Chromosomes and Cell Division Notes PDF

**Chromosomes and Cell Division Notes** - **Chromosomes in Eukaryotes Are Linear, While Those in Prokaryotes Are Circular.** **The Prokaryotic Chromosome =** Prokaryotic cells have a singular circular chromosome attached to the cell membrane. Containing a single, circular molecule of double-...

**Chromosomes and Cell Division Notes** - **Chromosomes in Eukaryotes Are Linear, While Those in Prokaryotes Are Circular.** **The Prokaryotic Chromosome =** Prokaryotic cells have a singular circular chromosome attached to the cell membrane. Containing a single, circular molecule of double-stranded DNA. (Binary Fission) **Eukaryotic Chromosomes =** Eukaryotic cells contain linear chromosomes within a nucleus. Containing a long, linear molecule of DNA tightly coiled around protein complexes called histones, forming a structure called chromatin, which is made up of repeating units called nucleosomes. (Mitosis) - **Binary Fission** Binary fission is a type of asexual reproduction where a single-celled organism divides into two identical daughter cells, essentially splitting itself in half, with each new organism receiving a copy of the parents genetic material. It's most commonly observed in bacteria and other prokaryotes (Asexual reproduction). - **Two Reasons Why Cells Divide.** Cells divide primarily for growth and development (to create more cells for an organism to grow larger) and repair and replacement (to replace damaged or dead cells). **Growth and Development =** As an organism grows, it needs more cells to support its increasing size, which is achieved through cell division. **Repair and Replacement =** When cells are damaged or die, cell division produces new cells to replace them, allowing for tissue healing and maintenance. - **Differentiate Between VOCAB.** **Somatic Cell =** Any cell of a living organism other than the reproductive cells. **Germ Cells =** A cell containing half the number of chromosomes of a somatic cell and able to unite with one from the opposite sex to form a new individual; a gamete. **Parent Cell =** A "parent cell" is a cell that divides to produce new cells, called "daughter cells," essentially acting as the source of new cells during cell division; in other words, it is the original cell that gives rise to other cells through reproduction or growth. **Daughter Cell =** A "daughter cell" is a cell that is produced as a result of cell division, essentially a new cell formed when a parent cell splits into two or more parts; in mitosis, daughter cells are genetically identical to the parent cell as they contain the same number and type of chromosomes. - **Differentiate Between VOCAB and Illustration.** **Homolog =** One of a pair of chromosomes that segregate from one another during the first meiotic division. A gene related to a second gene by descent from a common ancestral DNA sequence. **Illustration =** Look for two chromosomes that are the same size and shape, representing one chromosome from each parent. **Centromere =** A region of a chromosome to which the spindle fiber is attached during cell division. **Illustration =** The centromere is the constricted area where the two sister chromatids of a chromosome are joined together. **Sister Chromatids =** The two identical copies of a chromosome created during DNA replication, joined together at the centromere, and which separates during cell division to become individual chromosomes in daughter cells; essentially, they are two identical halves of a duplicated chromosome. **Illustration =** The two identical copies of a single chromosome attached at the centromere, appearing as the two arms of an "X" shape on a chromosome illustration. - **Cell cycle and phases (eukaryotic cell cycle).** 1. **Interphase** - **G1 phase (Gap 1) =** The cell grows in size and synthesizes various enzymes and nutrients that are needed for DNA replication and cell division. It's period of cell growth and preparation for DNA synthesis. - **S phase (Synthesis) =** DNA replication occurs, producing two copies of each chromosome. This ensures that each new cell will receive a complete set of genetic information. - **G2 phase (Gap 2) =** The cell continues to grow and produce proteins and organelles needed for mitosis. It also undergoes final preparations for division, such as checking for DNA errors and making necessary repairs. 2. **Mitotic (M) Phase** - **Prophase =** Chromosomes condense and becomes visible under a microscope. The nuclear membrane begins to disintegrate and spindle fibers start to form. - **Metaphase =** Chromosomes line up in the middle of the cell at the metaphase plate. Spindle fibers attach to the centromeres of each chromosome. - **Anaphase =** Sister chromatids are pulled apart by the spindle fibers, moving toward opposite poles of the cell. - **Telophase** = The separated chromosomes reach the cell poles, and new nuclear membranes form around each set, creating two separate nuclei. 3. **Cytokinesis** - This final step, where the cells cytoplasm divides, forming two daughter cells, each with a complete set of chromosomes. In animal cells, a cleavage furrow forms, and in plant cells, a cell plate forms. Each phase ensures that the cell divides properly and that each daughter cell receives the necessary cellular components and genetic information. - **Interphase and Mitotic Phase.** Interphase is made up of the G1, S, and G2 phases, while the Mitotic Phase consistent of mitosis (including prophase, metaphase, anaphase, and telophase) and cytokinesis; essentially, the cell spends most of its time in interphase preparing for cell division, and then the mitotic phase is when the actual cell division occurs. - **The Four Phases of Mitosis.