DNA Structure and Function

• Deoxyribonucleic acid (DNA) carries genetic information in cells.
• DNA is found in the nucleus, condensed into chromatin.
• Chromatin condenses further into chromosomes during cell division.
• DNA is grouped into genes, the basic units of inheritance.
• Genes are expressed into proteins, which perform cellular functions and determine observable traits.

DNA Structure

• DNA has a double helical structure, with two strands twisting around each other.
• Each strand contains covalently bonded monomers called nucleotides.
• Nucleotides consist of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.
• Nitrogenous bases are purines (adenine (A) and guanine (G)) or pyrimidines (thymine (T) and cytosine (C)).
• Adenine and thymine form two hydrogen bonds, guanine and cytosine form three hydrogen bonds.
• Base pairing is essential for DNA structure and function.
• During expression, DNA is transcribed into messenger RNA (mRNA) in the nucleus.
• mRNA is then translated into proteins in the ribosome.

DNA Replication

• The cell cycle includes G1, S, G2, and M stages, preparing the cell for division.
• DNA replicates during the S stage to produce two identical copies.
• DNA strands run in opposite directions (antiparallel).
• DNA replication is semi-conservative, producing daughter molecules with one parent and one new strand.
• Helicase unzips the DNA molecule by breaking hydrogen bonds, forming a replication fork.
• The unzipped strands become templates for new DNA molecules.
• The leading strand runs in the 3'-5' direction.
• RNA acts as a primer for DNA polymerase on the leading strand.
• DNA polymerase adds new complementary bases to the leading strand.
• The lagging strand runs in the 5'-3' direction.
• Multiple RNA primers attach to the lagging strand, allowing DNA polymerase to create Okazaki fragments.
• DNA ligase glues the Okazaki fragments together.

DNA vs RNA

• Ribonucleic acid (RNA) is similar to DNA but single-stranded.
• RNA contains the sugar ribose instead of deoxyribose, and includes uracil instead of thymine.
• RNA can act as a primer in DNA replication or as messenger RNA in gene expression.
• RNA molecules can also function as enzymes.

DNA Molecule

• The DNA in one cell, if stretched out, would be approximately 2 meters long.
• DNA compaction into chromatin using proteins called histones is essential for it to fit within the cell's nucleus.
• DNA was discovered by Frederich Miescher in the late 1800s.
• Rosalind Franklin, James Watson, and Francis Crick revealed DNA's double helical structure using X-ray diffraction in the 1950s.
• Genes, distinct units of inheritance, carry codes and instructions to make proteins.
• Genes can be up to 1 million bases long but constitute only 1% of the DNA sequence.
• Non-coding segments called introns control gene expression.
• Coding segments of DNA are called exons.

DNA Sequencing

• DNA sequencing uncovers the genetic makeup of a cell by revealing the base pairing content.
• Sanger sequencing, or the chain termination method, uses fluorescently tagged nucleotides.
• Next-generation sequencing has largely replaced Sanger sequencing due to efficiency and cost.
• Sanger sequencing remains the golden standard because of its high accuracy.
• DNA sequencing is used in settings such as diagnostics and research for genetically inherited diseases.
• The Human Genome Project completed in 2003, revealed the entire human genome sequence.

Explore DNA's structure: a double helix of nucleotides with bases adenine, thymine, guanine, and cytosine. Learn how DNA carries genetic information, forms genes, and is transcribed into mRNA for protein synthesis, which dictates traits and cellular functions.