Theme: Reproduction on a cellular level

Lecturer: prof. Fedonyuk L.Ya.

Plan of lecture:• Structure of nucleus• The levels of organization of eukariotic

chromosomes.• Сhromosomes types.• Normal human karyotype

characteristics.• Cell (mitotic) cycle, its stages. Mitosis, its

stages. • Cytological and cytogenetically

characteristics of meiosis.

Nucleus

consists of nuclear envelope nucleolus nucleoplasm chromatin

(chromosomes)

Nuclear envelope• surrounds the nuclear

material• consists of two parallel

membranes separated from each other by a narrow perinuclear cisternae

• is perforated at intervals by openings called nuclear pores

Nuclear Membrane or Envelope - two membranes which formthe nucleus, is porous. Allows RNA to leave nucleus.

Assembly and Disassembly of Nuclear Envelope

•Nuclear envelope is a cell cycle dependent structure that disperses at the onset of mitosis (late prophase) and reassembles around the reforming nucleus in the late telophase.

•The correlations of breakdown of the nuclear envelope, formation of chromosomes and mitosis are essential for cell division and the ability of cells to divide in an orderly manner.

Nucleoplasm

• is the portion of the protoplasm that is surrounded by the nuclear envelope

• is consists of a matrix and various types of particles.

Whole Mount Electron Microscopy Demonstrating Fibrogranular Structure of the Internal Nuclear

Matrix

Nucleolus• is a well-defined nuclear

inclusion (sometime more than one)

• is present in the cells that are actively synthesizing proteins

• become detectably only when the cell is in interphase

• is involved in the synthesis of rRNA and its assembly into precursors of ribosomes

Chromatin• is double-stranded DNA

complexed with histones and acidic proteins

• is responsible for RNA – synthesis, resides within the nucleus in two forms: heterochromatin and euchromatin

Heterochromatin• is a highly condensed portion

of chromatin• is visible in light microscope• appears in the light

microscope as basophilic clumps of nucleoprotein

• is not transcribed into RNA

Euchromatin• isn’t condensed portion of

chromatin during interphase• from which RNA is

transcribed, its genes can be activated, is transcriptionally active, mostly encodes proteins

• does not visible in light microscope.

Levels of organization of eukaryotic chromosomes:

1. The DNA is associated with basic proteins called histones to form nucleosomes, each of which consists of 8 histones bead with DNA wrapped around it.

2. The nucleosomes are organized into large coiled loops held together by nonhistone scaffolding proteins.

3. The chromonema is a single double-stranded DNA molecule with a protein coat

Chromosome = DNA (deoxyribonucleic acid) + associatedproteins (mainly histones) = “packaged” DNA

Organization of eukaryotic chromosomes• The chromosomes have already

doubled, and each now consists of two identical sister chromatid

• The chromatid is composed of a very fine filament, called as chromonema

• The two chromatids remain attached to each other at a point of primary constriction, the centromere.

• The centromere is a specific DNA sequence of about 220 nucleotides, to which is bound a disk of protein called a kinetochore.

• It is a place, where the spindle fibers attach during cell division.

• Regions on either sides of centromere are called arms.

• The long arm of a chromosome is designated “q” and the short arm – “p”.

Metaphase chromosome structure

1 - long arm 2 - short arm 3 - centromere 4 - secondary constriction 5 - satellite 6 - chromatids

Types of chromosomes

1 – Metacentric (the centromere divides it into two equal arms) 2 – Submetacentric (the centromere is slightly displace from the

center of chromosome)3, 4 – Acrocentric (the centromere establishes one long arm and one

short arm) а – centromereб - secondary constriction

Karyotype • is a diploid number of chromosomes • is represented in humans by the 22

pairs of autosomes and the 1 pair of sex chromosomes (either XX or XY) totaling 46 chromosomes

• Pair of chromosomes, with the same gene loci in the same order, are known as the homologous chromosomes.

• The chromosomes of each pair have characteristic size and shape.

• An ideogram is a karyotype, which displays chromosomes arranged in pairs in descending size order.

2n=44a+XX2n=44a+XX ((femalefemale karyotypekaryotype))2n=44a+XY2n=44a+XY ((malemale karyotype)karyotype)

• Two important characters of living organisms, Growth and Reproduction are due to Cell division.

• Cell division involves two phases – 1. Division of nucleus.– 2. Division of cytoplasm.

• Two major types of cell division– 1. Mitosis - similar daughter cells (2n)– 2. Meiosis - Haploid/gametes (n)

The cell cycle•Covers a time from one division of cells till other division or destruction (perish) of cell

•It has two major stages: 1) mitosis 2) interphase

Cell Cycle: Interphase• Before mitosis• Time of high metabolic activity• DNA replicated and synthesized• Three phases: G1, S, and G2• G1(gap 1)- longest stage of cell

cycle, RNA, protein sysnthesis• S (synthesis)- DNA replicated , 2

chromatids per chromosome, chromatids genetically identical

• G2(gap 2)- RNA synthesis, not well understood

The human cell cycle

G1

S

G2M

DNA synthesis synthesis

Growth and preparation forcell division

Rapid growth and preparation for DNA synthesis

phase

phase

phase

phase

Mitosis

Cell Cycle: Mitosis

• Process of cell division(nuclear division) which produces daughter cells genetically identical to the parent cell

• Four Phases (P-M-A-T): prophase, metaphase, anaphase, and telophase.

