Structure of DNA and RNA

DNA

all information for an organism

RNA

information needed at certain time

Protein

substance capable of
performing process

The genetic material for life must exhibit these characteristics

1. Information

2. Transmission

3. Replication

4. variation 

Information

Stores all information necessary to make an entire organism 

Transmission

Must be capable of passing information from parent to offspring

Replication

Must be copied

Variation

Must be capable of change ( Variation exist)

chromosomes

long, thread-like structures made of DNA and proteins that carry genetic information. They are found in the nucleus of most living cells.

Griffith studied

Streptococcus pneumoniae

groundbreaking experiment in 1928 that led to the discovery of the "transforming principle"—a key milestone in understanding that DNA is the genetic material.

Griffith experiment:

Griffith worked with two strains of Streptococcus pneumoniae bacteria:

Strain Type	Description	Effect on Mice
S strain (Smooth)	Had a protective capsule → Virulent (causes pneumonia)	🐭 Mice died
R strain (Rough)	No capsule → Non-virulent (harmless)	🐭 Mice lived

Griffith conclusion: 

• Something in the dead S bacteria transformed the live R bacteria into disease-causing S bacteria.

• He called this the "transforming principle."

Fractionated type S bacterial cells into the 4 major classes of macromolecules

(Avery, MacLeod and McCarty)

– Lipids
– Carbohydrates
– Proteins
– Nucleic acids – both DNA and RNA

Avery, MacLeod and McCarty Experiment:

1. Extracted substances (DNA, RNA, and proteins) from heat-killed S strain bacteria.

2. Treated each extract with specific enzymes to destroy one type of molecule at a time:

   • Protease → destroys proteins

   • RNase → destroys RNA

   • DNase → destroys DNA

3. Mixed each treated extract with live R strain bacteria and observed whether transformation occurred (i.e., whether the R bacteria turned into virulent S bacteria).

Avery, Macleod and McCarty concluded:

DNA carries genetic information, not proteins or RNA.

Hershey and Chase Experiment

provided final, definitive proof that DNA, not protein, is the genetic material passed from viruses to cells.

bacteriophage

virus that infects bacteria

Composed of only DNA and protein (radioactive labeling)

Hershey and Chase Experiment

step by step 

1. Label the Phages:

• One batch of phages had ³²P-labeled DNA.

• Another batch had ³⁵S-labeled protein coats.

2. Infect Bacteria:

• The labeled phages were allowed to infect E. coli bacteria.

3. Blender Step (Agitation):

• After infection, they used a blender to shake off the viral protein coats from the outside of the bacteria.

4. Centrifuge (Spinning):

• They spun the mixture to separate:

  • The bacteria (heavier, in the pellet at the bottom)

  • The phage coats (lighter, in the liquid

DNA contains

phosphorus

Proteins contain

sulfur

Hershey and Chase conclusion:

Label	Where Radioactivity Was Found	Conclusion

³²P (DNA)

Inside the bacteria (pellet)

DNA entered the cells

³⁵S (protein)

Outside the bacteria (liquid)

Protein did not enter

Mitochondria and chloroplast

perform genetic functions; DNA is present in these organelles

Nucleotides

building blocks of nucleic acid
Nucleic acids are formed by nucleotides

DNA and RNA are
types of

nucleic acids

Each nucleotide contains:

-Nitrogenous Base

-Pentose Sugar

-Phosphate group 

DNA contains

Deoxyribose sugar.
– “Deoxy” (without an oxygen)

RNA contains

ribose sugar

Two kinds of nitrogenous bases:

Purines (nine-member double ring)
– Pyrimidines (six-member single ring)

Purines (nine-member double ring):

• Adenine (A)
• Guanine (G)

Pyrimidines (six-member single ring)

• Cytosine (C)
• Thymine (T) in DNA only
• Uracil (U) in RNA only

Nucleotides are covalently linked
by

phosphodiester bonds

A phosphate connects

5′
carbon

nucleotide to the

3′ carbon

X-ray diffraction

When a purified substance, such as
DNA, is subjected to X-rays, it produces
a diffraction pattern that reveals the
regular structure of the molecule

Rosalind Franklin’s data suggested
several key structural features of DNA:

I. Helical structure
II. Too wide for a single-stranded helix
III. 10 base pairs per turn of the helix
Discovery of the Structure of DNA

The amount of adenine was always
similar to the amount of thymine

A=T

The amount of guanine was always
similar to the amount of cytosine

G=C

Two polynucleotide strands are held together
with

hydrogen bonding between the
nucleotide bases

A binds with T

2 hydrogen bonds

G binds with C

3 hydrogen bonds

sugar phosphate backbone

On outside

Hydrogen bonds

interaction between a hydrogen atom, covalently
bonded to another atom and a pair of non-bonded electrons on a separate
atom

The strands are

complementary

The 2 strands are directionally

antiparallel

Minor Groove

narrower of the two grooves that spiral around the DNA double helix.

Major Groove

wider and deeper groove formed by the twisting of the DNA double helix. It runs along the DNA molecule and is one of the two grooves created between the sugar-phosphate backbones

RNA is usually

single-stranded but
can be double stranded

rRNAs: ribosomal RNAs

Structural components of ribosomes for protein
synthesis during translation

mRNAs: messenger RNAs

Template for protein synthesis

tRNAs: transfer RNAs

Carry amino acids to ribosome for protein synthesis