In humans, chromosomes are categorized into autosomes (non-sex chromosomes) and sex chromosomes (which determine biological sex). Most human cells are diploid, meaning they contain two complete sets of chromosomes (one from each parent). However, reproductive cells, known as gametes (sperm and eggs), are haploid, containing only one set of chromosomes.
DNA Replication and Mutations
Before a cell can divide, it must copy its entire genome so that each daughter cell receives a complete set of instructions. This process is known as DNA replication. Replication follows a semiconservative model, meaning that when a new DNA molecule is formed, it consists of one original "parental" strand and one newly synthesized "daughter" strand.
The process is driven by specialized enzymes. DNA polymerase is the primary enzyme responsible for synthesizing the new DNA strand by adding nucleotides that are complementary to the template. However, DNA polymerase cannot start from scratch; it requires a primer—a short sequence of nucleotides that provides a starting point for synthesis.
While replication is incredibly accurate, errors can occur. These errors are known as mutations. Mutations can be categorized by how they change the DNA sequence:
- Substitutions: A single nucleotide is swapped for another.
- Frameshift Mutations: These occur when nucleotides are either inserted or deleted from the sequence. Because the genetic code is read in specific groupings, adding or removing a base shifts the entire "reading frame," often changing every amino acid that follows.
Exam Tip: When studying mutations, remember that a substitution might only change one amino acid, but a frameshift (insertion or deletion) usually changes the entire protein structure because it alters the triplet reading frame.
Gene Expression: From DNA to Protein
Having the blueprint (DNA) is not enough; the cell must actually build the proteins that perform biological work. This process is called gene expression, and it occurs in two major stages: transcription and translation.
Transcription
Transcription is the process of copying a specific segment of DNA into messenger RNA (mRNA). This is carried out by the enzyme RNA polymerase, which reads the DNA template and assembles a complementary RNA strand.
In eukaryotic cells (like human cells), transcription involves extra processing steps. The initial RNA transcript contains both exons (coding regions) and introns (non-coding regions). To create a functional mRNA, the introns must be removed and the exons joined together—a process called splicing. Sometimes, a cell can join different combinations of exons together, a phenomenon known as alternative splicing, which allows one gene to code for multiple different proteins.
Translation and the Genetic Code
Once the mRNA is processed, it moves to the ribosome for translation. Here, the information in the mRNA is converted into a chain of amino acids (a protein).
The "language" used here is the genetic code. The ribosome reads the mRNA in groups of three nucleotides called codons. Each codon corresponds to a specific amino acid. To bridge the gap between the RNA code and the amino acids, the cell uses transfer RNA (tRNA). Each tRNA molecule has an anticodon at one end—which matches the mRNA codon—and carries a specific amino acid at the other. Meanwhile, ribosomal RNA (rRNA) makes up the physical structure of the ribosome where this assembly takes place.
Cellular Reproduction: Mitosis and Meiosis
Cells reproduce to allow for growth, repair, and the continuation of a species. There are two primary methods: asexual reproduction (producing genetically identical clones) and sexual reproduction (producing genetically diverse offspring).
Mitosis (Asexual Reproduction)
In multicellular organisms, mitosis is the process used for somatic (body) cell division. The goal of mitosis is to create two daughter cells that are genetically identical to the parent cell, thereby maintaining the same number of chromosomes.
The cell cycle is regulated through several stages, with mitosis itself divided into four main phases:
- Prophase: Chromosomes condense and the spindle apparatus begins to form.
- Metaphase: Chromosomes line up in the middle of the cell.
- Anaphase: Sister chromatids are pulled apart toward opposite poles.
- Telophase: Two new nuclei form around the separated chromosomes.
Meiosis (Sexual Reproduction)
To produce gametes (sperm and eggs), cells undergo meiosis. Unlike mitosis, meiosis reduces the chromosome number by half, resulting in four haploid daughter cells.
The most critical feature of meiosis is the introduction of genetic diversity. During the early stages, homologous chromosomes (pairs of chromosomes containing the same genes, one from each parent) undergo crossing over. This is the exchange of genetic material between non-sister chromatids, ensuring that every gamete produced is genetically unique.
The Cell Cycle and Cancer
The cell cycle is a highly regulated sequence of events that ensures DNA is replicated correctly and divided equally. Cells pass through various checkpoints to verify that each stage is complete and error-free.
When these regulatory mechanisms fail, the consequences can be severe. If the cell cycle becomes dysregulated, cells may begin to divide uncontrollably, leading to the formation of tumors. This uncontrolled growth is often driven by mutations in specific genes:
- Oncogenes: Genes that, when mutated, promote excessive cell growth.
- Tumor Suppressor Genes: Genes that normally slow down cell division or repair DNA errors; when these are inactivated, the "brakes" on cell division are lost.
If these malignant cells gain the ability to break away from the original tumor and spread to other parts of the body, the process is known as metastasis.
