science quiz 2/13

• DNA (deoxyribonucleic acid) is a molecule present in all living cells that contains information determining inherited traits and necessary for life
• Nucleotide:

  • A subunit in a nucleic-acid chain consisting of a sugar, a phosphate, and a nitrogenous base
• Genes are parts of chromosomes found in the nucleus of most cells, made of protein and DNA
• DNA must be able to give instructions for building and maintaining cells, as well as be copied during cell division to ensure identical genes in each cell
• Scientists initially believed only complex molecules like proteins could perform these functions, but were surprised by the capabilities of the DNA molecule
• DNA is composed of nucleotide subunits, each containing a sugar, a phosphate, and a base
• There are four bases in DNA: adenine, thymine, guanine, and cytosine, each with a different shape
• Scientists commonly refer to bases by the first letter of their names: A, T, G, and C
• Erwin Chargaff found that the amount of adenine in DNA always equals the amount of thymine
• Erwin Chargaff found that the amount of guanine always equals the amount of cytosine
• Erwin Chargaff's findings are known as Chargaff's rules
• Chargaff's rules later helped scientists understand the structure of DNA
• Rosalind Franklin used X-ray diffraction to make images of DNA molecules
• In X-ray diffraction, X rays are aimed at the DNA molecule and bounce off when hitting a part of the molecule
• The pattern made by the bouncing rays is captured on film
• Franklin's images suggested that DNA has a spiral shape
• James Watson and Francis Crick, after seeing Franklin's X-ray images, concluded that DNA looks like a long, twisted ladder
• Watson and Crick built a model of DNA that perfectly fit with both Chargaff's and Franklin's findings
• Watson and Crick's model of DNA as a double helix helped explain how DNA is copied and functions in the cell
• A strand of DNA resembles a twisted ladder, known as a double helix
• The two sides of the ladder are made of alternating sugar parts and phosphate parts
• The rungs of the ladder are made of a pair of bases:

  • Adenine always pairs with thymine
  • Guanine always pairs with cytosine
• Watson and Crick matched Chargaff's observations by pairing the bases in DNA
• The width of the DNA ladder matches the combined width of the matching bases
• Only the correct pairs of bases fit within the ladder's width
• Pairing of bases allows the cell to replicate DNA
• Each base always bonds with only one other base
• Pairs of bases are complementary to each other
• Both sides of a DNA molecule are complementary
• During replication, a DNA molecule is split down the middle where the bases meet
• Bases on each side of the molecule are used as a pattern for a new strand
• Complementary nucleotides are added to each side of the ladder as the bases on the original molecule are exposed
• Two DNA molecules are formed during replication
• DNA is copied every time a cell divides
• Each new cell gets a complete copy of all the DNA
• Unwinding, copying, and re-winding of DNA is done by proteins within the cell
• DNA is usually found with several kinds of proteins
• Other proteins help with carrying out the instructions written in the DNA code
• DNA is often wound around proteins, coiled into strands, and bundled up
• In a cell without a nucleus, each DNA strand forms a loose loop