4.5 Nucleic Acids
Nucleic acids are vital organic macromolecules found in all living cells. They are polymer chains formed from nucleotide monomers and play a central role in storing and transmitting genetic information, as well as directing the synthesis of proteins required for life processes.
Learning Objectives
- State the elemental composition of nucleic acids.
- Identify the three main components of a nucleotide monomer.
- Compare the structures, pentose sugars, and nitrogenous bases of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
- Describe the complementary base pairing rule in the DNA double helix.
- State the three main types of RNA and their roles in protein synthesis.
- Explain the role of DNA as a carrier of hereditary information and how genetic codes determine polypeptide sequences.
- Describe the formation of chromosomes from DNA polynucleotide chains and histone proteins.
Structure of Nucleotides, DNA Double Helix, and Chromosome Packing
Compare DNA and RNA components, understand complementary base pairing, and trace the packaging of DNA into chromosomes.
Nucleotide
Monomer unit consisting of a pentose sugar, nitrogenous base, and phosphate group.
DNA
Double helix with deoxyribose sugar and bases A, T, G, C.
RNA
Single-stranded with ribose sugar and bases A, U, G, C.
Short Explanation
Definition
Nucleic acid — a vital organic macromolecule found in all living cells. Nucleic acids are polymer chains formed from nucleotide monomers and play a central role in storing and transmitting genetic information, as well as directing the synthesis of proteins required for life processes.
Elemental Composition
Nucleic acids consist of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P). They exist as one or two polymer chains made up of nucleotide monomers.
Structure of a Nucleotide
Each nucleotide monomer is formed by combining three components through a condensation process: a pentose sugar, a nitrogenous base, and a phosphate group.
Deoxyribonucleic Acid (DNA)
DNA consists of two polynucleotide chains intertwined in opposite directions to form a double helix. It contains deoxyribose sugar and the bases adenine (A), guanine (G), cytosine (C), and thymine (T). Nitrogenous bases from opposing chains are held together by hydrogen bonds. Adenine pairs with thymine, and guanine pairs with cytosine.
Ribonucleic Acid (RNA)
RNA is a single polynucleotide chain that is shorter than DNA. It contains ribose sugar and the bases adenine (A), guanine (G), cytosine (C), and uracil (U). Thymine in DNA is replaced by uracil in RNA. The three types of RNA — mRNA, rRNA, and tRNA — are all involved in protein synthesis.
Importance of Nucleic Acids
DNA determines the characteristics of living organisms, such as eye colour or height. DNA contains genetic codes carried by nitrogenous base sequences (A, G, C, T) for polypeptide synthesis. A codon is a series of three bases on mRNA that determines the amino acid sequence in proteins. For example, the AUG codon codes for the amino acid methionine.
Formation of Chromosomes
Histones are basic structural proteins around which DNA polynucleotide chains are wound. Histones do not carry genetic information. Nucleosomes are structural units formed by the combination of DNA molecules wrapped around histone proteins. Chromosomes are formed when nucleosomes coil and intertwine tightly inside the nucleus of a cell.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What are the three components that join together through condensation to form a nucleotide?
State the complementary base pairing rule for the nitrogenous bases in a DNA double helix.
What is a nucleosome and how is it related to chromosome formation?
Common Mistakes
- Writing that RNA contains thymine instead of uracil.
- Confusing the pentose sugars between DNA (deoxyribose) and RNA (ribose).
- Believing that histone proteins carry genetic information. Histones are structural proteins and do not carry genetic codes.
- Forgetting that base pairs in DNA are held together specifically by hydrogen bonds.
Concept Misunderstandings
Chromosomes consist entirely of DNA.
Chromosomes are composed of DNA polynucleotide chains tightly wound around histone proteins.
RNA is double-stranded like DNA.
RNA is a single polynucleotide chain and is shorter in length compared to DNA.
DNA directly forms polypeptides on its own.
DNA is first transcribed into mRNA codons, which are then translated into amino acid sequences to produce polypeptides.
Summary
- Nucleic acids contain C, H, O, N, and P, and consist of nucleotide monomers.
- A nucleotide = pentose sugar + nitrogenous base + phosphate group.
- DNA is a double helix with deoxyribose sugar and bases A, T, G, C.
- RNA is single-stranded with ribose sugar and bases A, U, G, C.
- Three types of RNA (mRNA, rRNA, tRNA) participate in protein synthesis.
- Genetic code is a three-base sequence transcribed from DNA to mRNA codons and translated into polypeptide chains.
- DNA coils around histones to form nucleosomes, which intertwine to form chromosomes.
Short Activity
Objective: test your understanding of nucleic acid structure, base pairing, and chromosome formation.
A. Objective Quiz
Which nitrogenous base is present in RNA but absent in DNA?
What is the structural unit formed when DNA winds around histone proteins?
B. Fill in the Blanks
In a DNA double helix, the nitrogenous base adenine always pairs with via hydrogen bonds.
A series of three nitrogenous bases on mRNA that codes for a specific amino acid is called a .
C. Matching / Drag and Drop
Match each nucleic acid term with its correct textbook description. Drag each term to its matching description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Term to Its Description
Write the correct term next to each description.
| Description | Term |
|---|---|
| 1. Protein that DNA winds around to form nucleosomes and does not carry genetic information | |
| 2. Carries transcribed codons from DNA to direct the sequence of amino acids | |
| 3. The 5-carbon pentose sugar found in DNA nucleotides | |
| 4. Monomer unit consisting of a pentose sugar, nitrogenous base, and phosphate group |
Answer choices: Histone • mRNA • Deoxyribose • Nucleotide
