Chapter 3
Explain how the sequence of a molecule of DNA, made up of many monomers of only four possible nucleotides, can encode the enormous amount of genetic information stored in the chromosomes of living organisms.
Just four nucleotides can give rise to the vast diversity of genetic information because the nucleotides can be in any order. Any base on a strand of DNA can be followed by any other base (or the same base), which gives rise to an enormous potential genetic diversity of any given gene.
Describe the usual flow of genetic information in a cell.
The usual flow of genetic information in a cell is from DNA to RNA and finally to protein. This tenet is known as the central dogma of molecular biology.
Explain how the functions of DNA emerge from the structure of its monomers and its antiparallel, double-helical, three-dimensional structure.
DNA has two major functions: storing information and transmitting that information from one generation to the next. DNA stores an enormous amount of genetic information due to its lack of restriction on the sequence of bases in the DNA molecule. Since there is no restriction on how the nucleotides can be ordered, the possibilities of gene‐encoded proteins are virtually endless. DNA molecules can also be of great length due to the fact that they form special shapes, supercoils in prokaryotic cells and chromatin in eukaryotic cells, which allow them to fit inside the cells. Without these structures, a cell would not be able to contain the amount of DNA necessary to maintain proper function. DNA is able to transmit information from one generation to the next due to specific hydrogen bonding between bases. This complementary pairing of one purine with one pyrimidine (Guanine to Cytosine, or Adenine to Thymine) is responsible for a key structural element of DNA―that two complementary strands of DNA interact to form a double helix. This complementary structure allows each strand of DNA to be the template for another strand of DNA or RNA. This ensures that the sequence of genes, and thus the function of the proteins, remain the same as cells grow and divide.
Describe how DNA molecules are replicated.
During DNA replication, the two strands of the double helix unwind and separate, disrupting the hydrogen bonds holding the strands together. Each parental strand serves as a template for a new complementary strand of DNA because of the specific pairing of the bases—an A on the template will always specify a T on the new strand, a G on the template will always specify a C on the new strand, and so on. The end result of DNA replication is two DNA molecules that have identical sequence to the original molecule.
List three types of noncoding RNA and describe their functions.
Five types of noncoding RNA are: (1) ribosomal RNAs (rRNA) that are found in all ribosomes and aid in translation; (2) transfer RNAs (tRNA) that carry individual amino acids for use in translation; (3) small nuclear RNAs (snRNA), which are involved in eukaryotic gene splicing, polyadenylation, and other processes in the nucleus; (4) microRNAs (miRNA) that inhibit translation; and (5) small interfering RNAs (siRNA) that destroy RNA transcripts.
Explain the relationship between RNA structure and function.
RNA exists in various forms (e.g., mRNA, tRNA, etc.) and its structure helps define its function. The first level of RNA structure is its base sequence, which is specified by the base sequence in the DNA template strand because of base pairing between A, T, G, and C in DNA with U, A, C, and G in RNA, respectively. Single‐stranded RNA molecules often form complex and varied three‐dimensional structures that can enhance their stability. Also, because the ribose sugar in RNA contains the 2' hydroxyl group that DNA lacks, RNA is more reactive than DNA and can have catalytic functions. The varied three-dimensional structures of RNA also contribute to the catalytic ability of RNA.
Name four differences between the structure of DNA and RN
Structural differences between DNA and RNA include: The sugar in RNA is ribose, while the sugar in DNA is deoxyribose. The base thymine in DNA is replaced by uracil in RNA. DNA molecules are double-stranded and often very long, while RNA molecules are single-stranded and also usually much shorter than DNA molecules.
Describe how a molecule of RNA is synthesized using a DNA molecule as a template.
The process by which a molecule of RNA is synthesized using a DNA template is called transcription. The DNA molecule first unwinds into its two strands. One of these strands is used as a template for the synthesis of a strand of RNA. The resulting RNA transcript is complementary in sequence to the DNA strand template (with the exception of uracil replacing thymine in the RNA). The polymerization is carried out by the enzyme RNA polymerase, which binds to the DNA template (usually at a promoter sequence) and transcribes the DNA nucleotides into RNA nucleotides. RNA polymerase adds nucleotides to the growing RNA strand in the 5' to 3' direction, and thus moves along the template strand in the 3' to 5' direction. For a full picture of the eukaryotic transcription complex,
Name and describe three mechanisms of RNA processing in eukaryotes, and explain their importance to the cell.
Three mechanisms of RNA processing (chemical modification of the primary transcript to generate the finished mRNA) in eukaryotic cells are: (1) Addition of the 5' cap. This modified nucleotide (7‐methylguanosine) allows the mRNA to be recognized by the ribosome complex. It also helps stabilize the mRNA. (2) Addition of the poly(A) tail. This is important in mRNA transcription termination as well as in the export of the mRNA into the cytoplasm. Like the 5ꞌ cap, it also helps stabilize the mRNA. (3) Excision of introns. The process of intron removal is called RNA splicing. A single transcript with multiple introns may be spliced in different ways to generate different mRNAs and different protein products with different functions. Thus, this alternative splicing is one more layer contributing to the diversity of the genetic information stored in DNA.