Lecture 8 Fundamentals of Life Sciences PDF

Fundamentals of Life Sciences Unit II – Lecture 8 (all slides are important in this lecture for Quiz and Class Test) 1  Lysosom...

Fundamentals of Life Sciences Unit II – Lecture 8 (all slides are important in this lecture for Quiz and Class Test) 1  Lysosom es Lysosomes are specialized, membrane-bound vesicles present within eukaryotic cells, predominantly in animal cells. These organelles are characterized by their capacity to house hydrolytic enzymes, facilitating the breakdown of a wide array of biomolecules. Structurally, lysosomes are dense granular entities enveloped by a lipid bilayer membrane. Their internal environment, or lumen, maintains an acidic pH (approximately 4.5–5.0), which is optimal for the hydrolytic enzymes they contain. 2 3 4  Vacuoles A vacuole is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution. 5 6 7 What is the main function of a centrosome? The major functions of the centrosomes are as follows: The centriole acts as MTOC (microtubule-organizing center) that arranges the microtubules array based on its ability to anchor, release, or nucleate microtubules. The position and the polarity of the centrosome control the positioning of microtubules. Centrioles can be transformed into basal bodies. Basal bodies formed from centrioles gives rise to cilia and flagella. 8 CELL DIVISION 9 10 11 In Meiosis 12 13 14 15 16 17 18 Mitosis is a process that involves the division of a cell's nucleus to create two identical daughter cells. The steps of mitosis are: Interphase: The cell grows and prepares for mitosis by replicating its DNA and producing proteins. Prophase: Chromosomes condense and become visible, the nuclear membrane disappears, and the centrosomes separate and move to opposite sides of the nucleus. Metaphase: Chromosomes line up in the middle of the cell, and the centromeres of all the chromosomes line up at the metaphase plate. Anaphase: Chromosomes and their copies are pulled to opposite ends of the cell. Telophase: New membranes form around the chromosomes at each end of the cell, and the mitotic spindle breaks down. Cytokinesis: The cell divides in two. During mitosis, there are several checkpoints to ensure that the process continues correctly. If these checkpoints aren't met, mitosis can halt or pathologies like cancer can occur. 19  Let’s start by looking at a cell right before it begins mitosis. This cell is in interphase (late G2phase) and has already copied its DNA, so the chromosomes in the nucleus each consist of two connected copies, called sister chromatids. You can’t see the chromosomes very clearly at this point, because they are still in their long, stringy, decondensed form. This animal cell has also made a copy of its centrosome, an organelle that will play a key role in orchestrating mitosis, so there are two centrosomes. (Plant cells generally don’t have centrosomes with centrioles, but have a different type of microtubule organizing center that plays a similar role.) 20  In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes. The chromosomes start to condense (making them easier to pull apart later on). The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the centrosomes as they move apart. The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made, disappears. This is a sign that the nucleus is getting ready to break down. 21  In late prophase (sometimes also called prometaphase), the mitotic spindle begins to capture and organize the chromosomes. The chromosomes become even more condensed, so they are very compact. The nuclear envelope breaks down, releasing the chromosomes. The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes. Microtubules can bind to chromosomes at the kinetochore, a patch of protein found on the centromere of each sister chromatid. (Centromeres are the regions of DNA where the sister chromatids are most tightly connected.) Microtubules that bind a chromosome are called kinetochore microtubules. Microtubules that don’t bind to kinetochores can grab on to microtubules from the opposite pole, stabilizing the spindle. More microtubules extend from each centrosome towards the edge of the cell, forming a structure called the aster. 22  In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide. All the chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up). At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles. Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed. 23  In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell. The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell. Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer. All of these processes are driven by motor proteins, molecular machines that can “walk” along microtubule tracks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they walk. 24  In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place. The mitotic spindle is broken down into its building blocks. Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear. The chromosomes begin to decondense and return to their “stringy” form. 25  Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase. In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The “drawstring” is a band of filaments made of a protein called actin, and the pinch crease is known as the cleavage furrow. Plant cells can’t be divided like this because they have a cell wall and are too stiff. Instead, a structure called the cell plate forms down the middle of the cell, splitting it into two daughter cells separated by a new wall. 26  When cytokinesis finishes, we end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell. The daughter cells can now begin their own cellular “lives,” and – depending on what they decide to be when they grow up – may undergo mitosis themselves, repeating the cycle. 27 Meiosis 28 29 30 31 End of Lecture 8 32