Molecular Radiobiology: Macromolecule Damage, DNA Replication & Protein Synthesis

What is molecular radiobiology?

The study of biochemical reactions that begin with radiation interactions at the atomic level and produce observable damage through molecular changes.

At what level does radiation initially interact with tissue?

At the atomic level.

At what level does observable damage from molecular changes occur?

At the molecular level.

What does in vitro mean?

Outside a living organism or outside the body.

What does in vivo mean?

Within a living organism or within the body.

According to the lecture, are molecules more radiosensitive in vitro or in vivo?

More radiosensitive in vivo; less radiation may produce the same damage inside an organism than outside the body.

What two factors were discussed as relevant to the difference in radiosensitivity in vitro versus in vivo?

Water and oxygen.

What are the three major effects of irradiation on macromolecules?

Main-chain scission, cross-linking, and point lesions.

What is a macromolecule?

A large molecule made of smaller molecular units; examples discussed include DNA and proteins.

What is main-chain scission?

Breakage of the backbone of a long-chain macromolecule, creating smaller molecules.

Main-chain scission

What happens to solution viscosity after main-chain scission?

It decreases because the long molecules are broken into smaller molecules.

How can main-chain scission be measured?

By measuring the decrease in solution viscosity.

What is cross-linking?

Radiation-induced spur-like structures form on a macromolecule and attach to another part of the same molecule or to a nearby molecule.

What happens to solution viscosity after cross-linking?

It increases.

Why does cross-linking increase viscosity?

Macromolecules become attached to themselves or neighboring molecules, behaving as though coated with a sticky substance.

Cross-linking

What is a point lesion?

Disruption of a single chemical bond.

Do point lesions measurably change solution viscosity?

No. Point lesions do not measurably change viscosity.

What can point lesions do to a molecule?

Modify the molecule, cause malfunction, or contribute to cell damage associated with stochastic effects.

Stochastic effect

A effect that has an increased probability of occurrence as a function of dose received without a threshold (generally linear)

Deterministic effect

have a clear dose threshold, do not occur below that threshold, and grow more severe as the dose increases (such as skin burns, acute radiation syndrome, or cataracts)

Point lesion

Which macromolecular effects may allow repair and recovery?

Main-chain scission, cross-linking, and point lesions may all be repairable according to the lecture.

What does the lecture say intracellular repair and recovery need?

Time.

What is anabolism?

Construction of macromolecules from smaller molecules; it requires energy and is critical to cell survival.

What is catabolism?

Breakdown of molecules, which releases energy.

Which process builds larger molecules: anabolism or catabolism?

Anabolism.

Which process breaks down molecules and releases energy?

Catabolism.

Do anabolism and catabolism occur continuously?

Yes. Both occur continuously.

What cellular energy molecule is associated with mitochondria and supports cellular processes?

ATP (adenosine triphosphate).

Why is protein synthesis important to cells?

Proteins are essential for cellular function and repair.

Which nucleic acids are particularly important to protein synthesis?

Messenger RNA (mRNA) and transfer RNA (tRNA).

What is transcription?

The process in which DNA serves as a template to make an mRNA copy.

What are the steps of protein synthesis?

1. DNA unwinds, exposing the gene responsible for the protein. 2. RNA polymerase initiates synthesis of RNA; occurs in nucleus. 3. mRNA leaves nucleus and enters cytoplasm. 4. Ribosome reads mRNA message 3 nitrogen bases(codons) at a time. 5. anticodons on tRNA match up w/ codons on mRNA trand, then amino acids come into close contact w/ one another. 6. amino acids join together through dehydration synthesis to form protein.

What happens to mRNA after transcription?

It leaves the nucleus and attaches to a ribosome.

What is translation?

The process in which a ribosome reads the mRNA sequence and uses it to assemble a polypeptide.

What is a codon?

A sequence of three bases on mRNA that corresponds to an amino acid.

How many bases make up one codon?

Three bases.

What is the role of tRNA in protein synthesis?

tRNA brings the corresponding amino acids to the ribosome.

What happens to amino acids during translation?

They are joined together to form a polypeptide chain.

What happens to a polypeptide chain after it is produced?

It folds into the shape needed to perform its function.

Why are proteins considered less radiosensitive than DNA?

Protein production is continuous, and damaged proteins can often be replaced by making new ones.

During which cell-cycle phase does the cell grow and accumulate materials needed to make DNA?

G₁ phase.

What materials accumulate during G₁ to form nucleotides?

Deoxyribose, phosphate, and bases.

During which phase is DNA copied?

S phase of interphase.

What happens first when DNA is copied?

The two DNA strands separate.

What happens after the DNA strands separate during replication?

Complementary nucleotides pair with the exposed bases on each strand.

What are the DNA base-pairing rules?

A pairs with T, and C pairs with G.

What is the result of DNA replication?

Two identical DNA molecules.

Which enzyme was identified in the lecture's explanation of DNA unzipping and replication?

DNA polymerase.

What does DNA polymerase do in the lecture's explanation?

It was identified with the process of unzipping DNA and breaking hydrogen bonds; it is also involved in DNA synthesis.

