DNA vs. RNA: Structure and Function

DNA vs. RNA

• DNA is a double helix composed of nucleotides with a deoxyribose sugar, phosphate group, and nitrogenous bases (Adenine, Thymine, Cytosine, Guanine).
• DNA resides in the nucleus as it is too large to leave.
• DNA codes for genes which make proteins.
• RNA acts as a messenger between DNA and ribosomes to build proteins from amino acids.
• RNA differs from DNA in its sugar (ribose vs. deoxyribose), number of strands (single vs. double), location (inside and outside the nucleus vs. only inside), and base pairing (uracil instead of thymine).

Types of RNA

• mRNA (messenger RNA): Copies the code from DNA in the nucleus and takes it to the ribosome.
• tRNA (transfer RNA): Transfers specific amino acids to the ribosome to match with mRNA and build a protein.
• rRNA (ribosomal RNA): Forms ribosomes along with proteins.

Gene Expression

• Gene expression refers to the active production of a protein from a gene.
• Not all genes are expressed simultaneously.
• Some genes are rarely expressed (e.g., adrenaline), some are constantly expressed (e.g., hair growth), and some are cyclically expressed (e.g., estrogen).
• Gene expression (protein synthesis) is essential for cell differentiation and function.

Protein Synthesis

• Protein synthesis is a two-stage process for producing proteins from DNA.
• Also known as gene expression or the central dogma.
• The two steps include transcription and translation.

Transcription

• Transfers information from DNA to mRNA.
• Necessary because DNA is too large to leave the nucleus, while RNA can.
• RNA polymerase binds to DNA, unzips it, and uses one strand as a template to create a single-stranded mRNA.

Regulation in Transcription

• mRNA is processed/edited before leaving the nucleus to remove non-coding regions (introns).
• Exons (coding regions) are spliced together to form the final mRNA that leaves the nucleus.

Translation

• Decodes mRNA into a protein.
• tRNA transfers specific amino acids to the ribosome based on mRNA codons.
• Amino acids are joined together to form a protein.

Codons

• A series of three mRNA bases that code for a specific amino acid.
• Start codon: AUG (codes for methionine)
• Stop codons: UAG, UAA, UGA

Role of tRNA in Translation

• tRNA has an anticodon that is complementary to the codon in mRNA.
• tRNA delivers the matching amino acid to the ribosome.

Steps of Translation

• Initiation: Ribosome assembles around mRNA, with the first tRNA attaching to the start codon.
• Elongation: tRNAs transfer amino acids to the tRNA of the next codon.
• Termination: When the stop codon is reached, the ribosome releases the polypeptide.

Gene Mutations

• Errors during DNA replication that change a single gene.
• Rare occurrence (1 in 100,000,000 bases).
• Can have a major effect on the protein being made.

Point Mutations

• Occur at a single point (change to one base).
• Types include substitution, deletion, and insertion.

Substitution Mutations

• One base is changed for another.
• Can be silent, missense, or nonsense.

Silent Mutations

• Do not affect the protein being made.

Missense Mutations

• Affect one amino acid in the protein chain, with varying effects on the protein's function.

Nonsense Mutations

• Cause an early stop codon, terminating the polypeptide chain sequence prematurely.
• Severely affect the protein being made.

Deletion/Insertion Mutations

• Base removed or inserted from the DNA sequence.
• Cause a frameshift mutation.

Frameshift Mutations

• Shift the reading frame of the genetic message.
• Can change every amino acid following the mutation point.
• May render the protein non-functional.

Causes of Gene Mutations

• Spontaneous occurrence during cell replication.
• Environmental agents (mutagenic agents): UV light, radiation.
• Cancer-causing agents (carcinogens): Chemicals (cigarette smoke), viruses (HPV).

This quiz explores the differences between DNA and RNA, including their structure, function, and types. Learn about the roles each molecule plays in gene expression and protein synthesis. Ideal for students studying molecular biology.