CHAPTER SUMMARY
3.1 DEOXYRIBONUCLEIC ACID (DNA) STORES AND TRANSMITS GENETIC INFORMATION.
- Experiments carried out by Griffith in 1928 demonstrated that bacteria can transmit genetic information from one strain to another.
- Experiments performed by Avery, MacLeod, and McCarty in 1944 showed that DNA is the molecule that transmits genetic information.
- DNA is copied in the process of replication.
- Ribonucleic acid (RNA) is synthesized from a DNA template.
- The central dogma of molecular biology states that the usual flow of genetic information is from DNA to RNA to protein. DNA is transcribed to RNA, and RNA is translated to protein.
3.2 DNA IS A POLYMER OF NUCLEOTIDES AND FORMS A DOUBLE HELIX.
- A nucleotide consists of a 5-carbon sugar, a phosphate group, and a base.
- The four bases of DNA are adenine, guanine, cytosine, and thymine.
- Successive nucleotides are linked by phosphodiester bonds to form a linear DNA molecule.
- DNA strands have polarity, with a 5′-phosphate group at one end and a 3′-hydroxyl group at the other end.
- Cellular DNA molecules consist of a helical spiral of two paired, antiparallel strands called a double helix.
- In a DNA double helix, A pairs with T, and G pairs with C.
- The structure of DNA relates to its function. Information is coded in the sequence of bases, and the structure suggests a mechanism for replication, in which each parental strand serves as a template for a daughter strand.
- Cellular DNA is packaged with evolutionary conserved proteins called histones. Histones form a nucleosome core particle around which DNA is wrapped to form a 10-nm and a 30-nm chromatin fiber.
3.3 TRANSCRIPTION IS THE PROCESS BY WHICH RNA IS SYNTHESIZED FROM A DNA TEMPLATE.
- RNA, like DNA, is a polymer of nucleotides linked by phosphodiester bonds.
- Some types of RNA can store genetic information and other types can catalyze chemical reactions. These characteristics have led to the RNA world hypothesis, the idea that RNA played a critical role in the early evolution of life on Earth.
- Unlike DNA, RNA incorporates the sugar ribose instead of deoxyribose, and the base uracil instead of thymine.
- RNA is synthesized from one of the two strands of DNA, called the template strand.
- RNA is synthesized by RNA polymerase in a 5′-to-3′ direction, starting at a promoter and ending at a terminator in the DNA template.
- RNA polymerase separates the two DNA strands, allows an RNA–DNA duplex to form, elongates the transcript, releases the transcript, and restores the DNA duplex.
3.4 THE PRIMARY TRANSCRIPT IS PROCESSED TO BECOME MESSENGER RNA (mRNA).
- In prokaryotes, the primary transcript is immediately translated into protein.
- In eukaryotes, transcription and translation are separated in time and space, with transcription first occurring in the nucleus and then translation in the cytoplasm.
- In eukaryotes, there are three major types of modification to the primary transcript—the addition of a 5′ cap, polyadenylation, and splicing.
- The 5′ cap is a 7-methylguanosine added to the 5′ end of the transcript.
- The poly(A) tail is a stretch of adenines added to the 3′ end of the transcript.
- Splicing is the excision of introns from the transcript, bringing exons together.
- Alternative splicing is a process in which primary transcripts from the same gene are spliced in different ways to yield different protein products.
- Some RNAs, called noncoding RNAs, do not code for proteins, but instead have functions of their own.
Self-Assessment Question 1
Explain how the function of DNA is attributable to and dependent on its structural features.
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Model Answer:
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 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 also incredibly stable 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 another key structural element of DNA—that two complementary strands of DNA interact to form a double helix. These attributes play a major role in the faithful replication of the DNA sequence. This ensures that the sequence of genes, and thus the function of the proteins, remains the same as cells grow and divide. Of course there are some occasions where a mistake is made in the DNA copying process, called a mutation, that can alter the function of the protein. In the majority of cases, this altered function is detrimental to the organism, but in a rare number of cases this mutation can actually be beneficial.
Self-Assessment Question 2
Describe how DNA molecules are replicated.
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Model Answer:
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. The order of bases in the new strand is determined by the base sequence in the parental strand. The end result of DNA replication is two DNA molecules that have identical sequence to the original molecule.
Self-Assessment Question 3
Explain how the sequence of just four nucleotide monomers found in DNA can encode the enormous amount of genetic information stored in the chromosomes of living organisms.
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Model Answer:
Just four nucleotides can give rise to the vast diversity of genetic information because the nucleotides can be in any order at each position in the sequence. This gives rise to an enormous potential genetic diversity of any given gene.
Self-Assessment Question 4
Draw a nucleosome, indicating the positions of DNA and proteins.
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Model Answer:
See also Figure 3.13.
Self-Assessment Question 5
Describe the usual flow of genetic information in a cell.
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Model Answer:
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.
Self-Assessment Question 6
Name two differences between the structure of DNA and RNA.
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Model Answer:
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.
Self-Assessment Question 7
Describe how a molecule of RNA is synthesized using a DNA molecule as a template.
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Model Answer:
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 that 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 please see Figure 3.17 and Figure 3.18.
Self-Assessment Question 8
Explain the relationship between RNA structure and function.
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Model Answer:
RNA exists in various forms (e.g., mRNA, tRNA, etc.) and its structure helps define its function. Short, single-stranded RNA molecules often form complex and varied three-dimensional structures that can enhance their stability. 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.
Self-Assessment Question 9
Name and describe three mechanisms of RNA processing in eukaryotes, and explain their importance to the cell.
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Model Answer:
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 5’ 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.
Self-Assessment Question 10
List three types of noncoding RNA and describe their functions.
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Model Answer:
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.