Human Biology Lecture 2 (Cell Division, Cleavage & Development) 2024/2025 PDF

Univ. of Mustansiriyah College of Pharmacy Branch of Clinical Laboratory Sciences HUMAN BIOLOGY Prof. Dr. Ahmed Ali Mohammed Lec. 2 Cell Division, Cleavage & Development 2024/ 2025 Introduction Cell division is the process by which the cell divides into...

Univ. of Mustansiriyah College of Pharmacy Branch of Clinical Laboratory Sciences HUMAN BIOLOGY Prof. Dr. Ahmed Ali Mohammed Lec. 2 Cell Division, Cleavage & Development 2024/ 2025 Introduction Cell division is the process by which the cell divides into two or more daughter cells to increase in number and produce new cells. It is an essential biological process used to grow, Lec. 2 Medical replenishParasitology (repair) or replace the old, dead, worn or damaged cells, and it is also used to reproduce. The divided cell is called LEC. 1the parent cell, and the newly formed cells are known INTRODUCTION as the daughter cells. Cell division usually occurs as part of a larger cell cycle. Cells divide and reproduce in two ways, mitosis and meiosis. Mitosis is the most dramatic event in the cell’s life and a fundamental process for life. It is responsible for growth and development. It is essentially a duplication process because it produces two genetically identical daughter cells from a single parent cell. The human body grows from a single embryonic cell through mitosis. Even after growth, mitosis replaces the cells that are lost everyday. The constant replenishment of the skin cells, for example, occurs through mitosis. Mitosis takes place in cells in all the parts of our body, keeping the tissues and organs in good working order. Meiosis, on the other hand, is quite different. It mixes the genetic contents and generates daughter cells that are distinct from one another and from the original parent cell. Although virtually all our cells can undergo mitosis, only a few special cells are capable of meiosis, those will later become eggs in females and sperm in males. So, basically, mitosis is for growth and maintenance, while meiosis is for sexual reproduction. While mitosis results in two identical daughter cells, meiosis results in four sex cells. This is simply mean that mitosis is a single step division while meiosis is a two-step division, and this results in meiosis having two phases namely meiosis one and meiosis two. 1 The Cell Cycle The cell cycle is a repeating series of events including growth, DNA synthesis and cell division. The cell cycle consists of four separate phases: G1 (gap 1), S (synthesis), G2 (gap 2) and M (mitosis and cytokinesis). The DNA replication occurs in the S phase, whereas the M phase is when the cell actually divides. The gap phases are less prominent but equally important. As it can be seen in the figure, mitosis only occupies a fraction of the cell cycle. The period between the mitotic divisions (G1, S and G2) is known as interphase. The interphase is the longest part of the cell cycle. During this phase, the cell grows and copies its DNA before moving into mitosis. Scientists used to think about this phase as a resting phase during which not much happened, but they now know that this is far from the truth. The cell cycle lasts roughly 24 hours, but depending on the type of cell, it can vary in length from less than 8 hours to more than a year. Most of the variability occurs in G1. In the G1 phase, metabolic changes are happening to prepare the cell for division. At a certain point (the restriction point), the cell is committed to division and moves into the S phase. While in the S phase, the DNA synthesis and replication of genetic material occur. Each chromosome consists of two sister chromatids. In the G2 phase, the metabolic changes assemble the cytoplasmic materials which are necessary for mitosis and cytokinesis. And, in the M phase, the nuclear division (mitosis) followed by the cytoplasm division (cytokinesis) occurs. 2 Cell division During mitosis, a cell duplicates all its contents, including its chromosomes, then splits to form two identical daughter cells. The primary goal of mitosis is to make sure that each daughter cell gets one copy of each chromosome. The other cellular components, like the ribosomes and mitochondria, are divided between the two daughter cells, but their equal partitioning is less important. Mitosis is divided into six phases: prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis. The first five phases do the job of nucleus splitting and its duplicated genetic information into two daughter nuclei which is called karyokinesis. While in the final step (cytokinesis), the parental cell cytoplasm divides into two parts; each will go to one of the daughter cells and causing the splitting of the entire cell into two identical daughter cells. The contractile ring shrinks at the equator of the cell, pinching the plasma membrane inward and forming what is called a cleavage furrow. Whereas in meiosis, the single cell divides twice to form four haploid daughter cells called gametes (haploid is the type of cell having a single set of chromosomes). Gametes are our reproductive cells; they are also referred to as sex cells. Female gametes are called ova or egg cells, and male gametes called sperm. Not all cells take the same time for division; human cells divide once every 24 hours. Some cells divide when there is damage in the present cells, while other cells do not divide at all like our nerve and heart cells. There are many differences between mitosis and meiosis, the main differences are: 1. Mitosis gives two nuclei, and hence two cells, while meiosis gives four. 2. Mitosis gives identical cells to each other and to the mother cell, while meiosis leads to genetic variation due to crossing over and independent assortment. 3. Mitosis includes one division, while meiosis includes two divisions. 4. Mitosis creates diploid daughter cells, while meiosis creates haploid daughter cells. 5. Mitosis creates body (somatic) cells , while meiosis creates germ (sex) cells. 