DNA Structure and Chromosome Organization – Comprehensive Study Notes
How DNA (the Genetic Material) Was Discovered
Griffith’s Transformation Experiment (1928):
He studied two types of Streptococcus pneumoniae bacteria:
S strain: Looked smooth, had a protective sugar coat, and killed mice.
R strain: Looked rough, lacked a sugar coat, and did not harm mice.
When he mixed heat-killed S cells (which don't kill mice) with live R cells (which also don't kill mice), something surprising happened: the mice died.
Outcome: The live R cells had changed into deadly S cells. This meant a "transforming substance" from the dead S cells made the R cells lethal.
Main Idea: A factor could transfer a trait (like being deadly) from one type of cell to another, suggesting this factor was the genetic material.
Avery, MacLeod, and McCarty’s Experiment (1944): Identifying the Transforming Substance
They wanted to find out what this transforming substance was.
They took heat-killed S cells and broke them down, then added live R cells, just like Griffith did. But this time, they specifically removed different types of molecules:
If they removed proteins (using Protease), transformation still happened.
If they removed lipids or carbohydrates, transformation still happened.
If they removed RNA (using RNase), transformation still happened.
But, if they removed DNA (using DNase), NO transformation happened. The R cells stayed R cells.
Conclusion: DNA was the only molecule whose removal stopped transformation. This proved that DNA carries genetic information.
Second Experiment: They showed that only the DNA from heat-killed S cells could make R cells become deadly S cells.
Reinforcement: DNA is indeed the genetic material, carrying instructions that can be passed on.
Hershey–Chase Experiment: Phage Labeling
This experiment further supported DNA as the genetic material.
They used a virus called bacteriophage T2, which infects bacteria.
How they did it:
They labeled the DNA of the virus with a radioactive tag called (phosphorus, found in DNA).
They labeled the proteins of the virus with a radioactive tag called (sulfur, found in protein).
The Question: Which part of the virus (DNA or protein) actually enters the bacterial cell to give instructions?
Result: They found that the DNA label () was inside the infected bacterial cells, while the protein label () stayed outside.
Conclusion: DNA is the genetic material that carries the instructions for making new viruses, not protein.
The Basic Parts of DNA and RNA (Nucleic Acids)
Nucleic acids (like DNA and RNA) are made of smaller units called nucleotides.
Each nucleotide has three parts:
A phosphate group.
A sugar: deoxyribose in DNA (and ribose in RNA).
A nitrogenous base (A, T/U, G, C).
The sugar has numbered carbons (like 5' and 3') which are important for how the DNA stands are put together.
Nucleotides connect to each other through strong bonds called phosphodiester bonds, forming a long chain. This chain has a direction, always going from the 5' end to the 3' end.
In DNA, the bases face inward and pair up, holding two strands together like a zipper.
DNA Nucleotides: The Different Bases
There are four different nitrogenous bases in DNA:
Purines: These are larger, two-ring structures.
Adenine (A): Forms Deoxyadenosine monophosphate (dAMP)
Guanine (G): Forms Deoxyguanosine monophosphate (dGMP)
Pyrimidines: These are smaller, single-ring structures.
Cytosine (C): Forms Deoxycytidine monophosphate (dCMP)
Thymine (T): Forms Deoxythymidine monophosphate (dTMP)
In Simple Terms:
The phosphate group is attached to the 5' carbon of the sugar. The chain grows by adding new nucleotides to the 3' carbon.
A Single Strand of DNA and Its Direction
One end of a DNA strand has a 5'-phosphate group (the start).
The other end has a 3'-hydroxyl group (the end).
The phosphodiester bonds connect the 5' phosphate of one nucleotide to the 3' hydroxyl of the next, building the strand in a 5' to 3' direction.
In a double-stranded DNA molecule, the two strands run in opposite directions (antiparallel). If one strand goes 5' to 3' from left to right, the other goes 3' to 5' from left to right.
Rosalind Franklin and Her Key DNA Picture
Rosalind Franklin took important X-ray images of DNA crystals. These images were crucial because they showed that DNA has a helical (spiral) shape and gave clues about its measurements. Her work greatly helped Watson and Crick.
Watson and Crick’s DNA Model (around 1953)
They proposed the famous double helix structure of DNA:
It's like a twisted ladder, with two strands spiraling around each other (a right-handed double helix).
The two strands run in opposite directions (antiparallel).
The twisting creates bigger gaps (major grooves) and smaller gaps (minor grooves) along the helix.
The steps of the ladder are made by bases pairing up and holding the two strands together with hydrogen bonds.
This structure immediately suggested how DNA could make copies of itself.
Key Measurements:
Each "step" (base pair) is long.
One full turn of the helix is about , containing about 10 base pairs.
Big Idea: The specific way bases pair up allows DNA to be copied accurately.
Rules for Base Pairing and Chargaff’s Observations
The bases always pair in a specific way:
Adenine (A) always pairs with Thymine (T), forming two hydrogen bonds.
Guanine (G) always pairs with Cytosine (C), forming three hydrogen bonds.
Chargaff’s Rules: In any living thing's DNA, the amount of A is always equal to T, and the amount of G is always equal to C.
So, ext{%A} = ext{%T} and ext{%G} = ext{%C}.
This also means the total amount of purines (A+G) equals the total amount of pyrimidines (T+C).
Different Shapes of DNA Helices
The most common form of DNA, found in our cells, is a right-handed helix called B-DNA.
