Key Concepts in Molecular Biology and Cell Division

Nucleic Acid Structure and Function

• Nucleoside: A part of a nucleotide made up of one sugar (ribose or deoxyribose) and one nitrogenous base (pyrimidine or purine) joined together by N-glycosidic bonds.
• Nucleotide: Simple molecules that form large molecules called nucleic acids. It is formed by a nitrogenous base, sugar, and three phosphate groups.
• Chromatin: Bundles of DNA and proteins; the normal state of the genetic material during interphase.
• Chromosome: Structures formed by the condensation of chromatin that appear when the cell is about to divide.
• Centromere: A bond that joins two sister chromatids; it's where the microtubules are attached to the chromosome during the division process.
• Telomere: The end of the arms of a chromosome, which prevents them from attaching to other chromosomes.
• Haploid cell: A cell that has a single set of chromosomes or n chromosomes (e.g., ovum).
• Diploid cell: A cell that has two sets of n chromosomes or 2n chromosomes (e.g., human somatic cells).
• Homologous chromosome: Equal pairs of chromosomes.

Gene Expression: Transcription and Translation

• Transcription: Takes place in the nucleus, where the information stored in the DNA is transferred to a messenger RNA (mRNA) molecule.
• Translation: The mRNA bonds to the ribosomes, and the free amino acids bond to a transfer RNA (tRNA), which transfers each amino acid to the mRNA. The amino acids then link and form protein chains.
• Codon: A sequence of three nucleotides that form a unit of genetic code.
• RNA polymerase: An enzyme that synthesizes the formation of RNA from a DNA template during transcription.

DNA Replication

• Replication: The process by which double-stranded DNA makes copies of itself, each strand, as it separates, synthesizing a complementary strand.
• Replication fork: The area where the replication of DNA will actually take place.
• Helicase: Enzymes that bind and may even remodel nucleic acid or nucleic acid-protein complexes.
• Topoisomerase: A class of enzymes that alter the supercoiling of double-stranded DNA.
• Single-Strand Binding (SSB) proteins: Proteins responsible for keeping the replication fork of DNA open.
• RNA primase: An enzyme that creates the primers, marking the starting point for the construction of the new strand of DNA.
• RNA primer: A short, single strand of RNA or DNA (generally about 18-22 bases) that serves as a starting point for DNA synthesis. It is required for DNA replication because the enzymes that catalyze this process, DNA polymerases, can only add new nucleotides to an existing strand of DNA.
• DNA polymerase: An enzyme that synthesizes DNA molecules from deoxyribonucleotides, the building blocks of DNA. It can only add DNA bases in one direction (5'-3').
• Exonuclease: An enzyme that removes all the primers from both strands of DNA.
• DNA ligase: An enzyme that joins DNA strands together by catalyzing the formation of a phosphodiester bond.

Cell Cycle and Cell Division

• Interphase: The longest phase in the cell cycle, during which the daughter cells grow to adult size. At the end of the phase, DNA replicates. In this phase, the centrioles also replicate.
• Mitosis: The process of nuclear division, through which two daughter cells with the same number of chromosomes as the parent cell are formed.
• Cytokinesis: When the cytoplasm divides during the cell division phase.
• Prophase: The phase where the chromatin fibers that were replicated during interphase condense and form chromosomes.
• Metaphase: The phase where the chromosomes reach maximum condensation.
• Anaphase: The phase where the sister chromatids of each chromosome separate.
• Telophase: The phase where the daughter chromosomes migrate to the end of their corresponding cells.