Mitosis and Meiosis Study Notes

Humans have 46 chromosomes grouped into 23 pairs. Each pair consists of one chromosome from the mother and one from the father. This pairing helps create genetic diversity, which explains why siblings can look different despite sharing the same genes. On average, siblings share about 50% of their genes, while other relatives like cousins share even fewer genes depending on their relationship.

Chromatin is a substance found in our cells made up of DNA and proteins. DNA is packed tightly around proteins called histones, allowing it to fit inside the nucleus of the cell. When a cell gets ready to divide, chromatin becomes condensed into structures known as chromosomes. These chromosomes are necessary for moving genetic material accurately during cell division.

Chromosomes can appear as long threads or loops before division but become shorter and thicker during the process, making it easier to move them where they need to go.

There are two types of chromosomes to note:

• Sister Chromatids: Once a cell copies its DNA, it forms two identical copies of each chromosome, known as sister chromatids. They are connected at a point called the centromere until the cell divides, ensuring that each new cell receives the same genetic information.

• Homologous Chromosomes: These are pairs of similar but not identical chromosomes. One comes from the mother and the other from the father. They contain the same genes but can have different variations, known as alleles, which lead to different traits in individuals. During a special type of cell division called meiosis, these homologous chromosomes can exchange parts, increasing genetic variety.

Definitions of Important Terms:

• Diploid (2N): Refers to cells that have two complete sets of chromosomes. In humans, this means we have 46 chromosomes arranged in 23 pairs. Most body cells are diploid.

• Haploid (N): Refers to cells that have one set of chromosomes. This includes reproductive cells, such as sperm and eggs, which have 23 chromosomes. When a sperm fertilizes an egg, they combine to restore the diploid number of 46.

Mitosis:

• Mitosis is the process by which a single cell divides to create two identical cells. This is essential for growth, repairing damage, and reproduction in some organisms.

• Mitosis consists of several phases:

  1. Prophase: The chromatin condenses into chromosomes, spindle fibers appear, and the nucleus starts to break down.

  2. Metaphase: Chromosomes line up at the cell's center, and spindle fibers connect to special areas called kinetochores.

  3. Anaphase: Sister chromatids are pulled apart to opposite sides of the cell.

  4. Telophase: Two new nuclei form around the separated chromosomes, and the cell prepares to divide into two.

Meiosis:

• Meiosis produces four unique haploid cells (sperm or eggs) from one diploid cell. It is crucial for sexual reproduction by reducing the chromosome number by half and increasing genetic variation.

• It has two main stages:

  1. Meiosis I: Homologous chromosomes are separated. The process starts with chromosomes pairing up (Prophase I) and can exchange segments of DNA (crossing over), adding to diversity.

  2. Meiosis II: Similar to mitosis, where each of the two haploid cells divides to separate sister chromatids, resulting in four genetically diverse gametes.

Genetic Diversity is achieved through:

• Crossing Over: During Prophase I of meiosis, homologous chromosomes exchange genetic material, creating new gene combinations.

• Independent Assortment: During Metaphase I, the alignment of chromosomes is random, resulting in different combinations being passed to gametes.

Karyotyping:

• A karyotype is a visual representation of an individual’s chromosomes. It organizes chromosomes in pairs by size and shape, helping to identify abnormal chromosome numbers or structures, useful for diagnosing genetic conditions like Down syndrome.

Chromosomal Disorders:

• Down Syndrome: Caused by having three copies of chromosome 21, leading to specific facial features and developmental delays. Early diagnosis is helpful for education and healthcare.

• Turner Syndrome: Affects females with only one X chromosome (45, X0), leading to short stature and infertility. Treatment often includes hormone therapy.

• Klinefelter's Syndrome: Affects males with an extra X chromosome (47, XXY), causing changes such as breast tissue development and fertility issues. Treatments may include hormone therapy and fertility assistance.

Key Differences between Mitosis and Meiosis:

• Mitosis: One division creates two identical diploid cells, preserving the original genetic information.

• Meiosis: Two divisions create four genetically diverse haploid cells, promoting variation essential for sexual reproduction.

Interphase: Interphase is the phase where the cell prepares for division and consists of three parts:

• G1 Phase (Gap 1): The cell grows and synthesizes proteins necessary for cell division.

• S Phase (Synthesis): DNA is replicated, creating two copies of each chromosome.

• G2 Phase (Gap 2): The cell continues to grow and prepares for mitosis, repairing any DNA errors.

Cytokinesis: Cytokinesis is the final step of the cell division process. It occurs after mitosis or meiosis and involves the division of the cytoplasm, resulting in two separate daughter cells. In animal cells, this is achieved through the formation of a cleavage furrow that pinches the cell membrane inwards, while in plant cells, a cell plate forms to divide the cells.

Interphase: Interphase is the phase where the cell prepares for division and consists of three parts:

• G1 Phase (Gap 1): The cell grows and synthesizes proteins necessary for cell division.

• S Phase (Synthesis): DNA is replicated, creating two copies of each chromosome.

• G2 Phase (Gap 2): The cell continues to grow and prepares for mitosis, repairing any DNA errors.

Cytokinesis: Cytokinesis is the final step of the cell division process. It occurs after mitosis or meiosis and involves the division of the cytoplasm, resulting in two separate daughter cells. In animal cells, this is achieved through the formation of a cleavage furrow that pinches the cell membrane inwards, while in plant cells, a cell plate forms to divide the cells.

Nondisjunction is a failure of homologous chromosomes or sister chromatids to separate correctly during cell division, which can lead to an abnormal number of chromosomes in daughter cells. This can occur during meiosis or mitosis and is a common cause of chromosomal disorders. When nondisjunction happens during meiosis, it can result in gametes with an abnormal number of chromosomes. If such a gamete participates in fertilization, it can lead to conditions such as Down syndrome (trisomy 21) or Turner syndrome (monosomy X). Nondisjunction affecting autosomes can lead to various syndromes, while that affecting sex chromosomes can lead to syndromes like Klinefelter syndrome (XXY).

Meiosis I and Meiosis II are two distinct stages of meiosis that contribute to the production of haploid cells for sexual reproduction. The key differences include:

• Meiosis I:

  1. Homologous Chromosomes Separated: In this stage, homologous chromosomes are paired and then separated into two different cells.

  2. Prophase I: Chromosomes pair up and exchange segments of DNA through crossing over, adding to genetic diversity.

  3. Result: Two haploid cells are created, each containing one chromosome from each homologous pair, but these chromosomes are still composed of sister chromatids.

• Meiosis II:

  1. Sister Chromatids Separated: In this stage, the sister chromatids are separated in each of the two haploid cells created from Meiosis I.

  2. Similar to Mitosis: The process is similar to mitosis where chromosome alignment and segregation occur without further DNA replication.

  3. Result: Four genetically diverse haploid cells are produced, each with a single set of chromosomes.