Under certain conditions, DNA can also form a left-handed helix called Z-DNA, often when cytosines have extra chemical tags (methylation).
How DNA is Organized in Eukaryotic Cells (Chromosomes)
Each eukaryotic chromosome contains one very long DNA molecule.
Chromatin: DNA is tightly packed with special proteins (histones and nonhistone proteins) to fit inside the cell's nucleus.
Banding patterns on chromosomes help scientists identify different regions.
Centromeres (the constricted middle part) and telomeres (the ends) are crucial for how chromosomes behave during cell division.
C-value paradox: The amount of DNA in an organism (genome size) doesn't always match how complex the organism is.
First Level of DNA Packaging: Nucleosomes (The Basic Bead)
DNA's first step of packing involves wrapping around proteins to form nucleosomes (like beads on a string).
The histone core is made of eight histone proteins (two copies each of H2A, H2B, H3, and H4).
The DNA wraps almost two times around this histone core, forming a nucleosome core particle.
Structure Details:
A nucleosome is about 11 nm wide and includes the DNA wrapped around the histone core plus a small piece of "linker" DNA connecting it to the next nucleosome.
Another histone protein, H1, acts like a clip, helping to hold the DNA onto the nucleosome core. This forms a chromatosome, which is slightly more compact (6 nm).
This first level of packing shortens the DNA by about 7 times.
Analogy: Imagine a string (DNA) wrapped around small beads (nucleosomes).
Second Level of DNA Packaging: The 30 nm Fiber (The Thicker Rope)
The "beads-on-a-string" nucleosomes then coil and fold together to form a thicker structure called the 30 nm fiber (like a solenoid or a zig-zag).
This step further compacts the DNA by another 6–7 times.
Analogy: This is like taking the string of beads and coiling it into a thicker rope.
Third Level of DNA Packaging: Radial Loop Scaffold and Rosette Model
The 30 nm fiber is organized further into even larger structures using a protein framework (a scaffold).
Specific DNA regions called scaffold-associated regions (SARs) attach the chromatin to these nonhistone scaffold proteins.
The chromatin then forms radial loops that stick out from this central scaffold, often organized into flower-like shapes called rosettes.
When a cell gets ready to divide (in metaphase), these radial loops pack even more tightly, forming very dense structures (around 240 nm).
Overall, DNA organization changes from looser forms during normal cell life (interphase) to extremely condensed forms during cell division.
Metaphase Chromosome: The Most Compact State
During metaphase, chromosomes become super condensed and easily visible. This tight packing is essential for them to be accurately separated into new cells.
Packaging Scale:
The chromatin bundles into a 240 nm fiber within the rosettes.
The final metaphase chromosome can be about 700 nm in diameter.
From its stretched-out form, DNA is compacted about 10,000 times to fit into a metaphase chromosome.
Chromatin Remodeling and Gene Control
For genes to be copied into RNA (transcription), the DNA needs to be accessible.
Chromatin remodeling is how the cell changes how tightly DNA is packed to control gene activity:
Histone acetylation: Adding chemical groups (acetyl groups) to histones makes the DNA loosen up, making it easier for the cell's machinery to read the genes.
Histone deacetylation: Removing these acetyl groups makes the DNA wind up more tightly, making genes harder to read.
This back-and-forth control of chromatin structure is vital for turning genes on and off.
Polytene Chromosomes and Chromosome Puffs
Polytene chromosomes are unusually large chromosomes found in certain cells (like insect salivary glands).
They often show swollen areas called chromosome puffs (also known as Balbiani rings).
These puffs are places where the DNA has unwound and is actively being transcribed (genes are being read).
They show that even in giant chromosomes, there are periods of intense gene activity.
Telomeres, Centromeres, and Chromosome Ends
Important parts of chromosomes include:
Telomeres: These are protective caps at the very ends of chromosomes. They safeguard the genetic information during DNA copying.
Centromeres: These are specific regions that are crucial for separating chromosomes correctly when cells divide.
Telomeres are rich in Guanine (G) residues, which can form special folded structures called G-tetraplexes.
Telomeres also have special proteins that bind to their ends to help maintain and regulate them.
Telomere End-Binding Proteins and Structure
Telomere End-Binding Proteins (TEBP) attach to the 3' end of telomeres, helping to keep chromosome ends stable.
Telomere Repeat-binding Factor 2 (TRF2) is another key protein that helps protect the telomere ends and allows them to form a special loop structure called a T-loop.
The T-loop structure is formed when the single-stranded 3' overhang of the telomere folds back and invades the double-stranded DNA, creating a protective loop. This loop, along with specific proteins like TRF2, stops the cell from seeing the chromosome end as damaged DNA.
The specific repeating G-rich sequences and the single-stranded overhang at telomeres are what allow these protective structures to form, ensuring that our genetic material stays safe and complete.
In summary: Telomeres and centromeres are critical for making sure chromosomes stay stable, divide properly, and don't "age" too quickly. Their structure and maintenance involve special protein teams that protect our genetic information every time a cell divides.
Note: Throughout these notes, special symbols (like for Angstrom, which measures tiny distances, and numbers for chemical elements like and ) are used. Measurements like per base pair and per helical turn describe the size and twist of DNA. Hydrogen bond counts for A–T (2) and G–C (3) show how tightly bases stick together. Concepts like the Beads-on-a-String model, the 30 nm fiber, radial loops, and rosettes are used to explain how DNA is progressively packed to fit inside a cell, from a loose string to a tightly folded chromosome.