DNA & RNA Structure and Function: A Comprehensive Guide

DNA and RNA: The Nucleic Acids

What are DNA and RNA?

DNA and RNA belong to the group of molecules called nucleic acids.

Functions of DNA and RNA

DNA

• Stores genetic information
• Provides instructions for protein synthesis
• Passes genetic code to daughter cells

RNA

Carries out instructions from DNA for protein synthesis.

Types of RNA and Their Functions

• mRNA (messenger RNA): Makes a copy of DNA and carries this copy from the nucleus to the cytoplasm for protein synthesis.
• tRNA (transfer RNA): Transfers amino acids to ribosomes for protein synthesis.
• rRNA (ribosomal RNA): Controls ribosome formation in the cytoplasm.

Structure of Nucleic Acids

Monomers and Components

The monomer that makes up nucleic acids is called a nucleotide. Each nucleotide consists of:

• Phosphate group
• Sugar
• Nitrogenous base

Nitrogenous Bases

• DNA: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
• RNA: Adenine (A), Guanine (G), Cytosine (C), Uracil (U)

Complementary Base Pairing

Nitrogenous bases join with hydrogen bonds. In DNA, A pairs with T, and G pairs with C. In RNA, A pairs with U.

Key Differences Between DNA and RNA

DNA Replication

What is DNA Replication?

DNA replication is the process where a DNA molecule makes an exact replica of itself before cell division.

Why is DNA Replication Necessary?

Each daughter cell must receive an exact copy of the DNA from the parent cell.

Where and When Does DNA Replication Occur?

DNA replication occurs in the nucleus during interphase, the stage before mitosis.

Steps of DNA Replication

1. The enzyme helicase "unzips" the DNA molecule, breaking the hydrogen bonds between nucleotides.
2. Each strand acts as a template for the production of the other side.
3. Complementary base pairing occurs as template nucleotides bond to complementary nucleotides by hydrogen bonds.
4. The enzyme DNA polymerase facilitates the bonding of phosphates and sugars in the new double strand.

Semi-Conservative Replication

DNA replication is called "semi-conservative" because each daughter molecule conserves half of each parent DNA molecule.

Recombinant DNA and Genetic Engineering

What is Recombinant DNA?

Recombinant DNA is DNA that is removed from one organism and combined with the DNA of another organism.

What is a Plasmid?

A plasmid is a simple ring of DNA found in bacteria. It can be used for creating recombinant DNA.

Creating Recombinant DNA

1. A restriction enzyme is used to cut open the plasmid.
2. The DNA from a cell containing a gene of interest is isolated and inserted into the plasmid.
3. The plasmid is put back into the bacterial cell and allowed to multiply with the gene of interest.
4. This gene is then placed in an organism where it is wanted, for example, in a plant to create a pest-resistant plant.

Uses of Recombinant DNA

• Treatment of cystic fibrosis
• Cancer treatment
• Creation of pest-resistant plants
• Creation of plants with higher nutritional value
• Production of hormones such as insulin and growth hormones

Transgenic Organisms

A transgenic organism is an organism that has been modified with genetic material from another species. Examples include genetically modified crops.

Examples of Genetically Modified Organisms

• Golden rice contains daffodil genes that produce beta-carotene (Vitamin A).
• Transgenic goats have been created by inserting a human gene so that the milk they produce contains an antibacterial enzyme found in human milk.

Cloning

Clones are exact genetic copies of organisms that can be created artificially. For example, scientists have created clones of sheep.

Protein Synthesis

Two Major Steps in Protein Synthesis

The two major steps in protein synthesis are transcription and translation.

Transcription

Transcription is the process of creating mRNA from the DNA template. It takes place in the nucleus.

Translation

Translation is the process of creating a polypeptide (protein) from the mRNA transcript. It takes place in the cytoplasm at a ribosome.

Role of Ribosomes in Protein Synthesis

Ribosomes control the synthesis of polypeptides. They are made up of protein and rRNA.

Steps of Transcription

1. Initiation: A promoter nucleotide sequence signals the start of transcription. RNA polymerase enzyme attaches to the promoter to start mRNA synthesis.
2. Elongation: Individual complementary mRNA nucleotides bond together to form a single mRNA strand.
3. Termination: RNA polymerase reaches a base called a terminator. mRNA is released from the DNA template.