3 Development Development of the human body is the process of growth to maturity. The growth and development occur after birth and include both physical and psychological development. It is influenced by genetic, hormonal, environmental and other factors. The development and the growth continue throughout life, through childhood, adolescence and adulthood to senility (ageing). Generally, the human body comprises a head, neck, trunk (which includes the thorax and abdomen), arms and hands, legs and feet. Development begins with the fertilization, where an egg released from the ovary of a female is penetrated by a sperm followed by the fusion of the nucleus of the male gamete (the sperm) with the nucleus of a female gamete (the egg), producing a new cell called a zygote, which contains all the chromosomes needed for a new human organism. Half the chromosomes will come from the egg and half from the sperm. Fertilization usually takes place in the fallopian tube then the zygote spends the next few days traveling down the fallopian tube toward the uterus, where it will take up residence. If the fertilization occurs, the egg will have all the instructions to grow into a new organism, and that one cell will become two, then four, then eight, then sixteen, and on and on. Cell division begins approximately 24 to 36 hours after conception. After about 9 months, that one cell will become a new baby. The zygote stage is brief and is followed by cleavage, which is a series of cell divisions that occurs in the early embryo, where the single-celled zygote becomes subdivided into smaller cells. Just a few hours after conception, the single-celled zygote begins making a journey down the fallopian tube to the uterus. As it travels, it divides by mitosis several times to form a ball of a compact mass of cells called the morula. The morula is a berrylike cluster of cells consisting of 16 cells and represents an early-stage embryo. After reaching the 64-cell stage as more cell divisions occur, a fluid-filled cavity called the blastocoele forms inside it, and this ball of cells will be called the blastocyst, which consists of a single layer of cells. The blastocyst is a distinctive stage of an embryo. It consists of an outer layer of cells called the trophoblast and an inner cell mass called the embryoblast. The outer layer of the blastocyst surrounds the inner cell mass and the fluid-filled cavity, whereas the embryoblast will subsequently form the embryo. In humans, the blastocyst formation begins about 5 days after fertilization when a fluid-filled cavity opens up in the morula and appears between the cells of the inner cell mass and the enveloping layer. 4  The blastocyst continues in traveling down the fallopian tube and reaches the uterus about 4 or 5 days after fertilization. When the outer cells of the blastocyst contact the cells of the endometrium, it embeds there. The process of embedding is called implantation, which generally occurs about 7 to 8 days after fertilization. After the implantation occurs, the blastocyst is called an embryo. 5  The embryonic stage lasts through the eighth week following fertilization, then gastrulation occurs. Gastrulation is defined as an early developmental process in which the embryo transforms from a one-dimensional layer of epithelial cells (the blastula) into a multilayered ball of cells and a multidimensional structure called the gastrula. During the second week after fertilization, cells in the embryo migrate to form three distinct cell layers, called the ectoderm, mesoderm, and endoderm. Each layer will soon develop into different types of cells and tissues, as shown in the Figure below. Before gastrulation, the embryo is a continuous epithelial sheet of cells, but by the end of gastrulation, the embryo begins the differentiation to establish distinct cell lineages set up the basic axes of the body. Gastrulation is followed by organogenesis. It is the process of developing individual organs from the newly formed germ layers. The germ layers develop specialized cells and tissues and start to form most of the organs. Each layer gives rise to specific tissues and organs in the developing embryo. During this time, the embryo grows in size and becomes more complex. 6  The ectoderm gives rise to the epidermis and the nervous system. Whereas the endoderm gives rise to the epithelium of the digestive system, respiratory system and the organs associated with the digestive system, such as the liver and the pancreas. The mesoderm gives rise to many cell types such as the muscle, bone and connective tissue cells, the notochord, the heart, blood and blood vessels, the cartilage of the ribs and vertebrae, and the dermis. Differentiation of Cells During the third week after fertilization, the embryo begins to undergo cellular differentiation. Differentiation is the process by which unspecialized cells become specialized. It occurs as certain genes are expressed (switched on) while other genes are switched off. Because of this process, cells develop unique structures and abilities that suit their specialized functions. 7 Organ Formation After cells differentiate, all the major organs begin to form during the remaining weeks of embryonic development. Most of the embryo's organs develop during weeks 4 through 8. As the embryo develops, it also grows in size. By the eighth week of development, the embryo reaches about 30 millimeters (just over 1 inch) in length. It may also begin to move. If the embryo is exposed to toxins during this period, the effects are likely to be very damaging. Growth and Development of the Embryo Almost all the organs are completely formed by about 10 weeks after fertilization (which equals 12 weeks of pregnancy), with the exceptions of the brain and the spinal cord, which continue to form and develop throughout pregnancy. Most malformations (birth defects) occur during the period when organs are forming. As a summary, the embryonic stage begins with implantation. The embryo then forms three distinct cell layers, each layer develops into different types of cells and organs. A few of the developments that occur in the embryo during weeks 4 through 8 are listed in Figure. 8 9