• Upon completion of the phases of mitosis (nuclear division) the cell “officially” divides into two by a process called cytokinesis - division of cytoplasm

Cell Cycle

Interphase

Not part of mitosisDNA is replicatedchromosomes start to condense

Chromosome duplication

Prophase

*Chromosomes coil and condense further.*Nucler membrane breaks down/ disappears.*Microtubules increase in number, spindle apparatusforms.

G2, Prophase

Metaphase*Nuclear membrane completely disappeared*Chromosomes move to equator of cell - begin to line up*Chromosomes attach to spindle via kinetochore

Centromeric Region

Anaphase*Movement of chromosomes via microtubulesto opposite sides of the cell. One chromatid to one end the otherChromatid to the opposite end

Chromatid Separation

• In anaphase, the sister chromatids separate.– Two daughter cells– Each has a complete and

identical set of chromosomes

Telophase

*Genetically identical info at each pole*Spindle fibers disappear*Chromosomes uncoil*Nuclear envelope reforms around

METAPHASE

Metaphaseplate

Spindle Daughterchromosomes

Nuclearenvelopeforming

ANAPHASE

Metaphase, Anaphase, Telophase

Mitosis in a plant cell

CytokinesisCytokinesis - is separate from mitosis, = pinching of cell/divison of cytoplasm.

Mitosis + Cytokinesis result in twoidentical daughter cells.

Mitosis

Histologic picture of Mitosis:

Mitosis:• Interphase: No morphological changes,

Replication of DNA and organelles.• Prophase: Visible chromosomes• Metaphase: equatorial plate formation• Anaphase: Separation of chromosome pairs• Telophase: Two separate nuclei formation.• Cytokinesis: Separation of daughter cells.

Chromosomes and Chromatids During Mitosis

BeginInterphase

AfterInterphase

AfterProphase

AfterMetaphase

AfterAnaphase

Aftertelophase

№ ofChromosomes

46 46 46 46 92 46

№ ofChromatids

46 92 92 92 92 46

Cell division

Molecular Basis of Carcinogenesis• Genes control cell division

by cytokines.• Four classes of regulatory

genes.1.Promotors – Proto-

oncogenes2.Inhibitors – Cancer-

suppressor genes – p533.Genes regulating Apoptosis.4.DNA repair genes.

Meiosis

• Cell division which results in halpoid “sex” cells (i.e., egg and sperm)

• One replication of the genetic material (DNA) during interphase, but two nuclear divisions (meiosis I and meiosis II).

• Results in haploid (N) cells (= gametes in animals) from an initial diploid (2N) cell

• Very similar to mitosis except that the cells produced are not genetically identical.

Overview of meiosis: how meiosis reduces chromosome number

Meiosis Meiosis I (reduction) Prophase I is divided into the

following five stages:• Leptotene -the chromatin

condenses into the visible chromosomes, each of which contain two chromatids joined at the centromere

• Zygotene - homologous maternal and paternal chromosomes pair and make physical contact (synapsis), forming a tetrad

Crossing-Over

. Prophase I • Pachytene - the chiasmata are formed and crossing over (random exchanging of genes between segments of homologous chromosomes) occurs – an event that is crucial for increasing generic diversity

• Diplotene - the chromosomes continue to condense and chiasmata can be observed, indicating where crossing over has taken place

• Diakinesis - the nucleolus disappears, chromosomes are condensed maximally, and the nuclear envelope disappears

Metaphase I• Spindle formation is

complete• Bivalents are aligned

at equator• Kinetochore

microtubules of the homologues point to opposite polesAnaphase I

• Homologues separate and move toward opposite poles

• Cytokinesis begins

The stages of meiotic cell division: Meiosis I

Telophase I

• Spindle dissolvesSpindle dissolves• Nuclear envelopes reformNuclear envelopes reform• Daughter nuclei are haploidDaughter nuclei are haploid• Each chromosome is still Each chromosome is still duplicatedduplicated• Cytokinesis results in 2 Cytokinesis results in 2 daughter cellsdaughter cells

Meiosis II Equatorial division -

begins soon after the completion of meiosis I, following a very brief interphase without DNA replication.

Prophase II • chromosomes are still

duplicated• spindle formation begins• nuclear envelope

fragments

Metaphase II• Spindle formation is

complete• Duplicated chromosomes

are aligned at equator• Kinetochore microtubules

of sister chromatids point to opposite poles

Anaphase II• Centromeres divide• Haploid sets of chromosomes

move toward poles• Cytokinesis begins

Telophase II

• Spindle dissolves• Nuclear envelopes

reform• Daughter nuclei are

haploid and genetically dissimilar from parent cell and each other

• Cytokinesis results in 4 daughter cells

The stages of meiotic cell division: Meiosis II

Independent Assortment

The biological significance of meiosis

• meiosis enables a species’ chromosome number to remain constant over generation.

• Meiosis produces novel combination of genes.

• Meiosis produces novel combination of non homologies chromosomes

“To a brave man, good and bad luck are like

his right and left hand, He uses both“

St Catherine of Siena

Thank you for attention !