RNA Polymerase

RNA polymerase is an enzyme that aids in transcription.

The Genetic Code

Each set of three bases in a DNA molecule codes for a specific amino acid.

Codons

A codon is the sequence of bases on an mRNA molecule that codes for a specific amino acid.

Number of Amino Acids

There are 20 different amino acids.

Why Codons are in Sets of Three

If the bases were in sets of two, that would only create 16 possible codes, which is not enough to code for the 20 amino acids that exist.

Examples of DNA to RNA to Amino Acid Sequences

• If the DNA code on the template strand is TACTCGACCGGCAATATC, the sequence of bases on the transcribed mRNA molecule will be AUG-AGC-UGG-CCG-UUA-UAG.
• If the RNA sequence is AAA-AUU-UAG, the original DNA sequence was TTT-TAA-ATC.
• If the DNA sequence is TAC-TCC-ACA-ATC, the sequence of amino acids in the polypeptide is Met (or Start)-Arg-Cys-Stop.

Translation: Roles of tRNA, mRNA, and Ribosomes

• tRNA: Carries the amino acid to the ribosome, where the codon is matched with the anticodon.
• mRNA: Provides the codons that determine the sequence of amino acids.
• Ribosomes: Provide the site of protein synthesis where mRNA and tRNA can match codon with anticodon.

Composition of Ribosomes for Translation

Ribosomes are composed of a small subunit and a large subunit. The large ribosomal subunit contains binding sites for the tRNA molecules to attach.

Steps in Translation

1. Each amino acid attaches to its proper tRNA.
2. Initiation: The mRNA, the first tRNA, and the ribosomal subunits come together.
3. Elongation: A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome one codon at a time.
4. Termination: The ribosome recognizes a stop codon. The polypeptide is terminated and released.

Anticodons

An anticodon is the three-base code on a tRNA that has a complementary codon on the mRNA. For example, if the codon is UCA, the complementary anticodon on tRNA is AGU.

Polyribosomes

Polyribosomes are multiple ribosomes translating at the same time off the same mRNA.

Mutations

How Mutations Affect Amino Acid Sequences

Mutations in DNA can change the sequence of amino acids in a polypeptide chain. If the sequence of nitrogenous bases in DNA is copied incorrectly during DNA replication, the DNA will not have the correct code for building a polypeptide chain with the correct amino acids.

Example of a Disease Caused by a Mutation

Sickle cell anemia is an example of a disease caused by a mutation in the DNA sequence.

Sickle Cell Anemia

In sickle cell anemia, DNA is copied incorrectly during DNA replication, leading to the wrong amino acids being coded for in the protein hemoglobin. This results in red blood cells having an abnormal sickle shape, causing fatigue, anemia, heart failure, brain damage, and other health problems.

The Specific Mutation in Sickle Cell Anemia

One base in the DNA sequence is changed from CTT to CAT when the DNA is replicated. As a result, the amino acid glutamic acid is replaced by valine in the hemoglobin molecule in blood.

Point Mutations

Point mutations involve a single nucleotide being added or deleted in a base sequence, causing a change in a specific codon.

Base Substitution

Base substitution is a point mutation where one nucleotide base is copied incorrectly in a DNA sequence.

Base Deletion

Base deletion is a point mutation where one nucleotide base is missing when DNA is copied.

Impact of Substitution vs. Deletion

Deletion generally causes more problems than substitution because it results in changes to all amino acids from the point of the mistake onward. Substitution causes a mistake in only one amino acid.

Spontaneous Mutations

A spontaneous mutation occurs during DNA replication or recombination without any external agent.

Mutagens

A mutagen is a physical or chemical agent that causes a mutation.

Examples of Environmental Mutagens

• Radiation: Ultraviolet, X-rays, nuclear radiation
• Chemical carcinogens: Tobacco, chemicals, pesticides, PCBs
• Viruses: For example, the HPV virus causes cervical cancer.

Type of Bond Between Base Pairs

The type of bond found between base pairs is a hydrogen bond.