Cell Growth and Division PDF

Cell Growth and Division All Cells Come from Cells The Cell Cycle Creates New Cells  Repair and Growth ◼ Cell reproduction enables your body to produce new skin cells that replace dead cells at your skin’s surface. ◼ SKIN → consists of two major regions ◼ Epid...

Cell Growth and Division All Cells Come from Cells The Cell Cycle Creates New Cells  Repair and Growth ◼ Cell reproduction enables your body to produce new skin cells that replace dead cells at your skin’s surface. ◼ SKIN → consists of two major regions ◼ Epidermis: outer layer of skin ◼ Dermis: Inner layer of skin ◼ The epidermis consists of two layers: ◼ The top portion consists of dead skin cells ◼ The bottom portion consists of living cells. ◼ The living skin cells in the bottom portion of the epidermis are constantly reproducing → the new cells gradually move outward towards the skin’s surface, replacing skins cells that have rubbed off. ◼ Human skin cells reproduce ever 24 hours. ◼ When skin is injured, additional cell reproduction helps heal the wound. ◼ The kind of reproduction that is involved with growth and repair of cells is MITOSIS. ◼ The replacement of lost or damaged cells is just one of the important roles cell reproduction plays- another is growth, simply increasing I size from a baby to a child to an adult. ◼ All of the trillions of cells in our bodies result from cell reproduction → began with single fertilized egg cell.  Reproduction ◼ The production of new cells can result in growth and repair within organisms, but cell division also has an essential role in the creation of entire organisms. ◼ Some less complex organisms reproduce by simple cell division → a single cell or group of cells each duplicates its genetic material and then splits into two genetically identical cells. ◼ This process is known as asexual reproduction. ◼ Definition: process in which a single cells or set of cells produces offspring that inherit all their genetic material from one parent. ◼ As a result, the offspring are genetically identical to one another and to their parent. ◼ When two parents are involved in the production of offspring, the process is called sexual reproduction. ◼ Definition: process in which genetic material from two parents combine and produces offspring that differ genetically from either parent. ◼ Sexual reproduction involves the union of sex cells, such as an egg and a sperm. ◼ In unicellular organisms, simple cell division results in the asexual reproduction of new organisms → most multicellular organisms reproduces sexually, but some may also reproduce asexually ◼ Whether an organism reproduces sexually or asexually, ALL multicellular organisms depend on cell division (mitosis) for growth.  Chromosomes and Cell Division ◼ Almost all the genes of a eukaryotic cell are located in the nucleus – this genetic material exists as a mass of very long fibers that are too thin to be seen under a light microscope. ◼ These fibers are made up of chromatin – as a cell prepares to divide, its chromatin fibers condense, becoming visible as structures called chromosomes. ◼ Definition: combination of DNA and protein molecules, in the form of long, thin fibers, making up the genetic material in the nucleus of a eukaryotic cell ◼ Definition: condensed threads of genetic material formed from chromatin as a cell prepares to divide ◼ The number of chromosomes in a eukaryotic cell depends on the species → (human body cells generally ach have 46 chromosomes- each chromosome may contain hundreds of genes) ◼ Before cell division can begin, a cell must first duplicate all of its chromosomes- each chromosome now contains two identical joined copies called sister chromatids ◼ Definition: one of a pair of identical chromosomes before a cell divides.  The Cell Cycle ◼ How often a cell divides depends on the type of cell → some divide more often, others less often, and some do not divide at all. ◼ Eukaryotic cells that do divide undergo a sequence of events known as the cell cycle. ◼ Definition: sequence of events from the production of a eukaryotic cell to the rime the cell itself reproduces. ◼ The cell cycle is a continuous cycle: that includes both the actual division of the cell (mitosis) and the time the cell needs to prepare itself for division (interphase). ◼ INTERPHASE ◼ The cell may spend as much as 90 percent of the cell cycle in interphase. ◼ Definition: Stage of the cell cycle during which a cell carries out its metabolic processes an preforms its functions in the body. ◼ During this phase: the cell is making duplicates many of its organelles and growing in size.  ◼ Interphase is broken down into THREE different parts: ◼ G1: cell grows in size ◼ S: Genetic material duplicates ◼ G2: cell prepares to divide ◼ G stands for “growth” / S stands for “DNA synthesis” ◼ At the end of the G2 phase, the cell is ready to begin mitosis. ◼ Humans and other mammals cell cycle lasts 10-20 hours. S phase- 3-6 hours, G2 phase is slightly shorter ◼ MITOTIC PHASE ◼ The mitotic phase includes two processes, mitosis and cytokinesis. ◼ Definition: Stage of the cell cycle when a cell is actively dividing. ◼ During mitosis → the nucleus and the duplicated chromosomes divide and are evenly distributed, forming two “daughter” nuclei. ◼ Cytokinesis usually begins before mitosis is complete → the combination of mitosis and cytokinesis  The Mitosis Dance ◼ During mitosis, the chromosomes’ movements are guided by a framework of microtubules called a spindle. ◼ Definition: framework of microtubules that guide the movement of chromosomes during mitosis and meiosis. ◼ The spindle microtubules grow from two centrosomes. ◼ Definition: region of cytoplasmic material that in animal cells contains structures called centrioles. ◼ Spindles would still work without centrioles. ◼ The mitotic phase is divided into four main stages: Prophase, Metaphase, Anaphase, Telophase ◼ PROPHASE ◼ The first stage of mitosis is prophase – first stage mitosis and meiosis 1 and 2 when they already replicated chromosomes and condense. ◼ The chromatin fibers have condensed and are thick enough to be seen under a light microscope ◼ DURING PROPHASE: ◼ Each chromosome consists of a pair of sister chromatids joined at the centromere ◼ The nucleolus disappears, and the cell stops making ribosomes ◼ The nuclear envelope breaks down. ◼ A football-shaped structure called the mitotic spindle forms and the spindle starts tugging the chromosomes toward the center of the cell ◼ METAPHASE ◼ The second stage of mitosis is metaphase – and the length of time a cell spends in it is very brief. ◼ Definition: : the second stage of mitosis and meiosis 1 and 2 when the spindle is fully formed and all of the chromosomes are held in place ◼ DURING METAPHASE: ◼ The chromosomes all gather in a plane across the middle of the cell ◼ The mitotic spindle is fully formed ◼ All of the chromosomes are attached to the spindle microtubules, with their centromeres lined up about halfway between the two poles (ends) of the spindle ◼ ANAPHASE ◼ The third stage of mitosis is anaphase – third stage of mitosis and meiosis 1 and 2, in which sister chromatids separate and move towards the poles of the spindle. ◼ DURING ANAPHASE: ◼ Sister chromatid separate from their partners and each is now considered a daughter chromosome ◼ Proteins at the centromeres help move the daughter chromosomes along the spindles towards the poles ◼ Microtubules, that are attached to the chromosomes shorten bringing chromosomes closer to the poles of the cell- microtubules not attached to chromosomes lengthen, pushing the poles further apart. ◼ TELOPHASE ◼ The fourth stage of mitosis is telophase – and when the chromosomes reach the poles of the spindle. ◼ Definition: final stage of mitosis and meiosis 1 and 2, in which the chromosomes reach the spindle poles, nuclear envelopes form around each set of daughter chromosomes, and the nuclei reappear. ◼ DURING TELOPHASE: ◼ The spindle disappears ◼ Two nuclear envelopes reform (one around each set of daughter chromosomes) ◼ The chromosomes uncoil and lengthen ◼ The nucleoli reappear.  Cytokinesis in Animals and Plants ◼ Cytokinesis (the actual division of the cytoplasm into two cells) typically occurs during telophase → This process completes the cell division process by dividing the cytoplasm into two daughter cells, each with a nucleus. ◼ ANIMAL CELLS: ◼ The first sign on cytokinesis is the appearance of an indentation around the middle of the cell. ◼ This indentation is caused by a ring of microfilaments in the cytoplasm just under the plasma membrane – this ring contracts like the pulling of a drawstring, deepening the indentation and pinching the parent cell in two. ◼ Because the two new nuclei are forming at the ends of the cell, cytokinesis results in two new cells. ◼ PLANT CELLS: ◼ A disk containing cell wall material called a cell plate forms inside the cell and grows outward- this new piece of cell wall divides the cell into two. ◼ The result is two daughter cells, each bounded by its own continuous membrane and its own cell wall.