Medical biology cytology Lec 6 nucleus cell cycle

Medical biology/ cytology Lec 6 - nucleus, cell cycle and cell divisions 15/03/1439 lecturer Farah Essa Ismaeel 1

The nucleus �The nucleus (nuclei, plural) the largest component of a cell, frequently appears as rounded, oval, flat, kidney shape, horse shoe shape, segmented or lobulated structure. �Position of nucleus often near the center of the cell but in some cells the nucleus located eccentric, basal or peripheral. 15/03/1439 lecturer Farah Essa Ismaeel 2

The nucleus �Found in all eukaryotic cells except mature red blood cells of mammals do not have a nucleus, or are nonnucleated. �Most cells have a single nucleus called mononucleated, some cells have two nucleus called binucleated as in liver cells (hepatocyte) or other cells may exhibit multiple nuclei called multinucleated as osteoclast and skeletal muscles. 15/03/1439 lecturer Farah Essa Ismaeel 3

�The nucleus stores genetic information. Every cell in the body contains the same genes. �The nucleus of a non dividing cell consists of the following components: 1. Nuclear envelop. 2. Chromatin material 3. Nucleolus. 4. Nucleoplasma 15/03/1439 lecturer Farah Essa Ismaeel 4

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Nuclear Envelope or nuclear membrane 15/03/1439 lecturer Farah Essa Ismaeel 6

Medical Application �Certain mutations in the gene coding for lamin A are associated with a subtype of the disorder progeria, which causes premature aging. 15/03/1439 lecturer Farah Essa Ismaeel 7

Functions of nuclear envelop �regulate the entry of proteins (histone and hormones) to the nucleus and export of RNAs from nucleus to the cytoplasm. � encloses the nucleus and separates the genetic material of the cell from the cytoplasm of the cell. � it serves as a barrier to prevent passage of macromolecules freely between the nucleoplasm and the cytoplasm. 15/03/1439 lecturer Farah Essa Ismaeel 8

Chromatin material 15/03/1439 lecturer Farah Essa Ismaeel 9

Nucleolus �Micrographs of a nucleus do show one or more dark regions of the chromatin with spongy appearance. These are nucleoli (sing. , nucleolus). �The nucleolus is a generally spherical, highly basophilic subdomain of nuclei in cells, actively making proteins. �The nucleolus is not surrounded by a membrane, it is a densely stained structure found in the nucleus. 15/03/1439 lecturer Farah Essa Ismaeel 10

Nucleoplasm or nuclear sap �Chromatin is immersed in a semi fluid medium called the nucleoplasm. �A difference in p. H suggests that nucleoplasm has a different composition from cytoplasm. 15/03/1439 lecturer Farah Essa Ismaeel 11

Functions of nucleus �It controls the heredity characteristics of an organism. �It is responsible for protein synthesis, cell division, growth and differentiation. �Stores heredity material in the form of deoxy-ribonucleic acid (DNA) strands. �Also stores proteins and ribonucleic acid (RNA) in the nucleolus. �It is a site for transcription process in which messenger RNA (m RNA) are produced for protein synthesis. �Nucleolus produces ribosomes and known as protein factories. �It also regulates the integrity of genes and gene expression. 15/03/1439 lecturer Farah Essa Ismaeel 12

The cell cycle 15/03/1439 lecturer Farah Essa Ismaeel 13

�G 1 phase. (8 Hr) Is the time gap between mitosis and DNA replication in which cells accumulate the enzymes and nucleotides required for DNA replication and cell grow in size also produce energy. �In specialized cells the G 1 is longer and in completely mature cells the cell escape from cell cycle to G 0 to do specific function �S phase. (8 Hr) called synthesis phase because the DNA duplicated and also centrioles duplicate �G 2 phase. (4 Hr) any error in DNA replication is corrected in this phase, happened second growth and RNA synthesis and storage of energy for next division finally synthesis of tubulin protein to build microtubules that form mitotic spindles. 15/03/1439 lecturer Farah Essa Ismaeel 14