** **Prophase =** Chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and the spindle fibers begin to form.  **Metaphase** = Chromosomes line up along the equator of the cell, attaching to the spindle fibers at their centromeres.  **Anaphase =** Sister chromatids of each chromosome separate and are pulled to opposite poles of the cell.  **Telophase =** Two new nuclear membranes form around the separated chromosomes, and the chromosomes begin to uncoil back into chromatin.  Cytokinesis is the process where the cell cytoplasm physically divides into two separate daughter cells, occurring after the completion of mitosis. Essentially, while mitosis is the division of the nucleus, cytokinesis is the division of the cell\'s cytoplasm.  - **Calculating Mitosis and Meiosis.** 1. **Mitosis** - **G1 Phase =** The cell has a diploid (2n) number of chromosomes, with each chromosome consisting of a single chromatid. - **S Phase =** Each chromosome is duplicated, so each now consists of two sister chromatids. However, the chromosome number remains the same (still 2n) because sister chromatids are not counted as separate chromosomes. - **G2 Phase =** The chromosome count remains the same as in G1 (2n), but each chromosome has two chromatids. - **Mitosis =** During mitosis, chromosomes are separated, and each daughter cell receives one chromatid from each chromosome. After mitosis, each daughter cell has the same number of chromosomes as the original cell in G1 (2n). 2. **Meiosis** - **Meiosis I (Reductional Division):** - Starts with a diploid cell (2n), with chromosomes each having two chromatids. - Homologous chromosomes separate, so each daughter cell receives one chromosome from each homologous pair, resulting in a haploid set of chromosomes (n), but each chromosome still has two chromatids. - **Meiosis II (Equational Division):** - The haploid cells undergo another division, where sister chromatids separate. - Each resulting gamete has a haploid number of chromosomes (n), with each chromosome now consisting of a single chromatid. - **Autosomes & Sex Chromosomes.** **Autosome =** One of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). **Sex Chromosome =** Chromosomes that determine an individual's biological sex. - **Females:** Have to X chromosomes (XX) - **Males:** Have one X and one Y chromosome (XY)  - **Role of DNA Helicase and DNA Polymerase in DNA Replication** In DNA replication, DNA helicase acts as the "unzipper" by breaking hydrogen bonds to separate the double stranded DNA into single strands, creating a replication fork, while DNA polymerase is the enzyme responsible for building new DNA strands by adding nucleotides complementary to the template strand, essentially copying the genetic information. - **Telomere** **Telomere =** Are sections of noncoding, repetitive DNA that acts as a protective cap on the tip of each chromosome. Damage to the telomeres can result in cell suicide (apoptosis) rather than risk dividing. (If telomere become too short, additional cell divisions cause loss of essential DNA and cell death) A telomere is a protective cap of repetitive DNA sequences located at the ends of chromosomes, and their length is closely linked to aging as they naturally shorten with each division, eventually reaching a point where the cell can no longer replicate, contributing to the process of cellular senescence and overall aging of an organism; essentially, shorter telomere indicate older cells and potentially shorter lifespan. - **Some cells rebuild their telomere:** - Single-celled eukaryotes - Cells that produce gametes - Embryos - **Too much telomere rebuilding can lead to uncontrolled cell division** - **Too little rebuilding is worse.** - Hoyeraal-Hreidarsson Syndrome: Telomeres are too short at birth. - Suffer premature organ and bone marrow failure. - Life Expectancy is 2-3 years. - **Cancer** **Cancer =** Is characterized by unrestrained cell growth and division that can damage adjacent tissues. Cancer is the second leading cause of death in the Unites States, and it occurs when disruption of the DNA interferes with a cell's ability to regulate cell division. It often results from mutations to genes important in controlling the cell cycle. **Benign Tumors =** Masses of normal cells; they don't spread and can be removed safely. **Malignant Tumors =** Shed and spread cancer cells: Metastasis (a metastatic growth). - Surgery **(Slash)** - Radiation **(Burn)** - Chemotherapy **(Poison)** **Comparison of Mitosis and Meiosis** **Characteristic** **Mitosis** **Meiosis** -------------------------------------- ----------------------------------------------------- --------------------------------------------------------------------- **Ploidy number of parent cell** Diploid (2n) Diploid (2n) **Ploidy of daughter cells** Diploid (2n) Haploid (n) **Number of daughter cells** 2 4 **Number of replication events** 1 (DNA replication happens once, during interphase) 1 (DNA replication happens once, during interphase) **Number of division events** 1 2 **Variation between daughter cells** Identical to each other and parent cell Genetically diverse due to crossing over and independent assortment **Purpose** Growth, repair, and asexual reproduction. Sexual reproduction (to produce gametes) - **1n & 2n Defined** **Haploid (1n):** A haploid cell contains one set of chromosomes (1n). Gametes (sperm and egg cells) are haploid cells, meaning they have only one set of chromosomes. This ensures that when two gametes fuse during fertilization, they restore the diploid (2n) state in the resulting zygote. **Diploid (2n):** A diploid cell has two sets of chromosomes (2n), one from each parent. Most cells in an organism are diploid, except