Overview
This lecture focused on the structure and function of DNA within cells, emphasizing how DNA codes for proteins, the importance of base pairing, and the basics of genes and chromosomes.
DNA Structure and Function
- DNA is the largest molecule in the body and stores genetic instructions for making all proteins.
- DNA is made of nucleic acids: adenine (A), guanine (G), thymine (T), and cytosine (C).
- Only cells with a nucleus contain DNA and can divide.
- A single strand of DNA is unstable; double-stranding with complementary base pairing makes it stable.
- Base pairing: A always pairs with T, C always pairs with G, forming the DNA double helix.
Genes, Chromosomes, and Chromatin
- Only one strand of the DNA ladder codes for proteins; the other strand provides stability.
- A gene is a section of DNA that codes for a specific protein by dictating amino acid type and sequence.
- Human cells contain about 25,000 genes.
- In dividing cells, DNA is packaged as chromosomes; in non-dividing cells, it's loose as chromatin.
- Humans have 23 pairs of chromosomes: 22 somatic (body traits) and 1 sex chromosome pair (XX or XY).
- The male's sperm determines the sex of the offspring.
Protein Synthesis and Cell Specificity
- Every cell contains all genetic information but only expresses what is needed (e.g., liver cells make liver proteins).
- Chemical signals from the blood affect cells based on protein receptors on their surface.
- Genes are selectively activated in response to cell-specific signals.
Transcription and mRNA
- DNA never leaves the nucleus.
- To make a protein, the cell unzips only the relevant gene, not the entire DNA strand.
- The process of transcription involves copying the gene onto messenger RNA (mRNA).
- mRNA then carries the genetic code to ribosomes, where proteins are made.
Key Terms & Definitions
- Nucleic Acids — Organic molecules that make up DNA: adenine, guanine, thymine, cytosine.
- Base Pairing — A always with T; C always with G.
- Gene — DNA segment coding for a specific protein.
- Chromosome — Packaged DNA visible only during cell division.
- Chromatin — Loose DNA found in non-dividing cells.
- Transcription — The process of copying a DNA section into mRNA.
- mRNA (Messenger RNA) — RNA that carries DNA’s instructions out of the nucleus.
Action Items / Next Steps
- Review the process of transcription for next class discussion.
- Study the differences between chromatin and chromosomes.
- Learn the obligatory base pairing rules.