�M phase. A nuclear division (mitosis in somatic cell and meiosis in germ cells) followed by a cell division (cytokinesis). 15/03/1439 lecturer Farah Essa Ismaeel 15

Mitosis 15/03/1439 lecturer Farah Essa Ismaeel 16

Mitosis The period of cell division, or mitosis (Gr. mitos, a thread), is the only cell cycle phase that can be routinely observed with the light microscope. During mitosis, a parent cell divides and each of the two daughter cells receives a chromosomal set identical to that of the parent cell. The events of mitosis can be subdivided into four phases. (Prophase, metaphase, anaphase and telophase) 15/03/1439 lecturer Farah Essa Ismaeel 17

prophase several changes occur: 1. The nucleolus disappears and the replicated chromatin condenses into discrete threadlike chromosomes. 2. At the centromere region of each chromosome, a large protein complex called the kinetochore serves as a site for attachment to microtubules. 3. The centrosomes with their now-duplicated centrioles separate and migrate to opposite poles of the cell. 4. The microtubules of the mitotic spindle polymerize between the two centrosomes. 5. Late in prophase, lamins and inner nuclear membrane are phosphorylated, causing the nuclear lamina and nuclear pore complexes to disassemble and disperse in cytoplasmic membrane vesicles. 15/03/1439 lecturer Farah Essa Ismaeel 18

Metaphase �chromosomes condense further and attach to the mitotic spindle at large electrondense protein complexes called kinetochores (Gr. kinetos, moving) at each centromere. The cell is now more spherical and the chromosomes are moved to equatorial plane. 15/03/1439 lecturer Farah Essa Ismaeel 19

Anaphase �sister chromatids (now called chromosomes themselves) separate and move toward opposite spindle poles by a combination of microtubule motor proteins and dynamic changes in the lengths of the microtubules as the spindle poles move farther apart. 15/03/1439 lecturer Farah Essa Ismaeel 20

Telophase �The two sets of chromosomes are at the spindle poles and begin reverting to their decondensed state. �The spindle depolymerizes �The nuclear envelope begins to reassemble around each set of daughter chromosomes. 15/03/1439 lecturer Farah Essa Ismaeel 21

cytokinesis A belt-like contractile ring of actin filaments associated with myosins develops in the peripheral cytoplasm at the cell’s equator. During cytokinesis at the end of telophase, constriction of this ring produces a cleavage furrow and progresses until the cytoplasm and its organelles are divided into two daughter cells, each with one nucleus. 15/03/1439 lecturer Farah Essa Ismaeel 22

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Most tissues undergo cell turnover with slow cell division and cell death. Nerve tissue and cardiac muscle are exceptions because their differentiated cells cannot undergo mitosis. The cell turnover rate is rapid in the epithelium lining the digestive tract and uterus or covering the skin. 15/03/1439 lecturer Farah Essa Ismaeel 25

Meiosis is a specialized process involving two cell divisions that occurs only in the cells that will form sperm and egg cells. 15/03/1439 lecturer Farah Essa Ismaeel 26

Two key features characterize meiosis � Early in the process the homologous chromosomes of each pair (one from the mother, one from the father) come together in an activity termed synapsis. During synapsis double-stranded breaks and repairs occur in the DNA, some of which result in reciprocal DNA exchanges called crossovers between the aligned maternal and paternal chromosomes. Crossing over produces new combinations of genes in the chromosomes in the germ cells so that few if any chromosomes are exactly the same as those in the mother and father. � The cells produced are haploid, having just one chromosome from each pair present in the body’s somatic cells. The union of haploid eggs and sperm at fertilization forms a new diploid cell (the zygote) that can develop into a new individual. 15/03/1439 lecturer Farah Essa Ismaeel 27

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Thank you and good luck 15/03/1439 lecturer Farah Essa Ismaeel 31