Cell Biology and Protein Synthesis Vocabulary

Nucleus vs. Nucleolus:
- Nucleus: The large organelle that serves as the location of the DNA. It is the control center of the cell.
- Nucleolus: A specific structure within the nucleus. It is explicitly identified as the location of RNA synthesis (specifically $rRNA$ ). Approximately $20\%$ of the nucleus is dedicated to the nucleolus.
Organelle Identification:
- Smooth ER: The smooth endoplasmic reticulum does not have ribosomes attached to its surface.
- Ribosomes: Visualized as "dots" in cell diagrams; these are the sites of protein synthesis.
- Cell Membrane: The outer boundary of the cell.
- Cytoplasm: The jelly-like substance filling the cell where organelles are suspended.

Strand Count:
- DNA: Consists of $2$ strands (double helix).
- RNA: Consists of $1$ strand.
Nitrogenous Bases:
- DNA: Contains Adenine ( $A$ ), Guanine ( $G$ ), Cytosine ( $C$ ), and Thymine ( $T$ ).
- RNA: Contains Adenine ( $A$ ), Guanine ( $G$ ), Cytosine ( $C$ ), and Uracil ( $U$ ).
Pentose Sugars:
- DNA: Utilizes Deoxyribose sugar.
- RNA: Utilizes Ribose sugar.
Cellular Location:
- DNA: Primarily restricted to the nucleus.
- RNA: Found in the nucleus and "all over the rest of the place" (cytoplasm and ribosomes).

Case Study: Substance X vs. Substance Y:
- In a hypothetical experiment, a radioactive molecule is added to a cell culture. Analysis shows substance $Y$ is radioactive, but substance $X$ is not.
- Identifying the Substances: Substance $X$ is identified as DNA because it presents a double helix structure. Substance $Y$ is identified as RNA because it is single-stranded.
- Determining the Radioactive Additive: If only the RNA (Substance $Y$ ) became radioactive, the additive must be a component unique to RNA. The two possibilities are:
  1. Uracil: The nitrogenous base present in RNA but absent in DNA.
  2. Ribose: The sugar present in RNA but absent in DNA.

Messenger RNA (mRNA):
- Because DNA is too important and large to leave the nucleus, it creates a messenger.
- The $mRNA$ takes the genetic information (the "recipe") from the DNA and carries it to the ribosome in the cytoplasm for protein production.
Transfer RNA (tRNA):
- The "t" stands for transfer.
- This molecule is responsible for moving amino acids to the ribosome.
- The amino acid attaches to the top of the $tRNA$ molecule, which then brings it to the $mRNA$ strand at the ribosome to facilitate bonding with other amino acids.

Ribosomal RNA (rRNA):
- The ribosome itself is composed of two parts: a protein part and the $rRNA$ part.
- These components combine in the nucleus to produce the functional ribosome which then migrates to the Rough ER ( $RER$ ) or the cytoplasm.

Definition: The process of producing an $mRNA$ strand from a DNA template occurring within the nucleus.
Step 1: Unzip and Unwind:
- The DNA double helix is opened by the enzyme DNA Helicase.
- Helicase breaks the hydrogen bonds between the nitrogenous bases.
Step 2: Complementary Base Pairing:
- Free-floating nucleotides pair with the exposed DNA template.
- Instead of Thymine ( $T$ ), Uracil ( $U$ ) pairs with Adenine ( $A$ ).
- The enzyme responsible for this is RNA Polymerase (it is crucial to specify "RNA" polymerase to differentiate from DNA replication).
- In RNA, only one strand (side) of the DNA is copied.
Step 3: Release:
- The newly formed $mRNA$ strand is released.

Splicing (The Student Metaphor):
- Transcription is like a student frantically copying a friend's French homework at the last minute without reading it. The initial copy contains everything, including errors or unnecessary parts.
- Once the student realizes they have more time, they go back to "edit" the work. This editing is splicing.
Introns vs. Exons:
- Exons: The "good stuff" or coding regions that are needed for protein synthesis. These exit the nucleus.
- Introns: The junk or unnecessary sequences. These stay inside the nucleus and are recycled into free-floating nucleotides.
Protective Caps:
- Because the $mRNA$ is thin and thread-like, it is fragile. To protect it as it leaves the nucleus, a poly-A tail (a sequence of multiple Adenine bases) is added to one end, and a cap is added to the other to keep the strand together.
- Advanced biology also notes the presence of promoters where the polymerase binds.

Definition: The process of converting the $mRNA$ sequence into a protein (polypeptide), occurring at the ribosome.
Step 1: Initiation:
- The $mRNA$ leaves the nucleus and attaches to a ribosome.
- The ribosome consists of a small unit and a large unit that "clamp" the $mRNA$ .
- The ribosome hunts for the start codon ( $AUG$ ).
- The first $tRNA$ brings the first amino acid (Methionine) to the ribosome.
Step 2: Elongation:
- The polypeptide chain gets longer as the ribosome reads the $mRNA$ sequence.
- The ribosome has three sites: the A site (Arrival), the P site (Placement), and the E site (Exit).
- As each $tRNA$ drops its amino acid, a peptide bond is formed through a condensation reaction, and the empty $tRNA$ leaves to pick up another amino acid.
Step 3: Termination:
- The process continues until the ribosome reaches a stop codon.
- The completed primary structure (polypeptide) is released from the ribosome.

Codons: Groups of three nucleotides on the $mRNA$ strand. There are $64$ possible combinations ( $4^3 = 64$ ).
- Start Codon: $AUG$ (Codes for Methionine). Every sequence must start here.
- Stop Codons: $UGA, UAA, UAG$ . These signal the end of the protein. Most importantly, stop codons do not code for an amino acid. One should not write "stop" in an amino acid sequence.
Anticodons: Groups of three nucleotides on the tRNA that are complementary to the $mRNA$ codons. This ensures the correct amino acid is placed in the sequence.
Rule for Translation: When looking up amino acids on a genetic code chart, always use the mRNA codon, not the tRNA anticodon.

Question (Radioactivity): "Miss Lee, you never taught me about radioactivity. Is this question fair?"
- Response: Yes, it is fair because it tests the fundamental differences between DNA and RNA (Uracil and Ribose) rather than the physics of radioactivity itself.
Question (Tonicity and Concentration): A student asks about 100% water vs 40% solutions.
- Response: If a cell is in a solution with a higher concentration of solute (e.g., salt), it is in a hypertonic environment, leading to the shrinkage of the cell. If it is in a solution with a lower concentration (hypotonic), it may end up swelling. These processes are essential for maintaining homeostasis, such as regulating blood glucose with insulin.
Instructional Note: On the upcoming test, students will be provided with the amino acid chart. They should practice writing the first four letters of amino acids for the sequence.