Biology: DNA and Genetic Engineering

DNA Structure and Composition

• DNA is made up of nucleic acid polymers, consisting of nucleotides as the building blocks.
• A nucleotide is composed of a phosphate group (negatively charged), deoxyribose (sugar), and a nitrogen base (either pyrimidine or purine).
• Pyrimidines are smaller nitrogen bases, consisting of C and T.
• Purines are larger nitrogen bases, consisting of A and G.

Patterns in DNA

• Purine always pairs with pyrimidine.
• A pairs with T, and C pairs with G.
• The bond between C and G is always 3 hydrogen bonds.
• The bond between T and A is always 2 hydrogen bonds.
• The backbone of DNA is made up of sugar and phosphate, and is antiparallel on the other side.

DNA-Related Terms

• Transformation: the process of transferring genetic material from dead smooth bacteria to alive harmless bacteria, transforming it.
• Gene: a part of DNA that codes for something specific.
• Chromosome: the overall shape of clumped DNA.
• Chromatin: strands of compacting DNA.
• Nucleosome: a loop of DNA around a histone protein.
• Bacteriophage: a virus that parasitizes a bacterium by infecting it and reproducing inside it.

Chromatin and Chromosomes

• Chromatin is DNA in its uncoiled state
• Chromosomes are DNA in its tightly coiled state
• Bacteria have one circular chromosome, while humans have 46 linear chromosomes in a cell
• A duplicated chromosome consists of two sister chromatids, which are two identical double helixes

DNA Replication

• DNA Polymerase is responsible for pulling apart the DNA helix and adding new DNA nucleotides during replication

Centromeres

• The number of centromeres is equal to the number of chromosomes

Griffith's Experiment (1928)

• Griffith discovered that a genetic material from a heat-killed disease-causing bacterium could transform a harmless bacterium into a disease-causing one
• This transforming factor was later found to be DNA

Avery's Experiment (1944)

• Avery isolated and inactivated different molecules (carbohydrates, proteins, lipids, DNA, and RNA) to determine which one was responsible for the transformation
• He found that DNA was the transforming factor, and concluded that DNA is the genetic material that stores and transmits genetic information

Hershey and Chase's Experiment (1952)

• Hershey and Chase used radioactive isotopes to determine whether the genetic material of a bacteriophage was DNA or protein
• They found that the DNA core of the bacteriophage, not the protein coat, was responsible for the genetic material

Chargaff's Rules

• The percentages of guanine and cytosine are almost equal in any sample of DNA
• The percentages of adenine and thymine are also almost equal in any sample of DNA
• Chargaff's rules: A = T and G = C

Franklin's Experiment

• Franklin used x-ray diffraction to reveal an X-shaped pattern in DNA, showing that the strands are twisted around each other (helix)
• Her findings suggested that the nitrogen bases are near the center of the molecule

Watson and Crick's Model

• Watson and Crick built a 3D model of DNA using cardboard and wire, based on Franklin's data
• They discovered the double helix structure of DNA