Chapter 15 - DNA, RNA, and DNA Replication

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• It's worth noting that the characteristic can be handed on from mother to son or mother to daughter.

  • Males that have the characteristic do not pass it on to their children.
  • It is critical to distinguish between nonnuclear maternal inheritance and sex-linked inheritance.
  • Sex-linked genes are situated on a sex chromosome (typically the X chromosome) in the nucleus and can be inherited from either dads or mothers in sex-linked inheritance.

• Nonnuclear inheritance involves genes that are found in the mitochondria or chloroplasts and can only be transmitted from the mother.

  • The environment can influence gene expression and, as a result, an organism's phenotype.
  • For example, in the blossoms of the hydrangea plant, a basic soil pH results in pink flowers, but an acidic soil pH results in blue flowers.
  • A basic soil pH, for example, leads in pink hydrangea blossoms, whereas an acidic pH results in blue hydrangea flowers.
  • UV radiation can induce the expression of genes involved in the formation of melanin in humans.

• Phenotypic plasticity refers to the ability of the same genotypes to create distinct phenotypes in response to diverse environmental variables.

  • In living creatures, nucleic acids, such as DNA and RNA, are the bearers of genetic information.
  • %%These nucleic acids are responsible for the transmission of genetic information from one generation to the next.%%
  • While DNA is normally used as the transporter of genetic information between generations in prokaryotes and eukaryotes, certain viruses employ RNA as their major genetic material.

• The five-carbon sugar deoxyribose is found in DNA, while the five-carbon sugar ribose is found in RNA.

  • At the 2′ carbon, there is a significant variation between these sugars.
  • The 2′ carbon in deoxyribose is bonded by a hydrogen atom, whereas the 2′ carbon in ribose is attached by a hydroxyl (–OH) group. Because of this little distinction, DNA is far more stable than RNA.
  • This enhanced durability of DNA may explain why it is the bearer of genetic information between generations, whereas RNA is typically used for more transient purposes.
  • ^^In transcription, for example, mRNA transports genetic information from the nucleus to the ribosome.^^

• DNA is a double helix with two antiparallel strands.

  • One strand of DNA is orientated with the 5′ phosphate group at the beginning.
  • One strand of DNA has the 5′ phosphate group at the beginning of the strand, whereas the opposite strand has the 3′ hydroxyl (–OH) group at the beginning of the strand.
  • A purine on one strand is always followed by a pyrimidine on the opposing strand.
  • Purines have a double-ringed structure, but pyrimidines have a single-ringed structure, hence the breadth of the double helix remains constant.

• RNA is normally single-stranded (in mRNAs and microRNAs), but it may fold to form three-dimensional structures in ribosomal rRNAs and tRNAs.

  • ^^Topoisomerase enzymes create transient nicks in the sugar-phosphate backbone of DNA to alleviate supercoiling and subsequently seal these nicks.^^
  • The enzyme RNA polymerase then uses this information to create an RNA primer.
  • The enzyme RNA polymerase then uses a few complementary RNA nucleotides to create an RNA primer.
  • This RNA primer can then be supplemented with new DNA nucleotides.
  • The enzyme DNA polymerase is responsible for adding additional nucleotides to the 3′ hydroxyl group at the end of this RNA primer.

• DNA polymerase adds new nucleotides in the 5′ to 3′ orientation, always attaching the new nucleotide's 5′ phosphate to the 3′ hydroxyl on the developing nucleotide strand.

  • DNA polymerase reads the original strand in the 3′ to 5′ direction on one strand of the double helix and can add new nucleotides in the 5′ to 3′ direction constantly.
  • This is known as leading strand replication.
  • The second strand of the double helix, on the other hand, is orientated in the 5′ to 3′ orientation, making reproduction on this strand more difficult.
  • In order to read the strand from 3′ to 5′, DNA polymerase must move in a reverse way on this strand.
  • This strand undergoes discontinuous replication, resulting in small pieces known as lagging strand fragments (also known as Okazaki fragments), which are eventually linked together by the enzyme ligase.
  • This is referred to as lagging strand replication.

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