Cell Division 1 PDF

# Cell Reproduction ## Genomic DNA - Human body (somatic) cells have 46 chromosomes. - Human gametes (sperm or eggs) have 23 chromosomes each. - A typical body cell contains two matched or homologous sets of chromosomes, a configuration known as diploid. A diploid organism is designated 2n. - Huma...

# Cell Reproduction ## Genomic DNA - Human body (somatic) cells have 46 chromosomes. - Human gametes (sperm or eggs) have 23 chromosomes each. - A typical body cell contains two matched or homologous sets of chromosomes, a configuration known as diploid. A diploid organism is designated 2n. - Human cells that contain one set of chromosomes are called gametes, or sex cells; these are eggs and sperm, and are designated 1n, or haploid. - Matched pairs of chromosomes are called **homologous chromosomes**. Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus. - **Genes** are the functional units of chromosomes; they determine specific characteristics by coding for specific proteins. - **Traits** are the variations of those characteristics. For example, hair color. - Each copy of a homologous pair of chromosomes originates from a different parent; therefore, the different genes (alleles) themselves are not identical, although they code for the same traits such as "hair color." - The variation of individuals within a species is due to the specific combination of the genes inherited from both parents. - Even a slightly altered sequence of nucleotides within a gene can result in an alternative trait. For example: there are three possible gene sequences on the human chromosome that code for blood type: sequence A, sequence B, and sequence O. ## Eukaryotic Chromosomal Structure and Compaction - DNA must be tightly packaged to fit in the cell's nucleus. - The long strands of DNA are condensed into compact chromosomes. ### Levels of Chromosomes Compaction 1. Short stretches of the DNA double helix wrap around a core of eight **histone proteins**. The DNA-histone complex is called **chromatin**. The beadlike, histone DNA complex is called a **nucleosome**, and DNA connecting the nucleosomes is called **linker DNA**. 2. The nucleosomes and the linker DNA between them coil into a chromatin fiber. This coiling further condenses the chromosome about 50 times shorter than the extended form. 3. A variety of fibrous proteins is used to (pack the chromatin). - After replication, the chromosomes are composed of two linked sister **chromatids**. - When fully compact, the pairs of identically packed chromosomes are bound to each other by cohesion proteins. - The connection between the sister chromatids is called the **centromere**. ## The Cell Cycle - The **cell cycle** is an ordered series of events involving cell growth and cell division that produces two new daughter cells. - Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and nuclear and cytoplasmic division that produces two identical (clone) cells. ### The Cell Cycle Has Two Major Phases 1. **Interphase**: the cell grows and DNA is replicated. 2. **Mitotic phase**: the replicated DNA and cytoplasmic contents are separated, and the cell cytoplasm is typically partitioned by a third process of the cell cycle called **cytokinesis**. - A diagram of the cell cycle is shown, consisting of the following stages: Interphase (G1, S, G2), and Mitotic Phase (Mitosis, Cytokinesis) ## Interphase ### G1 Phase (First Gap) 1. The first stage of interphase is called the G1 phase (first gap). 2. During the G1 stage, the cell is quite active at the biochemical level. 3. The cell building blocks of chromosomal DNA and the associated proteins and sufficient energy reserves to complete the task of replicating each chromosome in the nucleus. ### S Phase (Synthesis of DNA) - DNA replication can proceed through formation of identical pairs of DNA molecules (sister chromatids). - The centrosome is also duplicated during the S phase. - The two centrosomes of homologous chromosomes will give rise to the mitotic spindle. The mitotic spindle orchestrates the movement of chromosomes during mitosis. - Centrioles help organize cell division. ### G2 Phase (Second Gap) 1. The cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation and movement. 2. Some cell organelles are duplicated. 3. There may be additional cell growth during G2. 4. The final preparations for the mitotic phase. ## The Mitotic Phase - The first portion of the mitotic phase is called **karyokinesis**, or nuclear division. - The second portion of the mitotic phase is called **cytokinesis**, the physical separation of the cytoplasmic components into the two daughter cells. ## Karyokinesis (Mitosis) - **Mitosis** is divided into a series of phases that result in the division of the cell nucleus. 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase ### Prophase - Chromosomes condense and become visible. - Spindle fibers emerge from the centrosomes. - Nuclear envelope breaks down. - Nucleolus disappears. ### Prometaphase - Chromosomes continue to condense. - Kinetochores appear at the centromeres. - Mitotic spindle microtubules attach to kinetochores. - Centrosomes move toward opposite poles. ### Metaphase - Mitotic spindle is fully developed; centrosomes are at opposite poles of the cell. - Chromosomes are lined up at the metaphase plate. - Each sister chromatid is attached to a spindle fiber originating from opposite poles. ### Anaphase - Cohesin proteins binding the sister chromatids together break down. - Sister chromatids (now called chromosomes) are pulled toward opposite poles. - Non-kinetochore spindle fibers lengthen, elongating the cell. ### Telophase - Chromosomes arrive at opposite poles and begin to decondense. - Nuclear envelope material surrounds each set of chromosomes. - The mitotic spindle breaks down. ### Cytokinesis - Animal cells: a cleavage furrow separates the daughter cells. - Plant cells: a cell plate separates the daughter cells. ## Go Phase - Cells in Go phase are not actively preparing to divide. - The cell is in a **quiescent** (inactive) stage that occurs when cells exit the cell cycle. - Some cells enter Go temporarily due to environmental conditions such as availability of nutrients, or stimulation by growth factors. - The cell remains in Go until conditions improve or until an external signal triggers the onset of G1. - Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in Go permanently. ## Images - An image depicting the human karyotype, which displays the 46 chromosomes of a typical human somatic cell. The chromosomes are organized in pairs from largest to smallest, with the sex chromosomes (X and Y) located at the bottom of the diagram - A diagram that illustrates the levels of compaction of eukaryotic chromosomes, showcasing the transition from a DNA double helix to a duplicated chromosome. - A diagram illustrating the stages of mitosis, including Prophase, Prometaphase, Metaphase, Anaphase, and Telophase - A diagram comparing cytokinesis in animal and plant cells, highlighting the different processes involved in physically separating the daughter cells.