【PEP】High School Biology · Compulsory Vol. 2: The Messenger of Life

If we compare a cell to a highly automated modern factory,DNA is the "master blueprint" locked in the CEO's office (the nucleus) safe. To protect the original while directing production, the secretary makes multiple copies of the blueprint into "work orders" — that is, mRNA.

1. Structural Foundations: The Chemical Contrast Between RNA and DNA

The basic unit of RNA is theribonucleotide. Its subtle chemical differences from DNA determine its entirely distinct biological fate:

Change in Pentose Sugar: RNA contains ribose, which has a hydroxyl group (-OH) at the 2' position, while DNA contains deoxyribose. This makes RNA chemically more reactive.
Base Substitution: RNA uses uracil (U) in place of DNA's thymine (T), pairing with adenine (A).
Physical Form: Due to its chemical reactivity, RNA typically exists as a single-stranded molecule with a smaller molecular weight, allowing it to freely pass through nuclear pores.

2. Division of Labor: The Three Major Types of RNA

Three distinct types of RNA work together within the cell to carry out the transformation from instruction to product:

Messenger RNA (mRNA): This RNA acts as DNA's messenger and is responsible for carrying genetic instructions.
Transfer RNA (tRNA): Shaped like a cloverleaf, it is responsible for transporting specific amino acids according to the instructions.
Ribosomal RNA (rRNA): It combines with proteins to form the ribosome — the "workshop" where proteins are made.

QUESTION 1

Which of the following accurately describes a common feature of transcription and DNA replication?

Both require DNA polymerase

Both use a DNA strand as a template and follow the base-pairing rules

Both produce double-stranded nucleic acid molecules

Both use four types of deoxyribonucleotides as raw materials

QUESTION 2

What is the main physical reason RNA molecules can pass through nuclear pores into the cytoplasm, while DNA cannot?

RNA contains uracil (U)

RNA is usually single-stranded and has a relatively small molecular weight

RNA contains ribose

QUESTION 3

During protein synthesis, which molecule acts as the "transporter" that carries specific amino acids to the ribosome?

mRNA

tRNA

rRNA

QUESTION 4

Which of the following is NOT a difference in chemical composition between RNA and DNA?

Type of phosphate group

Type of pentose sugar

Type of bases (T vs. U)

QUESTION 5

During transcription, which base on the RNA pairs with adenine (A) on the DNA template strand?

Deep Dive: The Evolutionary Logic and Chemical Instability of RNA

Read the passage and analyze the role of RNA in regulating life processes

As a temporary carrier of genetic information, the discovery of RNA broke the old notion that "DNA directly directs protein synthesis." Compared to DNA, which serves as a long-term storage medium, RNA is like a self-destructing instruction sheet. From a chemical standpoint, the hydroxyl group (-OH) at the 2' position of ribose gives RNA a strong chemical reactivity, making it highly susceptible to hydrolysis and far less stable than DNA. Yet, this very "instability" is a masterstroke of evolution, allowing cells to rapidly turn specific genes on or off in response to environmental signals.

1. [Reading Comprehension] Why is RNA described as a "self-destructing instruction sheet"? What significance does this have for cellular life activities?

Answer:
As a temporary carrier of genetic information, RNA's instability allows mRNA to be rapidly degraded after completing its translation instructions. The significance is that this enables cells to dynamically refresh gene expression, ensuring that life activities can quickly respond to environmental changes and preventing excessive synthesis of specific proteins due to an accumulation of instructions.

2. [In-depth Analysis] What are the dual effects of the 2'-OH group in ribose on the biological functions of RNA?

Answer:
Dual effects: On one hand, it reduces RNA's stability, making it easy to degrade to meet regulatory needs. On the other hand, its reactive chemical nature allows RNA to fold into complex spatial structures and even exhibit catalytic functions (such as ribozymes), supporting the hypothesis that an "RNA World" may have existed in the early stages of life's origin.

3. [Synthesis & Application] What do transcription and DNA replication have in common? How does this help ensure the accurate transmission of genetic information?

Answer:
Common features: ① Both processes require a template (the DNA strand); ② Both require energy (ATP); ③ Both follow the base-pairing rules. Significance: The base-pairing rules ensure that genetic information is accurately transferred from DNA to RNA, maintaining the stability of life processes.