CHAPTER 12 SUMMARY

The form of DNA is tied to its function

  • The two strand of DNA are aligned in the helix with the 5’ end of one strand pairing with the 3’ end of its complementary strand.
  • The sugar-phosphate backbones of the double helix are on the outside of the helix, and the nitrogenous bases point toward the center of the helix.
  • Base pairs re complementary; they consist of a pyrimidine pairing with a purine via hydrogen bonding (A pairs with T, G pairs with C)
  • The specific base sequences in DNA are used to store genetic information. Altering the base sequence in the DNA by mutation may cause a change in the genetic information. Faithful replication of the DNA molecule is precisely generated using complementary base pairing. The expression of genetic information in the DNA results in specific phenotypes.

DNA must be able to be replicated in order to be heritable.

  • During replication DNA is unwound, and the new strand is synthesized by complementary base pairing of each incoming nucleotide to the parental strand via hydrogen bones.
  • Prokaryotes have one replication origin; most eukaryotes have many. Replication proceeds in both directions from the replication origin, forming replication forks.

Many mistakes are made during initial synthesis of DNA; therefore, DNA proofreading and repair mechanisms are essential.

  • DNA polymerase proofreads and repairs base mismatches during replication.
  • Mismatch repair systems scan for possible errors in newly synthesized DNA>

THE BIG PICTURE

  • Knowing that DNA is the genetic material has allowed scientists to understand how hereditary information is passed on at the molecular level and how mutations can alter that hereditary information.
  • Advances in our knowledge of how DNA is replicated provide new technologies for understanding genes, their function their expression, and genetic relatedness in different organisms.
  • Focus on HOW the experiments provide evidence to prove that DNA is the genetic material.

The central dogma of molecular biology says that information flows in only one direction.

  • In an eukaryotic cell, a message (the messenger RNA or mRNA) is needed to carry the genetic information (the DNA) from the nucleus to the cytoplasm. The process of synthesizing mRNA from DNA is called transcription.
  • An adapter (transfer RJA or tRNA) is needed to translate the nucleic acid (in the messenger RNA) into proteins. Translation is the process of synthesizing protein from mRNA
  • The information flow is unidirectional, from gene (DNA), to message (RNA) to protein.

Transcription is the process of synthesizing RNA from a DNA template.

  • RNA polymerase is the enzyme that synthesizes RNA using one strand of the DNA as a template. The resulting RNA molecule can be messenger RNAs which are used to synthesize proteins or they can become transfer RNA or ribosomal RNA.
  • RNA polymerase initiates transcription at special sequences on the DNA called promoters. The promoter tells the RNA polymerase where to start transcription, which strand of DNA to read, and which direction to move on the DNA to begin synthesizing RNA.
  • RNA polymerase unwinds about 20 base pairs in the DNA as it reads and synthesizes the complementary RNA strand.
  • RNA polymerase terminates transcription at special sequences in the DNA
  • In prokaryotes, translation of the mRNA can begin before transcription terminates.
  • In eukaryotes, the mRNA is processed in the nucleus and must be transported from the nucleus to the cytoplasm before translation can begin.

Translation is the process of synthesizing proteins from an mRNA template.

  • The genetic code provides the key to “reading” mRNA sequence.
  • Genetic information in the messenger RNA is read three bases at a time; each three-base sequence is called a codon.
  • There are four possible bases that can be used in a codon, so there are 64 different codons.
  • Sixty-one codons specify specific amino aids so the genetic code is redundant, meaning that there is more than one codon for many of the 20 amino acids.
  • Three codons, the stop codons UAG, UAA, UGA, are used to terminate translation.
  • The codon used to start translation is AUG, which also specifies the amino acid methionine.
  • Transfer RNA is the link between a codon and its amino acid.
  • The anticodon of the tRNA base-pairs with the codon of the mrNA. Theis base pairing occurs on the ribosome.
  • The ribosome is the site in the cell where protein translation occurs.
  • Ribosomes are composed of a large subunit and a small subunit; both subunits consist of different ribosomal proteins and RNA molecules.
  • Th important sites on a ribosome are the T site (where the incoming charged tRNA enters the ribosome at the A site (where the anticodon of the charged tRNA binds the codon of the mRNA), the P site (where the tRNA with the growing polypeptide chain is located) ad the E site (where the tRNA exits from the ribosome).
  • Initiation of translation requireds the small subunit of the ribosome, the initiator tRNA, the mRNA with the start codon AUG positioned correctly on the ribosome, and the initiation factors.
  • When all these components are in place, the large subunit of the ribosome joins the small subunit of the ribosome, and the incoming charged tRNA enters the T site and is positioned at the A site. The anticodon of this tRNA base-pairs with codon on the mRNA that is at the A site of the ribosome.
  • Base pairing between the anticodon and the codon is always complementary in the first two positions (A to T and C to G). At the third position of the codon (and the first position of the anticodon) base pairing does not have to be exact. This position is called the wobble position and allows the cell to use one tRNA to base pair with up to four different codons.
  • A peptide bond is formed between amino acids and the ribosome shifts down the messenger RNA so that the next codon is placdd at the A site. The tRNA that was in the P site is moved to the E sites and will exit the ribosome.
  • During elongation, the messenger RNA is read with each new codon being moved into the A site during elongation so that the next incoming charged tRNA can try to base pair with it.
  • Termination occurs when a stop codon is moved into the A site of the ribosome. Enzymes will be released so that the polypeptide is released from the ribosome and the small and large subunits of the ribosome dissociate.

Mutations are heritable changes in genetic information.

  • Mutations can occur in germ-line cells and thus be passed on to offspring, or they can occur in somatic cells. Some mutations are easy to observe, others are difficult.
  • Point mutations are changes of one or two base pairs and include nonsense mutations and frameshift mutations.
  • Nonsense mutations are single-base changes that change a codon to a stop codon.
  • Deletions or insertions of one or two bases are frameshift mutations, which alter the reading frame of he genetic message.
  • Chromosome mutations are large deletions, duplication, inversions, or translocations that occur in the chromosome.
  • Spontaneous mutations can be caused by errors during DNA replication and errors during meiosis leading to unequal crossovers or unequal separation of the chromosomes.
  • Induced mutations are cause by chemicals and radiation.
  • Many mutations are fatal but some mutations may actually benefit the organism by creating an altered gen product that may improve the survival of the organism.