for gametes, which are produced through a process called meiosis. During meiosis, a diploid cell undergoes two rounds of division to produce four haploid (1n) gametes. Therefore, from one diploid (2n) cell, meiosis results in the production of four haploid (1n) gametes. Each gamete has half the number of chromosomes, which allows for the restoration of the diploid state upon fertilization. - **Karyotyping** **Karyotype =** a visual representation of an individual's complete set of chromosomes, organized in pairs according to size and banding pattern, allowing scientists to identify any abnormalities in chromosome number or structure, such as missing or extra chromosomes, which can be used to determine if an individual is male or female and diagnose potential genetic disorders; a normal human karyotype contains 46 chromosomes arranged in 23 pairs, with a sex chromosome (XX for female, XY for male) being the last pair. **Be able to answer questions like; are there any whole chromosome aberrations or is this individual male or female or how many chromosomes are there?** - **Sex in Mammals.** **Sex Chromosomes X and Y:** **Males:** XY **Females:** XX - **Crossing Over.** **Crossing over** is a process where homologous chromosomes exchange segments of genetic material. This genetic recombination results in chromosomes with a mix of genes from both the mother and father, creating new allele combinations. Here's more about how and when it happens and its effects on genetic variation: 1. **When Crossing Over Occurs:** - Crossing over happens during prophase I of meiosis, the specialized cell division that produces gametes (sperm and egg cells). - In this stage, homologous chromosomes (one from each parent) pair up closely in a process called synapsis. While paired, sections of these chromosomes can swap places at points called chiasmata. 1. **How Crossing Over Affects Variation:** - By creating chromosomes with new combinations of alleles (gene variants), crossing over increases genetic diversity in gametes. - This diversity means offspring have unique combinations of traits that differ from either parent, enhancing variation within a population. This variation is essential for evolution because it provides a basis for natural selection to act upon, allowing populations to adapt over time to changing environments. In summary, crossing over during prophase I of meiosis is a crucial source of genetic variation in sexually reproducing organisms, contributing to the unique genetic makeup of each individual. - **Meiosis in Males and Females.** 1. **In Males:** - Meiosis occurs in the testes and produces sperm cells. - During meiosis, each diploid cell (2n) undergoes two rounds of division to produce four haploid (1n) sperm cells. - All four of these cells become functional sperm, which means that each meiosis cycle in males results in four functional gametes. 2. **In Females:** - Meiosis occurs in the ovaries and produces egg cells (ova). - However, during meiosis in females, cytokinesis (the division of the cell's cytoplasm) is unequal. This process produces one large cell, and one smaller cells called polar bodies. - The result is that only one functional egg is produced, while the other three cells (polar bodies) are usually not functional as gametes and typically degenerate. - Thus, each meiosis cycle in females results in one functional gamete (egg cell). **Summary:** In males, meiosis produces four functional sperm cells, while in females, it produces once functional egg cell and three polar bodies. This difference contributes to the production of large numbers of small sperm in males and fewer, large eggs in females. - **Nondisjunction** Nondisjunction is when chromosomes don't separate properly during meiosis, leading to gametes with too many or too few chromosomes. - If a gamete with an extra chromosome (24 instead of 23) combines with a normal gamete (23), the resulting zygote will have 47 chromosomes. - Down Syndrome when there is an extra copy of chromosome 21 (trisomy 21), instead of 46. - This extra chromosome causes the traits and health issues seen in DS We typically see extra or missing chromosomes in the higher-numbered pairs (21, 18, 13) because they contain fewer genes, and the body can better tolerate the effects of having an extra or missing copy of these chromosomes. Here's why: 1. **Gene Content:** The higher-numbered chromosomes (such. As 21, 18, and 13) have fewer genes compared to lower-numbered chromosomes. This means that an extra or missing copy has a less severe impact on the overall functioning of the body. 2. **Viability:** When nondisjunction results in an extra or missing copy of a lower-numbered chromosome (which has many more genes), the genetic imbalance is so severe that it usually leads to early miscarriage or non-viable embryos. For example, trisomy involving larger chromosomes like chromosomes 1 or 2 is typically not compatible with life. 3. **Examples:** - Trisomy 21 (Down Syndrome) is the most common viable trisomy because chromosome 21 is one of the smallest human chromosomes with fewer genes. - Trisomy 18 (Edwards Syndrome) and Trisomy 13 (Patau Syndrome) are also seen but often come with more sever effects and high mortality rates because three chromosomes have more genes that chromosomes 21 but fewer than the larger chromosomes. **Summary:** Extra or missing copies of lower-numbered chromosomes (which have more genes) usually cause such sever genetic imbalances that they are not compatible with life, leading to early miscarriages. The higher-numbered chromosomes contain fewer genes, so the body can sometimes survive with an extra or missing copy, though often with significant developmental challenges.