Why is DNA considered the most radiosensitive and important macromolecule?

It is relatively scarce and has a unique structure; damage can alter genetic information.

Why is DNA an especially important target molecule?

It contains genetic information for each cell, controls growth and development, and determines characteristics.

Why can damage to DNA be more consequential than damage to a protein?

A damaged protein can often be replaced, but DNA damage can alter genetic instructions and may be passed to daughter cells.

What percentage of the body was DNA said to make up in the lecture?

Less than 1%.

What are the DNA damage effects described in the lecture?

Main-chain scission, cross-linking, rung breakage, and point mutation; main-chain scission may affect one or both side rails.

What is DNA cross-linking?

Abnormal joining of DNA strands or sections of DNA.

What is rung breakage in DNA?

A break that separates the bases across the DNA ladder.

What is a point mutation?

A change or loss of a DNA base that alters genetic information.

What can a changed or missing DNA base cause during copying?

An abnormal codon may be produced.

Can a point mutation be passed to a daughter cell?

Yes. Altered genetic information may be passed to a daughter cell.

How did the lecture distinguish point mutations from the other DNA damage discussed?

It described point mutations as not reversible.

What are possible consequences of radiation damage to DNA?

Chromosome aberrations, cytogenetic damage, abnormal metabolic activity, uncontrolled cell proliferation, tumors or cancer, cell death, and genetic damage in germ cells that may affect offspring.

What is a chromosome aberration?

An abnormality or structural change involving a chromosome.

What is cytogenetic damage?

Damage involving chromosomes or genetic material that can be evaluated at the cellular/chromosomal level.

What can uncontrolled cell proliferation potentially result in?

A tumor or cancer.

What can happen if radiation damages germ cells?

Genetic damage may occur, with effects potentially appearing in offspring.

What is radiolysis of water?

The dissociation or breakdown of water after it is exposed to ionizing radiation.

Why is water the principal radiation interaction in the body?

The body is largely an aqueous solution, so radiation is more likely to interact with abundant water than with relatively scarce DNA.

What approximate percentage of the body was described as water in the lecture?

Approximately 80%; the exact estimate depends on the source.

What happens when radiation ionizes a water molecule?

An electron is ejected, leaving a positively charged water molecule (a cation).

What is a cation?

A positively charged ion.

What is the initial radiolysis reaction for water?

H₂O + radiation → HOH⁺ + e⁻.

What are the possible outcomes after water is ionized?

The ejected electron may rejoin the cation, attach to another water molecule, or the unstable ions may dissociate into smaller products including free radicals.

What happens if the ejected electron rejoins the original water cation?

The water molecule is restored and no damage occurs from that interaction.

What is the rejoining reaction in water radiolysis?

HOH⁺ + e⁻ → H₂O.

What happens if the ejected electron attaches to another water molecule?

A negatively charged water ion is formed: H₂O + e⁻ → HOH⁻.

Why can ionized water molecules dissociate?

They are unstable.

What are the dissociation reactions shown for ionized water?

HOH⁺ → H⁺ + OH•; and HOH⁻ → OH⁻ + H•.

What is a free radical?

An uncharged molecule or atom with a single unpaired electron in its outer shell.

Are free radicals charged or uncharged?

Uncharged.

Why are free radicals highly reactive?

They contain an unpaired outer-shell electron and excess energy, making them unstable and reactive.

How long was the lifetime of a free radical given in the lecture?

Less than 1 millisecond.

What can free radicals do while they exist?

Diffuse through the cell and transfer excess energy to other molecules, disrupting bonds and producing point lesions.

How does radiation affect free-radical production?

Radiation increases free-radical production.

Approximately how many free-radical interactions per cell per day did the lecture attribute to normal bodily processes?

About 10,000 interactions daily.

What happens when two hydroxyl radicals combine?

They form hydrogen peroxide: 2 OH + 2 OH → H₂O₂.

Why is hydrogen peroxide harmful to the cell?

The lecture describes hydrogen peroxide as poisonous to the cell.

What forms when a hydrogen radical combines with oxygen?

A hydroperoxyl radical: H• + O₂ → HO₂•.

What can hydroperoxyl radicals form when they combine?

Hydrogen peroxide and oxygen: 2 HO₂• → H₂O₂ + O₂.

What types of organic molecules can radiation interact with?

Proteins, lipids, and carbohydrates.

What approximate percentage of the body did the lecture estimate is protein?

About 15%.

What approximate percentage of the body did the lecture estimate is lipid?

About 2%.

What can ionization of an organic molecule produce?

An organic free radical and a hydrogen free radical.

What is the general reaction for radiation interacting with an organic molecule?

RH + radiation = e- + RH → R + H*

What can happen when an organic radical reacts with oxygen?

It can form a peroxyl radical: R• + O₂ → RO₂•.

How can an organic peroxyl radical contribute to cell damage?

It is a reactive product that can contribute to cellular damage.

What is the direct effect of radiation?

The initial ionization occurs on the target molecule itself, less likely

Why are direct hits on DNA considered less likely?

DNA makes up less than 1% of the body according to the lecture.