Human Genetics - Part 1 PDF Fall 2023

Human genetics- Part 1 Nermin Eissa, Ph.D. College of Health Sciences Abu Dhabi University Fall-2023  Learning Outcomes: Distinguish between a chromosome and chromatin. Explain the purpose of a karyotype. List the stages of the cell cycle and...

Human genetics- Part 1 Nermin Eissa, Ph.D. College of Health Sciences Abu Dhabi University Fall-2023  Learning Outcomes: Distinguish between a chromosome and chromatin. Explain the purpose of a karyotype. List the stages of the cell cycle and state the purpose of each. Describe the purpose of the checkpoints in the cell cycle. Distinguish between mitosis and cytokinesis. 2 ©2020 McGraw-Hill Education  Chromosomes 1 The nucleus holds all the genetic material to direct all the functions in the body. Chromosomes—made of DNA. The instructions in each chromosome are contained within genes, which in turn are composed of DNA. 3 ©2020 McGraw-Hill Education  Chromosomes 2 Chromosomes, continued. Contain proteins that assist in the organizational structure. Collectively, the DNA and proteins are called chromatin. Humans have 46 chromosomes, in 23 pairs. 22 of these pairs are called autosomes—found in both males and females. One pair is called the sex chromosomes, because they contain genes that control gender. 4 ©2020 McGraw-Hill Education  Chromosomes 3 Males have the sex chromosomes X and Y, and females have two X chromosomes. The Y chromosome contains the SRY gene that causes testes to develop. 5 ©2020 McGraw-Hill Education  A Karyotype 1 A display of the chromosomes present in a cell. When a cell divides, chromatin condenses to form chromosomes. Staining causes the Karyotype chromosomes to have dark and light cross-bands of varying widths, and a computer uses these, in addition to size and shape, to pair up the chromosomes. 6 ©2020 McGraw-Hill Education  A Karyotype of Human Chromosomes Access the text alternative for these images 7 ©2020 McGraw-Hill Education (photo): ©CNRI/SPL/Science Source  A Karyotype 2 Mitosis—cell division that begins when the fertilized egg starts dividing. Ensures that every cell is diploid (has 46 chromosomes). In dividing cells, each chromosome is composed of two identical parts called sister chromatids. These are said to be replicated or duplicated chromosomes because the two sister chromatids contain the same genes. 8 ©2020 McGraw-Hill Education  A Karyotype 3 Centromere—holds the chromatids together until a certain phase of mitosis, when the centromere splits. Daughter chromosomes separate, the new cell gets one of each type (a full set of chromosomes). 9 ©2020 McGraw-Hill Education  Check Your Progress Explain the purpose of chromosomes in a cell. Describe how a karyotype can be used to determine the number of chromosomes in a cell. Explain why sister chromatids are genetically the same. 10 ©2020 McGraw-Hill Education  The Cell Cycle 2 Cell cycle—has two parts: interphase and cell division. When a cell is not undergoing division, the chromatin appears to be a tangled mass of thin threads. 11 ©2020 McGraw-Hill Education  Interphase 1 Most of the cell cycle is spent in interphase. Organelles carry on their usual functions. The cell gets ready to divide: it grows larger, the number of organelles doubles, and the amount of chromatin doubles (DNA replication). Divided into three main stages: G1, S, G2. 12 ©2020 McGraw-Hill Education  Stages of the Cell Cycle Access the text alternative for these images 13 ©2020 McGraw-Hill Education  Interphase 2 Phases of interphase: G1 stage—the cell performs its normal function. Also doubles its organelles and accumulates the materials needed for DNA synthesis. S stage—DNA replication. After the S stage, each chromosome consists of two identical sister chromatids. G2 stage—synthesizes the proteins needed for cell division. 14 ©2020 McGraw-Hill Education  Interphase 3 The amount of time the cell spends in interphase varies widely. Some cells, such as nerve and muscle cells, typically do not complete the cell cycle and are permanently arrested in G1. they won’t ever continue to the S and G2phases, they are instead said to be in a G0 stage. Embryonic cells spend very little time in G1 and complete the cell cycle in a few hours. 15 ©2020 McGraw-Hill Education  Mitosis and Cytokinesis 1 Following interphase is cell division. Cell division has two stages: M (for “mitosis”) stage and cytokinesis. Mitosis is a type of nuclear division. Also referred to as duplication division since each new nucleus contains the same number and type of chromosomes as the former cell. Cytokinesis—division of the cytoplasm. 16 ©2020 McGraw-Hill Education  Mitosis and Cytokinesis 2 The cell cycle occurs continuously in certain tissues. Mitosis is balanced by the process of apoptosis, or programmed cell death. Apoptosis occurs when cells are no longer needed or have become excessively damaged. 17 ©2020 McGraw-Hill Education  Cell Cycle Control The cell cycle is controlled by checkpoints, which delay it until certain conditions are met. That is, G1 checkpoint, G2 checkpoint, and the mitotic checkpoint. The cell cycle may also be controlled by external factors, such as hormones and growth factors. Failure of the cell cycle control mechanisms may result in unrestricted cell growth, or cancer. 18 ©2020 McGraw-Hill Education  Control of the Cell Cycle Access the text alternative for these images 19 ©2020 McGraw-Hill Education  𝐆𝟏Checkpoint 1 G1 checkpoint—if the cell cycle passes this checkpoint, the cell is committed to divide. If the cell does not pass this checkpoint, it can enter G0, where it performs normal functions but does not divide. Proper growth signals, such as growth factors, must be present for a cell to pass the G1 checkpoint. 20 ©2020 McGraw-Hill Education  𝐆𝟏Checkpoint 2 G1 checkpoint, continued. The integrity of the DNA is also checked. If DNA is damaged, proteins such as p53 can stop the cycle at this checkpoint and place the cell in G0. If the DNA can be repaired, it may reenter the cell cycle; if not, it may undergo apoptosis. 21 ©2020 McGraw-Hill Education  𝐆2Checkpoint G2 checkpoint The cell cycle halts here until the cell verifies that DNA has replicated. Prevents the initiation of the M stage unless the chromosomes are duplicated. If DNA is damaged, arresting the cell cycle allows time for the damage to be repaired so that it is not passed on to daughter cells. 22 ©2020 McGraw-Hill Education  Mitotic Checkpoints Mitotic checkpoint—occurs between metaphase and anaphase to make sure the chromosomes are properly attached to the spindle so can be distributed accurately to the daughter cells. 23 ©2020 McGraw-Hill Education  External Control of the Cell Cycle External control. An external signal, such as a hormone or growth factor, can stimulate a cell to divide. It binds to a receptor in the plasma membrane of a target cell. The signal is then relayed from the receptor to proteins inside the cell. The proteins form a pathway called the signal transduction pathway; they pass the signal from one to the next. 25 ©2020 McGraw-Hill Education  External Controls of the Cell Cycle Access the text alternative for these images 26 ©2020 McGraw-Hill Education  External Control 2 External control, continued. The last signal of the signal transduction pathway activates genes in the nucleus. The expression of these genes may stimulate or inhibit the cell cycle. Genes called proto-oncogenes stimulate the cell cycle, and genes called tumor suppressor genes inhibit the cell cycle. 27 ©2020 McGraw-Hill Education  Check Your Progress Describe the cell cycle, and list the locations of each phase and checkpoint. Explain the purpose of the S phase in the cell cycle. Explain how checkpoints help protect the cell against unregulated cell growth. Summarize why external controls may be necessary to regulate the cell cycle. 28 ©2020 McGraw-Hill Education