7. DNA and Chromosome Structure

DNA Structure

7. DNA and Chromosome Structure

DNA Structure: Videos & Practice Problems

Topic summary

DNA, or deoxyribonucleic acid, consists of three main components: a phosphate group, deoxyribose sugar, and nitrogenous bases (adenine, guanine, cytosine, thymine). Nucleotides, which include all three components, form the DNA backbone through phosphodiester bonds. Complementary base pairing occurs between purines (adenine, guanine) and pyrimidines (cytosine, thymine) via hydrogen bonds. The strands are antiparallel, running in opposite directions, and twist into a double helix structure, featuring major and minor grooves. Understanding these elements is crucial for grasping genetic information storage and transmission.

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DNA Structure

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DNA Structure Video Summary

DNA, or deoxyribonucleic acid, is a vital molecule that carries genetic information in living organisms. It consists of three main components: a phosphate group, a sugar (deoxyribose), and a nitrogenous base. The sugar in DNA differs from that in RNA, which contains ribose. The distinction lies in the hydroxyl group present at the 2' carbon position in ribose, while deoxyribose has a hydrogen atom at this position. This subtle difference contributes to the unique properties of DNA and RNA.

Nitrogenous bases can be categorized into two groups: purines and pyrimidines. Purines, which include adenine (A) and guanine (G), have a double-ring structure, while pyrimidines, such as cytosine (C) and thymine (T), have a single-ring structure. Importantly, purines always pair with pyrimidines in DNA, following Chargaff's rules, which state that A pairs with T and C pairs with G. This complementary base pairing is crucial for the stability and integrity of the DNA structure.

The structure of DNA is characterized by two types of bonds: phosphodiester bonds and hydrogen bonds. Phosphodiester bonds link nucleotides together to form the backbone of a single DNA strand, while hydrogen bonds connect complementary strands, forming the double helix structure. The strength of these bonds varies; for instance, G-C pairs are held together by three hydrogen bonds, making them stronger than A-T pairs, which are connected by two hydrogen bonds.

DNA strands are described as anti-parallel, meaning they run in opposite directions. This orientation is indicated by the 5' and 3' ends of the sugar molecules. The 5' end has a phosphate group attached to the fifth carbon of the sugar, while the 3' end has a hydroxyl group on the third carbon. The anti-parallel arrangement is essential for proper base pairing and the overall stability of the DNA molecule.

The double helix structure of DNA features a major groove and a minor groove, which are important for protein binding and interactions. The major groove is wider and contains more base pairings, while the minor groove is narrower. Understanding the structure and function of DNA is fundamental to genetics, molecular biology, and the study of life itself.

Problem

Which of the following is not a component of a nucleotide?

Phosphate

Pentose Sugar

Starch sugar

Nitrogenous base

Problem

Chargoff's rules states that which nucleotide pairings occurred?

A and G, C and T

T and U, C and G

G and U, C and A

A and T, C and G

Problem

Which of the following is NOT true regarding the structure of the DNA double helix?

The two strands run parallel

The two strands are complementary

The nucleotides are held together via Phosphodiester bonds

There is a major groove and minor groove

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Here’s what students ask on this topic:

What are the main components of DNA?

DNA, or deoxyribonucleic acid, consists of three main components: a phosphate group, a deoxyribose sugar, and nitrogenous bases. The nitrogenous bases include adenine (A), guanine (G), cytosine (C), and thymine (T). These components form nucleotides, which are the building blocks of DNA. Nucleotides link together through phosphodiester bonds to create the DNA backbone. The nitrogenous bases pair specifically (A with T and G with C) through hydrogen bonds, forming the double-stranded structure of DNA.

How do purines and pyrimidines differ in DNA structure?

Purines and pyrimidines are two types of nitrogenous bases found in DNA. Purines, which include adenine (A) and guanine (G), have a double-ring structure. Pyrimidines, which include cytosine (C) and thymine (T), have a single-ring structure. In DNA, purines always pair with pyrimidines: adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). This complementary base pairing is crucial for the stability and function of the DNA double helix.

What is the significance of antiparallel strands in DNA?

In DNA, the two strands are antiparallel, meaning they run in opposite directions. One strand runs from the 5' (5 prime) to the 3' (3 prime) end, while the complementary strand runs from 3' to 5'. This orientation is crucial for the formation of the double helix structure and for the processes of DNA replication and transcription. The antiparallel arrangement allows the nitrogenous bases to pair correctly (A with T and G with C) and ensures the proper functioning of enzymes involved in DNA synthesis and repair.

What types of bonds are involved in the DNA structure?

Two main types of bonds are involved in the DNA structure: phosphodiester bonds and hydrogen bonds. Phosphodiester bonds link the nucleotides together in a single strand, forming the DNA backbone. These bonds connect the phosphate group of one nucleotide to the deoxyribose sugar of the next. Hydrogen bonds, on the other hand, connect the complementary nitrogenous bases of the two DNA strands. Adenine (A) pairs with thymine (T) via two hydrogen bonds, while guanine (G) pairs with cytosine (C) via three hydrogen bonds. These hydrogen bonds stabilize the double helix structure.

Why is DNA with more G-C pairs stronger than DNA with more A-T pairs?

DNA with more guanine-cytosine (G-C) pairs is stronger than DNA with more adenine-thymine (A-T) pairs because G-C pairs are connected by three hydrogen bonds, while A-T pairs are connected by only two hydrogen bonds. The additional hydrogen bond in G-C pairs provides extra stability and makes the DNA molecule more resistant to denaturation (separation of strands) under conditions such as high temperature or chemical stress. This increased stability is particularly important in organisms that live in extreme environments.

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