associated along their lengths.

Meiosis I - Separates Homologous Chromosomes

• Prophase I - centrosome movement, spindle formation, and nuclear envelope breakdown occur as in mitosis

  • Chromosomes condense progressively throughout prophase I

  • During early prophase I, each chromosome pairs with its homolog, aligned gene by gene, and crossing over

  • The DNA molecules of nonsister chromatids are broken and are rejoined to each other

  • Each homologous pair has one or more X-shaped regions called chiasmata. Where crossovers have occurred

• Metaphase I - pairs of homologous chromosomes are now arranged at the metaphase plate, with one chromosome of each pair facing each pole

  • Each pair has lined up independently of other pairs (this arrangement is called independent assortment, to be discussed later)

  • Both chromatids of one homolog are attached to kinetochore microtubules from one pole

  • The chromatids of the other homolog are attached to microtubules from the opposite pole

• Anaphase I - breakdown of proteins that are responsible for sister chromatid cohesion along chromatid arms allows homologs to separate

  • The homologs move toward opposite poles, guided by the spindle apparatus

  • Sister chromatid cohesion persists at the centromere, causing the two chromatids of each chromosome to move as a unit toward the same pole

• Telophase I and Cytokinesis - when telophase begins, each half of the cell has a complete haploid set of duplicated chromosomes.

  • Each chromosome is composed of two sister chromatids; one or the other chromatids include regions of nonsister chromatid DNA

  • Cytokinesis - division of cytoplasm usually occurs simultaneously with telophase I, forming two haploid daughter cells

  • In animal cells like these, a cleavage furrow forms; in plant cells, a cell plate forms

  • In some species, chromosomes decondense, and nuclear envelopes form

  • No chromosome duplication occurs between meiosis I and meiosis II

• Prophase II - a spindle apparatus forms

  • In late prophase II, chromosomes, each still composed of two chromatids associated at the centromere, are moved by microtubules toward the metaphase II plate.

• Metaphase II - the chromosomes are positioned at the metaphase plate as in mitosis.

  • Because of crossing-over in meiosis, the two sister chromatids of each chromosome are not genetically identical.

  • The kinetochores of sister chromatids are attached to microtubules extending from opposite poles.

• Anaphase II - breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to separate and move toward opposite poles

  • Each chromatid has now become an individual chromosome

• Telophase II and Cytokinesis - nuclei form, the chromosomes begin decondensing, and cytokinesis occurs

  • The meiotic division of one parent cell produces four daughter cells, each with a haploid set of unduplicated chromosomes

  • The four daughter cells are genetically distinct from one another and from the parent cell

Cross-Over and Synapsis During Prophase I

• During propphase I, two members of a homolohous pair associate loosely along their length and is broken by specific proteins at precisely matching points → synaptonemal complex holds one homology tightly to the other → synapsis, the DNA breaks are closed up so that each broken end is joining to the corresponding segment of the nonsister chromatid → the points where crossing over has become visible as chiasmata after the synaptonemal complex disassembles and the homologs move slightly apart.

A Comparison of Mitosis and Meiosis 

1. Synapses and crossing over - duplicated homologous pairs pair up, and crossing over occurs

2. Alignment of homologous pairs at he metaphase plate - pairs of homologs are positioned at the metaphase plate, rather than individual chromosomes, as in metaphase of mitosis

3. Separation of homologs - at anaphase I of meiosis, the duplicated chromosomes of each homologous pair move toward opposite poles, but the sister chromatids of each duplicated chromosome remain attached

• Metastasis - the spread of cancer cells to locations distant from their original site

• Interphase - the cell grows in preparation for cell division, the chromosomes are duplicated, with the genetic material (DNA) copied precisely.y

• Mitosis - the chromosomes' copies are separated from each other and moved to opposite ends of the cell

• Cytokinesis - the cell divides into two daughter cells, genetically identical to each other and to the parent cell

Meiosis: Morphologic Differentiation

• A process that allows the generation of gametes in the gonads

  • Male gonads - testes (spermatogenesis) → spermatozoa or sperm

  • Female gonads - ovaries (oogenesis) → oocytes or eggs

1. Generate haploid cells

2. Increase genetic diversity