Cell Cycle and Cell Division PDF 2023-2024

Cell cycle and cell division  Objectives List the stages of the cell cycle and state the purpose of each. List the phases of mitosis and explain what happen during each phase. List the phases of meiosis and explain what happen during each phase. Differentiate between mitosis and meio...

Cell cycle and cell division  Objectives List the stages of the cell cycle and state the purpose of each. List the phases of mitosis and explain what happen during each phase. List the phases of meiosis and explain what happen during each phase. Differentiate between mitosis and meiosis.  The Cell Cycle Cell cycle: a series of events that takes place in a cell as it grows and divides. Has two parts: 1 Interphase 2 Cell division 1- Interphase Most of the cell cycle is spent in interphase. ▪ The cell gets ready to dived, it grows larger, the number of organelles doubles, and the amount of chromatin doubles (DNA replication) ▪ It is divided into three main stages: G1, S, G2 Phases of interphase: ▪ G1 Stage: The cell doubles its organelles and accumulates the materials needed for DNA synthesis. ▪ S Stage: DNA replication (makes a copy of its DNA). After the S stage, each chromosome consists of two identical sister chromatids. ▪ G2 Stage: synthesizes the proteins needed for cell division. 2- Cell division ▪ Cell division following interphase and it has two stages: Mitosis and Cytokinesis 1. Mitosis (M) stage: type of nuclear division , one copy of the DNA is distributed into each of its daughter cells. 2. Cytokinesis: it is the division of the cytoplasm; the cytoplasm divides and organelles are distributed into the two new cells. ▪ 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. The Cell Cycle  Cell Cycle Control ▪ The cell cycle is controlled by checkpoints, which delay it until certain conditions are met. They are G1 checkpoint, G2 checkpoint, and mitotic checkpoint. Failure of the cell cycle control mechanisms may result in unrestricted cell growth, or cancer.  Cell division The body of each living individual consists of a large number of cells. Each multicellular organism starts its life as a single cell, the fertilized egg or zygote. This cell undergoes successive divisions giving rise to the whole individual. Two main types of cell division take place in higher organisms, Mitosis and Meiosis.  Chromosome s ▪ The nucleus holds all the genetic material to direct all the functions in the body. ▪ DNA molecule is packaged into thread like structure called chromosomes. ▪ Genes are pieces of DNA that contain the instructions for building a protein and tell the cell what to do and when to grow and divide. ▪ Chromosomes also contain proteins that assist in the organizational structure of the DNA. ▪ Collectively, the DNA and proteins are called chromatin.  Chromosomes ▪ When a cell is not undergoing division, the chromatin appears to be a tangled mass of thin threads. ▪ When a cell divides, chromatin condenses to form chromosomes. ▪ In dividing cells, each chromosome is composed of two identical parts called sister chromatids attached by a centromere. ▪ These are said to be replicated or duplicated chromosomes because the two sister chromatids contain the same genes.  Chromosomes ▪ Human has 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. Males have the sex chromosomes X and Y (XY) , and females have two X chromosomes (XX).  Mitosis ▪ Creates new cells in the developing embryo. ▪ Responsible for replacement of cells in adults. ▪ During mitosis, the cell that divides is called the parent cell, and the new cells are called daughter cells. Phases of Mitosis ▪ Mitosis is divided into phases: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase  Prophase ▪ The centrosomes have duplicated and move toward opposite ends of the nucleus. ▪ Spindle fibers appear. ▪ The nuclear envelope begins to fragment. ▪ The nucleolus disappears. ▪ The chromosomes condense (are now visible). Each is composed of two sister chromatids held together at a centromere.  Prometaphase ▪ The spindle fibers attach to the centromeres as the chromosomes continue to shorten and thicken. ▪ Chromosomes are randomly placed in the cell. Metaphase ▪ The chromosomes, attached to spindle fibers, line up at the metaphase plate. Anaphase ▪ Centromeres divide and the sister chromatids separate and move toward opposite poles of the cell. ▪ Sister chromatids are now called chromosomes.  Telophase ▪ Begins when chromosomes arrive at the poles. ▪ Chromosomes become indistinct into chromatin again. ▪ The spindle fibers disappear. ▪ The nuclear envelope reappears. ▪ The nucleolus reappears. ▪ Characterized by the presence of two daughter nuclei. Stages of Mitosis Stages of Mitosis  Cytokinesis Cytokinesis: it is the division of the cytoplasm and organelles ( overlaps with the final stage of mitosis). Formation of the cleavage furrow. Actin filaments form a contractile ring; as the ring becomes smaller, the cleavage furrow pinches the cell in half. At the end of Mitosis you have: Two daughter cells Each daughter cell gets a complete set of chromosomes and is diploid (2n) (46 chromosomes). The daughter cells are genetically identical to each other and to the parent cell.  Meiosis Meiosis : reduction division. ▪ It has two cell divisions without an interphase in between. ▪ The two cell divisions of meiosis are called meiosis I and meiosis II. ▪ At the start of meiosis, the parent cell is diploid (2n), and the chromosomes occur in pairs. ▪ The members of a pair are called homologous chromosomes, or homologues. They look alike and carry genes for the same traits.  Meiosis I First Meiotic Division Prophase I Synapsis occurs: homologous chromosomes come together in pairs to form a tetrad. Crossing over occur which is the exchange of genetic material from homologous chromosomes. This causes genetic variations. The nuclear membrane disappears, and the spindle fibers are formed. Synapsis and Crossing Over Metaphase I : ▪ Homologous chromosome pairs align at the metaphase plate. Anaphase I : ▪ The two chromosomes, each still consisting of two chromatids, are drawn apart and pulled towards the opposite poles of the cell. Telophase I : Each set of chromosomes at one pole is surrounded by a nuclear membrane. At the end of Meiosis I You have 2 cells ; each cell contains a haploid number of chromosomes (23 chromosomes).  Meiosis II Second Meiotic Division Meiosis II is similar to mitosis Prophase II : Each centriole moves to the opposite poles of the cell. The spindle fibers are formed The nuclear membrane and nucleolus disappear. Each chromosome is still composed of two chromatids. Metaphase II : The chromosomes become arranged along the metaphase plate. Anaphase II : The centromere binding every 2 chromatids splits and each is moved towards the corresponding pole of the cell. Telophase II : The chromosomes elongate, become thinner and a nuclear membrane is formed around each group containing half the original number of chromosomes (n). Meiosis II At the end of Meiosis II you have: Four daughter cells Each daughter cell gets a haploid number of chromosomes (n) (23 chromosomes). The daughter cells are genetically different to each other and to the parent cell. The daughter cells mature into gametes (sperm and egg). o Fertilization restores the diploid number of chromosomes in the zygote. Comparison between Mitosis and Meiosis Mitosis vs. Meiosis Thank You