DNA Structure

DNA Structure

Middle School

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Learning Objectives

• Describe the double helix structure of a DNA molecule.

• Identify the three main parts that make up a nucleotide.

• Explain the rules for how DNA bases pair with each other.

• Tell the difference between the bonds holding the DNA molecule together.

Key Vocabulary

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DNA

A molecule carrying the genetic instructions for the growth, development, and reproduction of all living organisms.

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Nucleotide

The basic building block of DNA, made of a sugar, a phosphate group, and a nitrogenous base.

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Double Helix

The shape of a DNA molecule, which looks like a twisted ladder with two intertwined strands.

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Base Pairing Rule

The principle that adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

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Antiparallel Strands

A key feature of DNA where the two strands of the double helix run in opposite directions.

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DNA inside the Cells

DNA in Eukaryotic Cells

• DNA holds genetic instructions for complex organisms like plants and animals.

• It is safely stored within a membrane-bound structure called the nucleus.

• This organized storage helps manage the complex functions of eukaryotic organisms.

DNA in Prokaryotic Cells

• DNA carries the essential genetic code for simpler organisms like bacteria.

• It is located in the cytoplasm in a region known as the nucleoid.

• This simpler arrangement allows for rapid growth and reproduction in prokaryotic cells.

Multiple Choice

What is the primary role of DNA and where is it located within a eukaryotic cell?

• To create energy; found in the mitochondria

• To store genetic information; found in the nucleus

• To transport materials; found in the cytoplasm

• To build proteins; found in the ribosomes

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The Structure of DNA

• DNA is shaped like a twisted ladder, a structure called a double helix.

• The building blocks that make up the DNA chain are called nucleotides.

• A nucleotide contains a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

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Multiple Choice

What are the three components that make up a single nucleotide?

• Deoxyribose sugar, phosphate group, and nitrogenous base

• Ribose sugar, amino acid, and phosphate group

• Deoxyribose sugar, lipid, and nitrogenous base

• Phosphate group, fatty acid, and amino acid

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Assembling the DNA Ladder

The Backbone

• ​The sides of the ladder are made of a sugar-phosphate backbone.

• ​​The sugar and phosphate groups are held together by strong covalent bonds.

• ​These strong bonds make the DNA molecule stable and durable.

The Rungs

• ​The rungs of the ladder are made of pairs of nitrogenous bases.

• ​​These base pairs are connected to each other by weak hydrogen bonds.

• ​The weak bonds allow the DNA strands to separate for important processes.

Multiple Choice

What forms the 'backbone' of the DNA ladder, and what type of bond holds it together?

• Nitrogenous bases, held by hydrogen bonds

• Sugar and phosphate groups, held by covalent bonds

• Deoxyribose sugars only, held by ionic bonds

• Phosphate groups only, held by hydrogen bonds

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Base Pairing Rules in DNA

Adenine & Thymine

• In DNA, the base Adenine (A) consistently pairs up with the base Thymine (T).

• This base pair is held together by two chemical connections called hydrogen bonds.

• This complementary relationship is a fundamental component of Chargaff's rules of base pairing.

Guanine & Cytosine

• The base Guanine (G) consistently connects with the base Cytosine (C) within the DNA molecule.

• Three hydrogen bonds are formed between Guanine and Cytosine, holding them securely together.

• The presence of three bonds makes this pair slightly stronger than the Adenine-Thymine pair.

Multiple Choice

According to the rules of base pairing, which base pairs with Guanine (G) and how many hydrogen bonds are formed between them?

• It pairs with Thymine (T) via 2 hydrogen bonds.

• It pairs with Adenine (A) via 3 hydrogen bonds.

• It pairs with Cytosine (C) via 3 hydrogen bonds.

• It pairs with Guanine (G) via 2 hydrogen bonds.

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Antiparallel Strands

• The two strands of a DNA double helix are antiparallel.

• This means the two DNA strands run in opposite directions.

• This structure lets the nitrogenous bases meet in the center.

• It allows complementary bases (A-T and G-C) to pair correctly.

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Multiple Choice

What is the significance of the DNA strands being 'antiparallel'?

• It prevents the DNA from replicating.

• It allows the base pairs to properly connect in the center of the molecule.

• It makes the sugar-phosphate backbone stronger.

• It ensures that both strands have the exact same sequence.

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Common Misconceptions

Misconception	Correction
The two strands of DNA are identical.	The strands are complementary, not identical.
All bonds in DNA are equally strong.	Covalent bonds are strong, while hydrogen bonds are weak.
The order of bases does not matter.	The sequence of bases contains genetic instructions.

Multiple Choice

Why is it structurally important for the hydrogen bonds between bases to be weak while the covalent bonds of the backbone are strong?

• The weak backbone and strong base connections allow the DNA to be flexible.

• The strong backbone provides stability, while the weak bonds between bases allow the strands to separate for replication.

• Both types of bonds are actually equal in strength.

• The weak covalent bonds allow the DNA to be easily broken down for energy.

Multiple Choice

If a scientist determines that a DNA sample contains 30% Adenine, what percentage of the sample should be Guanine?

• 30%

• 60%

• 40%

• 20%

Multiple Choice

A hypothetical drug is developed that strengthens the hydrogen bonds between base pairs, making them as strong as covalent bonds. What is the most likely outcome for the cell?

• The cell's DNA would become more stable and function better.

• The cell would be unable to separate its DNA strands for replication, preventing cell division.

• The sugar-phosphate backbone would break down.

• The cell would produce proteins more rapidly.

Multiple Choice

How do the concepts of 'complementary base pairing' and 'antiparallel strands' work together to define the structure of the DNA double helix?

• They both describe the sugar-phosphate backbone exclusively.

• Antiparallel strands allow the complementary bases to align correctly in the center to form the hydrogen-bonded 'rungs' of the ladder.

• Complementary pairing means the two antiparallel strands are identical in sequence.

• Antiparallel strands cause Adenine to pair with Guanine, which is what complementary pairing describes.

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Summary

• DNA has a double helix structure, often described as a twisted ladder.

• A strong sugar-phosphate backbone forms the sides of the DNA ladder.

• Complementary base pairs (A-T and G-C) form the rungs of the ladder.

• The two DNA strands are antiparallel, running in opposite directions.

Poll

On a scale of 1-4, how confident are you about the structure of DNA after this review?

• 1

• 2

• 3

• 4