AP Biology 1.6: Nucleic Acids

AP Biology 1.6: Nucleic Acids

Welcome to AP Biology Unit 1.6! In this section, we delve into the fascinating world of nucleic acids, the large complex molecules that store, transmit, and express hereditary information. These molecules are vital for the existence of life. 

What Are Nucleic Acids?

Nucleic acids are made up of monomers called nucleotides, each consisting of a 5-carbon sugar, a nitrogenous base, and a phosphate group. There are two main types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA is the genetic material that stores instructions for building proteins, while RNA helps carry out these instructions by aiding in protein synthesis.

• Genes are specific units of inheritance composed of DNA. The sequence of nucleotides in a gene determines the sequence of amino acids in a protein, which ultimately determines its function.

Deoxyribonucleic Acid (DNA)

DNA is a molecule that stores and transmits genetic information, playing a key role in replication, synthesis of mRNA, and control of protein synthesis. The sugar in DNA is deoxyribose.

DNA is composed of a linear sequence of nucleotides linked by covalent bonds. The sequence is defined by the presence of a hydroxyl group (-OH) on the 3′ carbon of one nucleotide and a phosphate group on the 5′ carbon of the next nucleotide. During DNA synthesis, nucleotides are added to the 3′ end of the growing strand, forming covalent bonds between adjacent nucleotides.

DNA is structured as a double helix composed of two antiparallel strands, held together by hydrogen bonds between complementary nitrogenous bases. The sugar-phosphate backbone forms the sides of the helix, while the bases form the rungs. The antiparallel arrangement of the two strands (one running from 5′ to 3′ and the other from 3′ to 5′) is crucial for the stability and accurate replication of genetic material.

Purines and Pyrimidines

The nitrogenous bases in nucleic acids are vital components of the genetic code. There are five nitrogenous bases in total:

• Cytosine (C), Thymine (T), Uracil (U), Adenine (A), and Guanine (G).

• Uracil is found only in RNA, while thymine is found only in DNA.

• The bases can be divided into purines and pyrimidines:

  • Purines: Adenine (A) and Guanine (G), with a double-ring structure.

  • Pyrimidines: Thymine (T), Cytosine (C), and Uracil (U), with a single-ring structure.

Each purine pairs with a pyrimidine through hydrogen bonding. Adenine pairs with thymine (A-T) with two hydrogen bonds, while guanine pairs with cytosine (G-C) with three hydrogen bonds. This base pairing ensures the stability and accuracy of DNA replication and genetic transmission.

Chargaff’s Rule

Chargaff’s Rule states that in DNA, the amount of adenine equals the amount of thymine (A=T), and the amount of guanine equals the amount of cytosine (G=C). This rule helps to maintain the complementary base pairing that is essential for DNA’s structure and function.

DNA vs. RNA

Although DNA and RNA share some similarities, they also have distinct differences that contribute to their respective functions.

Key Similarities:

• Both are nucleic acids composed of nucleotides.

• Both contain a sugar, phosphate group, and nitrogenous base.

• Both contain adenine (A), guanine (G), and cytosine (C).

• Both are involved in the transmission of genetic information and protein synthesis.

Key Differences:

• Sugar Molecule: DNA contains deoxyribose, while RNA contains ribose.

• Nitrogenous Bases: DNA has thymine (T), while RNA has uracil (U) instead.

• Structure: DNA is usually double-stranded, forming a double helix, while RNA is typically single-stranded.

• Orientation: In double-stranded DNA, the two strands are antiparallel, running in opposite directions (5′ to 3′ and 3′ to 5′).

These structural differences are significant because they determine the unique roles that DNA and RNA play in genetic information storage, transmission, and protein synthesis.

Study Resources

For a deeper dive into nucleic acids and their function, check out the AP Bio Unit 1 replays or watch the 2021 Unit 1 Cram session. Understanding nucleic acids is crucial for comprehending how life stores, transmits, and uses genetic information.