Cellular-reproduction-and-genetics.docx

CELLULAR REPRODUCTION AND GENETICS

Arabidopsis thaliana (mouse ear cress) Oryza sativa (rice) Zea mays (corn)

INHERITANCE AND VARIATION OF TRAITS o o

Heredity = it is where traits are passed on from parents to offspring. Variations = it demonstrates differences among individuals.

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CHROMOSOMES -

Are rod-like structures made of DNA and protein Storage units of genes

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2 Kinds of proteins in chromosomes A. Histones - Proteins that DNA wraps around - Helps hold shape of chromosome and allows DNA to pack tightly B. Nonhistones o Do not aid in packing of DNA, instead they control specific regions of the DNA o o o o

Chromosomes consist of two identical halves (each half is called a chromatid) Chromatids form when DNA makes a copy of itself prior to cell division When the cell divides, each new cell receives one chromatid Chromatids are held together by a centromere

Chromosome numbers of selected organisms Organisms Saccharomyces cerevisiae (common yeast) Caenorhabdites elegans (roundworm) Drosophila melanogaster (fruitfly) Canis lupus familiaris (dogs) Homo sapiens (man)

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24 20

Every living thing undergoes reproduction. The cellular level of reproduction, in the for of cell division, provides growth in living organism. Growth is a permanent increase in size resulting from cell division and cell differentiation to form tissues and organs in multicellular organism. Cell can increase in size to a certain point, after which most of them undergo division.

THE ROLE OF CHROMOSOMES IN CELL DIVISION o o o

Cell division is an essential aspect of life. It enables a multicellular organism to grown and reach the adult size. It makes possible the replacement of worn out or damaged cells and keeps the total number of cells in an adult organism relatively constant.

THE TYPES OF HUMAN CELLS o

The human body is composed of two general types of cells: 1. Body cells or somatic cells 2. Sex cells

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The human body cells or somatic cells o Like blood cells, bone cells, skin cells and muscle cells o Are diploid in number (abbreviated as 2n) o Has 23 pairs of chromosomes o 22 pairs = autosomal chromosomes 1 pair = sex chromosomes XY = male sex chromosome XX = female sex chromosome Every cell of an organism produces by sexual reproduction has 2 copies of each autosome (one from each parent) The two copies of each chromosome are called homologous chromosomes – same shape and size and carry genes form same trait.

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Chromosome number 32

10

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DIPLOID vs. HAPLOID CELLS 8 52 46

DIPLOID CELLS -

Cells with two sets of chromosomes (2n)

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Diploid cells have both chromosomes for each pair as well as the sex chromosomes Diploid cells in humans have 23 pairs, or 46 chromosomes 2n = 46

HAPLOID CELLS -

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Human gametes or sex cells are haploid Sperm - male gamete Egg - female gamete Haploid cells contain only one set of chromosomes (n) Have half the number of chromosomes as diploid cells and only have one sex chromosome

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One or more nucleoli are visible inside the nucleus. The chromosome cannot be clearly seen. They appear as an irregular mass that is grainy in appearance because the DNA they contain are stretched out thinly in the nucleus. This facilitates the replication of DNA during S phase. Nuclear division is divided further into: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Cytokinesis = is the cytoplasmic division.

KARYOTYPE

CELL DIVISION

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Is a picture showing the complete set of chromosome in one’s cell In karyotyping class, chromosomes are isolated, stained and examined under a powerful microscope Before cell starts dividing, the chromosomes are duplicated This process produces sister chromatids

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CELL CYCLE o

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The series of events in which cells grow and divide 2 main parts: A. Interphase – cell growth and DNA replication B. Mitosis – nuclear and cytoplasmic division The cell cycle is divided into several stages:  G1 Phase  S Phase  G2 Phase  M Phase

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In order for multicellular organisms like plants and animals to grow, they must produce new cells from pre-existing cells. The cells produced must be the same or identical to the original or parent cell. This type of cell is Mitosis. However, there is another type of cell division which produces daughter cells with half number of chromosomes as the original parent cell, the resulting are not similar. This type of cell division is called Meiosis. In humans, mitosis cell division is important in the growth of organisms an in repair of worn-out tissues. Somatic cells / body cells are involved in mitosis. All daughter cells are diploid (2n) and identical to its parent cell.

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INTERPHASE o o

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Most of cell’s time spent in this part of cycle G1 Phase (Growth 1) = the cell marks the onset of its cycle there is an increase in the number of cell organelles. S Phase (Synthesis Phase) = DNA replication takes place G2 Phase (Growth 2) = enzymes and proteins required during M phase are synthesized M Phase = is cell division phase / meiosis / mitosis; nuclear and cytoplasmic division takes place. Some cells enter G0 phase where they remain and no longer grow or divide (Ex: nerve cells) The nucleus is clearly visible as a distinct membrane – bound organelle/

46 2n

46 2n

PROPHASE o o o o

(early) Each chromosome consists of two sister chromatids attached at the centromere; Structure called asters start to form around the centrioles; The two centrioles move apart to opposite poles; (later) Nucleolus disappears;

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Nuclear envelope disappears and spindle fibers appear.

