Chromosomes, DNA Replication, and Genetic Processes

Posted on Dec 20, 2024 in Biology

Chromosomes

Chromosomes are small, filamentous, rod-shaped structures that appear when a cell is dividing.

Parts of a Chromosome

  • Chromatids: Two chromatids joined at the centromere form a chromosome.
  • Telomeres: The terminal part of a chromosome, associated with aging and cellular senescence.
  • Secondary Constriction and Satellites: Specific regions on some chromosomes.

Types of Chromosomes

  • Metacentric: The centromere is in the center, and the two arms are equal.
  • Submetacentric: The centromere is slightly displaced from the center, resulting in one shorter arm and one longer arm.
  • Acrocentric: The centromere is near one end, resulting in one very long arm and one very short arm.
  • Telocentric: The centromere is located at one end, resulting in only one arm.

Chromosome Number

The number of chromosomes is specific to each species. Living things have a basic chromosomal set called the haploid set. Chromosomes are divided into two types:

  1. Autosomes: 22 pairs (44 chromosomes).
  2. Sex Chromosomes: Pair 23, determining the sex of the individual.

There are two types of cells:

  • Somatic Cells: Body cells that divide by mitosis.
  • Gametes: Reproductive cells with half the number of somatic chromosomes (23 chromosomes) and divide by meiosis.

DNA Replication

The DNA double helix must separate, breaking the hydrogen bonds linking the nitrogenous bases. Each strand serves as a template to form new strands, called daughter strands. Replication occurs due to the complementarity of the nitrogenous bases.

Mechanism of Replication

  1. The double helix opens, separating the two strands, forming a loop.
  2. Deoxyribonucleotides (adenine, guanine, cytosine, and thymine) enter the loop and bind to the template strand.
  3. Nucleotides are joined by an enzyme called DNA polymerase until a complete complementary chain is formed, resulting in two double helices with the same information. Each double helix carries one old strand and one new strand. Each double helix will form each of the chromatids that make up the chromosome.

This process of replication only occurs when the cell is about to begin mitosis.

Transcription and Translation

RNA reads the genetic information possessed by DNA and takes it from the nucleus to the cytoplasm. RNA consists of a single chain of nucleotides with ribose and the same bases as DNA, except that thymine is replaced by uracil. The process of RNA synthesis is called transcription. RNA leaves the nucleus carrying the genetic message to direct protein synthesis; this is called messenger RNA. Messenger RNA binds to a ribosome, which translates the message and produces a protein through translation, using the genetic code to decipher the language of proteins.

Structure of DNA

DNA consists of two long chains of nucleotides twisted into a double helix. The two chains are complementary, with nitrogenous bases falling into the interior of the double helix and pentose and phosphoric acid to the outside. Adenine is complementary to thymine, forming two hydrogen bonds. Guanine is complementary to cytosine, forming three hydrogen bonds. The union between the bases is known as base pairing.

Genetic Concepts

  • Gene: A sequence of nucleotides or a stretch of DNA that carries genetic information.
  • Genotype: The set of hereditary factors, i.e., the set of genes of an individual.
  • Phenotype: The observable expression of the genotype.
  • Character: Each individual characteristic, determined by at least one pair of genes.
  • Alleles: Each of the forms in which a gene can exist.
  • Homozygous: When both alleles are the same.
  • Heterozygous: When the two alleles determining the gene are different.
  • Dominant Gene: One that dominates over others determining the same character.
  • Recessive Gene: One that is hidden by a dominant gene but is present in the genotype and only manifests when homozygous.
  • Locus: The location of a gene on a chromosome.

Stem Cells

Stem cells are undifferentiated cells that can divide indefinitely to produce new stem cells. Under appropriate conditions, they can differentiate into one or more types of specialized cells, such as blood cells, hepatocytes, or neurons.

Types of Stem Cells

  • Fetal Stem Cells: Extracted from a fetus.
  • Umbilical Cord Stem Cells: Extracted during birth.
  • Embryonic Germ Stem Cells: Germ cells that produce eggs and sperm. If handled before maturation, they can differentiate into cells of any tissue.