7.1 Chromosomes, genes and DNA

• Chromosomes are thread-like structures found in the nucleus of cells that carry genetic information in the form of DNA.
• DNA, or deoxyribonucleic acid, is a molecule that contains the instructions for the development, functioning, growth, and reproduction of living organisms.
• Genes are segments of DNA that encode specific traits or characteristics, serving as the basic units of heredity. Each gene contains a specific sequence of nucleotides that determines the sequence of amino acids in proteins, which ultimately influence an organism's traits and characteristics.
• What: Chromosomes are thread-like structures found in the nucleus of cells that contain genetic information.
• Why: Chromosomes play a crucial role in controlling the activities of cells and passing genetic traits from one generation to another.
• When: Chromosomes were discovered in the nineteenth century when scientists developed powerful microscopes capable of viewing small cellular structures.
• How: Chromosomes become visible under a light microscope when a cell is dividing and are stained with special dyes to enhance visibility. They are numbered based on their length, and organisms have varying numbers of chromosomes in their cells, with humans typically having 46 chromosomes.
• Chromosomes are typically not visible in cells under a microscope because they are only observable during certain stages of the cell cycle, particularly during cell division. Additionally, chromosomes are extremely thin and translucent structures that are difficult to distinguish without specialized staining techniques. Without these stains, chromosomes blend into the background of the cell and are not easily discernible.
• No, red blood cells do not contain any chromosomes. This is because red blood cells undergo a process called enucleation during their maturation in the bone marrow, where they lose their nucleus and other organelles. As a result, they lack the genetic material required to maintain chromosomes. The absence of a nucleus in red blood cells means they do not possess the DNA or genetic information typically found in cells that contain chromosomes.
• Humans have 46 chromosomes
• Chromosomes are typically numbered based on their length, with the longest being designated as chromosome 1, followed by chromosome 2, and so forth
• In humans, each chromosome has a duplicate, resulting in pairs of chromosomes
• Fruit flies have 8 chromosomes
• Mango trees have 40 chromosomes
• A photograph of human chromosomes often depicts them lined up by size, although they don't naturally align this way
• Each chromosome resembles a cross shape because, just before cell division, they replicate themselves, and the copies remain attached along their length
• Each chromosome contains numerous genes arranged in a specific sequence
• Each gene governs a particular characteristic in the organism
• Chromosome 1 contains around 2000 genes
• Chromosome 15 contains about 600 genes
• Two genes on chromosome 15 contribute to determining eye color
• Variations in these genes result in different eye colors among individuals
• What: Genes are segments of DNA that serve as the basic units of heredity, containing instructions for building and maintaining an organism's cells, tissues, and organs.
• Why: Genes encode specific traits and characteristics, dictating everything from physical appearance to biochemical processes within an organism. They play a crucial role in determining an organism's development, growth, and functioning.
• When: Genes are present in every cell of an organism's body, inherited from its parents during reproduction. They are active throughout an organism's lifespan, orchestrating various biological processes from embryonic development to aging.
• How: Genes are composed of DNA sequences that encode information in the form of nucleotide bases (adenine, thymine, cytosine, and guanine). These sequences are transcribed into RNA molecules, which are then translated into proteins—the building blocks of cells and tissues. Proteins carry out specific functions based on the instructions encoded in the genes, ultimately determining the organism's traits and characteristics. Mutations and variations in genes can lead to genetic diversity and contribute to evolutionary processes.
• What: Chromosomes are thread-like structures composed of DNA and proteins found in the nucleus of eukaryotic cells. They carry genetic information in the form of genes, which are segments of DNA responsible for encoding specific traits and characteristics.
• Why: Chromosomes serve as vehicles for the storage, replication, and transmission of genetic information from one generation to the next. They ensure the accurate inheritance and expression of genetic traits during cell division and reproduction.
• When: Chromosomes are present in the nucleus of cells throughout an organism's life cycle, from the embryonic stage to adulthood. They are most visible and prominent during cell division when they condense into distinct structures visible under a microscope.
• Chromosomes are composed of long strands of DNA wrapped around proteins called histones
• DNA-protein complexes undergo various levels of coiling and folding to form the characteristic chromosome structure
• During cell division, chromosomes condense further into compact structures that are easily visible under a light microscope
• Each species has a characteristic number of chromosomes
• Chromosomes occur in pairs in most eukaryotic organisms
• Human cells typically contain 46 chromosomes organized into 23 pairs
• Chromosomes replicate and segregate during cell division
• This ensures that each daughter cell receives a complete set of genetic information
• Mutations, rearrangements, or abnormalities in chromosomes can lead to genetic disorders and diseases
• DNA is composed of nucleotides, which consist of:

  • A sugar molecule (deoxyribose)
  • A phosphate group
  • One of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), and thymine (T)
• Nucleotides are arranged in specific sequences along the DNA strands