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METAPHASE o o

The chromosomes line up along the equatorial plate; Each chromosome is attached to a spindle fiber at the centromere.

ANAPHASE o o

Meiosis I 23

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23 n

Each chromosome splits; The spindle fibers pull the chromatids apart to opposite poles of the cell.

23 n

TELOPHASE

PHASES OF MEIOSIS

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PROPHASE I

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Spindle fibers disintegrate; nuclear envelope reforms; Nucleolus appears in each nucleus; Chromosome uncoils After telophase, cytokinesis or division of the cytoplasm follows. The cytoplasm divides forming two identical cells. In animal cells, cytoplasm divides by forming cleavage furrow. In plant cells, cleavage of the cytoplasm doesn’t occur. Instead, it forms a cell plate.

Cytokinesis differs for plant and animal cells o o

In animals, cytokinesis occurs by cleavage. This process pinches the cell apart.

MEIOSIS o

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Also in humans, meiosis occurs in reproductive organs (female and male gonads) these are the ovaries and testes respectively. Sex cells / gametes (egg cell and sperm cell) are being produced through meiosis cell division. A diploid cell produces four haploid sex cells at the end of two successive cell division. The sex cells are not identical in genetic make up.

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23 n

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Meiosis II

Homologous chromosome undergoes synapsis pairing (homologous chromosomes have the same shape, genes and length) Asters appear around the centrioles and centrioles move apart opposite poles. Chromatids of each chromosomes cross and twist around each other (crossing over) the point of crossing is called chiasma (chiasmata, plural) this allows exchange of genetic materials (crossing ovetr is very important because this produces new combinations of genes, thus VARIATION occurs. Nuclear envelope and nucleolus disappear; spindle fibers form

METAPHASE I o

Homologous chromosomes align themselves along the equatorial plate.

ANAPHASE I o

Homologous chromosomes separate and move to opposite poles of the cell.

TELOPHASE I AND CYTOKINESIS o

Nuclear membrane forms, nucleus and cytoplasm divides forming two haploid daughter cells; centrioles replicate.

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Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells

PROPHASE II o

Centrioles move to opposite poles;

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Nuclear envelope disintegrates; Spindle fibers form again

METAPHASE II o

Chromosomes arrange along the equatorial plate along with the spindle fibers.

ANAPHASE II o

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Sister chromatids separate; they are pulled towards the opposite poles.

TELOPHASE II o

Spindle fibers disappear, nuclear envelops form nucleus and cytoplasm divide, forming four haploid daughter cells.

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Either homologous pairs fail to separate during meiosis I. Or sister chromatids fail to separate during meiosis II. When chromosomes in a pair fail to separate from each other during meiosis I. The resulting gametes acquires both members of a pair of chromosomes. If this involves chromosome pair 21, for ex. one of the gametes contains two copies of the chromosome, then the individual produced will have 47 chromosomes in his or her cells (wit three copies of chromosome 21) This condition is known as Down’s Syndrome. Named after Dr. Langdon Down – who first studied the condition.

SPERMATOGENESIS AND OOGENESIS

DOWN’S SYNDROME

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Meiosis is important in the production od sex cells or gametes. Spermatogenesis and Oogenesis = are processes resulting to the production of sperm cells and egg cells respectively.

FERTILIZATION AND VARIATION o

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The production of haploid sex cells ensures that when fertilization (union of egg cell (n) and sperm cell (n) happens, the zygote contains a diploid number (2) of chromosomes. Meiosis also ensures variation. It is made possible through the process of crossing over, during the Prophase I, crossing over ensures the formation of gametes with a varied set of genetic characteristics.

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People with this condition suffer from variable degrees of mental retardation, sterility and increased risk beyond the age 40 of Alzheimer’s disease, which affects the functioning of the brain. It is usually associated with pregnancy in women above 35 years if age. Connection: an extra copy of chromosome 21 causes down syndrome

CRU DI CHAT SYNDROME o o o

Sometimes, a piece of chromosome breaks off and gets lost. The effects of this accident depend on the particular genetic material lost. For ex. when a part of chromosome is lost, the afflicted individual will have a face that is round, moonlike, cries feebly and is mentally and physically retarded. Cru di chat is a French for cat’s cry, which is the sound of a baby with this condition makes when he / she cries.

WHEN SOMETHING GOES WRONG DURING MEIOSIS

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KLINEFELTER SYNDROME

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Meiosis may not always proceed normally. Accidents sometimes happen. These accidents may affect the functioning of the spindle fibers or the movement of one of more chromosomes. In humans, some accidents have been known to cause abnormal conditions.

Accidents during meiosis can alter chromosome number o

Abnormal chromosome count is a result of nondisjunction.

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A male child with this condition has an extra X chromosome. Lower IQ than sibs Tall stature Poor muscle tone Reduced secondary sexual characteristics Gynaecomastia (male breasts) Small testes / infertility

TURNER SYNDROME o

A female child with this condition lacks an X chromosome.

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Several drugs may cause breaks or other abnormalities in the chromosomes. Chlorpromazine (a popular tranquilizer) Diphenhydramine (an antihistamine) Lysergic acid diethylamide or LSD (a hallucinogen)

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