chapter 4

DNA Structure and Function

Basic Structure: DNA consists of a double helix structure resembling a spiral staircase, with a consistent width of about 20 nanometers. The strands are held together by hydrogen bonds between base pairs.

Bases pair in a specific manner:
- Guanine (G) pairs with Cytosine (C) through 3 hydrogen bonds.
- Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.
Purine (A, G) pairs with Pyrimidine (C, T), maintaining the double helix uniformity.

The backbone of DNA is alternated between phosphate groups and sugars, with the phosphate of one base, connecting to the sugar of another. This complementary base pairing ensures that the information of one strand is mirrored in the other, allowing replication and transcription.
Genes: A gene is a sequence of DNA that encodes instructions for synthesizing a protein or RNA molecule. The human genome consists of roughly 24,000 genes, which represent about 2% of our total DNA, the rest being intergenic sequences that contain regulatory information.

Chromatin: DNA is organized into chromatin, a complex of DNA and proteins (histones) that condense to form chromosomes during cell division.
Histones: These proteins help package DNA into structural units, facilitating the organization of DNA within the nucleus.
Centromeres and Kinetochores: During cell division, proteins attach at the centromere regions, forming kinetochores, which anchor to spindle fibers to move chromosomes segregating them accurately into daughter cells.

Difference from DNA: RNA has an extra hydroxyl group (OH) on its ribose sugar, making it more reactive and preventing it from forming the double helix structure seen in DNA.
Types of RNA:
- Messenger RNA (mRNA): Carries the information from the DNA to ribosomes for protein synthesis.
- Ribosomal RNA (rRNA): A component of ribosomes that helps facilitate the synthesis of proteins.
- Transfer RNA (tRNA): Transfers specific amino acids to the growing polypeptide chain during protein synthesis.
Transcription and Translation: The process of using DNA to synthesize RNA (transcription) and then translating that RNA to synthesize proteins (translation).
- Transcription involves RNA polymerase binding to DNA and synthesizing a complementary mRNA strand.
- Translation occurs in the ribosome, where the mRNA strand is read in codons (three-nucleotide sequences) to assemble the corresponding amino acids into proteins.

Protein Production: Ribosomes read mRNA in sets of three bases (codons), each specifying an amino acid. The presence of start (AUG) and stop codons regulate translation.
Silent Mutations: Sometimes mutations occur but do not affect the amino acid sequence, due to redundancy in the genetic code.
Gene Regulation: Gene expression is tightly regulated, affecting which proteins are made in a given cell type. Regulatory sequences linked to regulatory proteins modify gene expression based on cellular needs.

Cell Division: Cells undergo a cycle including phases of cell growth (G1), DNA synthesis (S), and preparation for division (G2) before undergoing mitosis (M phase).
Mitosis and Meiosis:
- Mitosis results in two identical daughter cells for growth and repair.
- Meiosis produces gametes (sperm and eggs) with half the amount of DNA (haploid), combining during fertilization to form a complete set (diploid).

Prophase: Chromatin coils into visible chromosomes and the nuclear envelope dissolves.
Metaphase: Chromosomes align along the equatorial plane.
Anaphase: Sister chromatids separate and are pulled to opposite poles.
Telophase: New nuclear membranes form around each set of chromosomes.

Meiosis I and Meiosis II both consist of similar phases as mitosis but result in four unique cells with half the original DNA.

Tissue Types: The body is organized into four main tissue types:
- Epithelial Tissue: Covers surfaces and forms glands.
- Connective Tissue: Supports, binds, and protects tissues and organs.
- Nervous Tissue: Involved in communication through electrical impulses.
- Muscle Tissue: Responsible for movement.