DNA Structure and genetic information

• Meiosis and mitosis are cell division processes crucial for genetic diversity, reproduction, and growth in organisms.
• DNA structure provides the blueprint for these processes, ensuring accurate replication and transmission of genetic information.

DNA Structure

• DNA (deoxyribonucleic acid) is the molecule that carries genetic information.
• It contains the instructions for the development and function of all known living organisms.
• DNA is a double-stranded helix.
• The helix resembles a twisted ladder, with the sides of the ladder made of a sugar-phosphate backbone and the rungs made of complementary nucleotide base pairs.
• Nucleotides are the building blocks of DNA, each consisting of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
• There are four types of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
• Adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C).
• These specific pairings are due to the number of hydrogen bonds that can form between the bases; A and T form two hydrogen bonds, while G and C form three.
• The sequence of these bases along the DNA molecule encodes genetic information.
• The double helix structure of DNA allows for efficient storage of genetic information and provides a mechanism for accurate replication.
• The two DNA strands are antiparallel, meaning they run in opposite directions; One strand runs from 5' to 3', while the other runs from 3' to 5'.
• The 5' and 3' refer to the carbon atoms on the deoxyribose sugar molecule.
• DNA is organized into structures called chromosomes.
• In eukaryotes, chromosomes are housed within the nucleus.
• DNA must be tightly packed to fit inside the nucleus, achieved through association with histone proteins, forming a complex called chromatin.
• Chromatin can be further compacted into higher-order structures.

Mitosis

• Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.
• Mitosis is used for growth, repair, and asexual reproduction in some organisms.
• It ensures that each new cell receives a complete and identical set of chromosomes.
• Mitosis is divided into distinct phases: prophase, prometaphase, metaphase, anaphase, and telophase (followed by cytokinesis).

Phases of Mitosis

• Prophase:
  • The chromatin condenses into visible chromosomes.
  • The nuclear envelope breaks down.
  • The mitotic spindle begins to form from the centrosomes.
• Prometaphase:
  • The nuclear envelope completely disappears.
  • Spindle fibers attach to the kinetochores of the chromosomes.
  • Kinetochores are protein structures on the centromere of each chromosome.
• Metaphase:
  • The chromosomes align along the metaphase plate (the equator of the cell).
  • Each chromosome is attached to spindle fibers from opposite poles.
• Anaphase:
  • The sister chromatids separate and move to opposite poles of the cell.
  • The spindle fibers shorten, pulling the chromatids apart.
• Telophase:
  • The chromosomes arrive at the poles and begin to decondense.
  • The nuclear envelope reforms around each set of chromosomes.
  • The mitotic spindle disappears.
• Cytokinesis:
  • Division of the cytoplasm to form two separate daughter cells.
  • In animal cells, cytokinesis occurs through the formation of a cleavage furrow.
  • In plant cells, a cell plate forms between the two daughter cells.

Meiosis

• Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.
• It is essential for sexual reproduction.
• Meiosis involves two rounds of cell division: meiosis I and meiosis II.
• Meiosis I separates homologous chromosomes, while meiosis II separates sister chromatids.

Phases of Meiosis I

• Prophase I:
  • The chromatin condenses into visible chromosomes.
  • Homologous chromosomes pair up in a process called synapsis, forming tetrads (bivalents).
  • Crossing over occurs: genetic material is exchanged between homologous chromosomes, leading to genetic recombination.
  • The nuclear envelope breaks down.
  • The spindle apparatus forms.
• Metaphase I:
  • Tetrads align along the metaphase plate.
  • Each homologous chromosome is attached to spindle fibers from opposite poles.
• Anaphase I:
  • Homologous chromosomes separate and move to opposite poles of the cell.
  • Sister chromatids remain attached.
• Telophase I:
  • The chromosomes arrive at the poles.
  • The cell divides, resulting in two haploid daughter cells.
  • Each daughter cell contains one chromosome from each homologous pair.
• Cytokinesis I:
  • Division of the cytoplasm to form two separate daughter cells.

Phases of Meiosis II

• Meiosis II is similar to mitosis, but it starts with a haploid cell.
• Prophase II:
  • Chromosomes condense.
  • The nuclear envelope breaks down (if it reformed during telophase I).
  • The spindle apparatus forms.
• Metaphase II:
  • Chromosomes align along the metaphase plate.
  • Sister chromatids are attached to spindle fibers from opposite poles.
• Anaphase II:
  • Sister chromatids separate and move to opposite poles of the cell.
• Telophase II:
  • The chromosomes arrive at the poles and begin to decondense.
  • The nuclear envelope reforms around each set of chromosomes.
• Cytokinesis II:
  • Division of the cytoplasm to form four separate haploid daughter cells.

Key Differences Between Mitosis and Meiosis

• Mitosis produces two diploid daughter cells, while meiosis produces four haploid daughter cells.
• Mitosis involves one round of cell division, while meiosis involves two rounds.
• Mitosis does not involve synapsis or crossing over, while meiosis does.
• Mitosis results in genetically identical daughter cells, while meiosis results in genetically diverse daughter cells due to crossing over and independent assortment of chromosomes.
• Mitosis is used for growth, repair, and asexual reproduction, while meiosis is used for sexual reproduction.