DNA

DNA

Middle School

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Learning Objectives

• Define DNA and its role as the 'blueprint of life'.

• Describe the double helix structure of DNA, including its main components.

• Explain the base-pairing rule that governs how DNA is formed.

• Identify the key scientists who helped discover the structure of DNA.

Key Vocabulary

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DNA

Deoxyribonucleic acid is a molecule that carries the genetic instructions for all living organisms.

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Nucleotide

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

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Double Helix

The shape of a DNA molecule, which resembles a twisted ladder with two strands wound together.

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Nitrogenous Base

The part of a nucleotide that carries genetic information; the four types are A, T, C, and G.

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Base-Pair Rule

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

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DNA in different types of Cells

Eukaryotic Cells

• DNA contains the genetic information for making proteins and directs most cellular activities.

• Eukaryotic cells are complex and are found in organisms such as plants and animals.

• In these cells, the DNA is stored inside a dedicated structure called the nucleus.

Prokaryotic Cells

• DNA serves as the genetic blueprint that determines the traits of all living things.

• Prokaryotic cells are much simpler and are found in single-celled organisms like bacteria.

• In these simpler cells, the DNA is not contained and floats throughout the cell.

Multiple Choice

What is the primary function of DNA in living organisms?

• It serves as the blueprint for traits.

• It provides immediate energy for cell activities.

• It forms the structural components of the cell wall.

• It helps with the digestion of food.

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The Building Blocks of DNA

• DNA is a polymer, a large molecule of repeating smaller units.

• These repeating units are monomers, known as nucleotides in DNA.

• Each nucleotide has a sugar, a phosphate group, and a base.

• The sugar and phosphate groups connect to form the DNA's backbone.

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Multiple Choice

What are the three components that make up a nucleotide?

• Sugar, phosphate, and nitrogenous base

• Sugar, protein, and amino acid

• Phosphate, lipid, and nitrogenous base

• Sugar, phosphate, and a protein

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DNA's Structure and Pairing Rules

• DNA has a double helix shape, which looks like a twisted ladder.

• Its rungs are made of four bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).

• The base-pair rule is that Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).

• The sequence of these base pairs forms an organism's unique genetic code.

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Multiple Choice

According to the base-pairing rules, which base always pairs with Cytosine (C)?

• Guanine (G)

• Adenine (A)

• Thymine (T)

• Cytosine (C)

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Discovery of the Double Helix

Rosalind Franklin

• ​Rosalind Franklin was a molecular biologist who studied the complex structure of the DNA molecule.

• ​​She used a technique called X-ray crystallography to capture images of the DNA molecule.

• ​Her 'Photo 51' revealed for the first time that DNA had a helical, or spiral, shape.

Watson and Crick

• ​James Watson and Francis Crick used Franklin's findings to continue the research on DNA.

• ​​They successfully built the first accurate three-dimensional model of the DNA double helix.

• ​Their model showed how all the different components of the DNA molecule fit together.

Multiple Choice

Which scientists are credited with building the first 3-D model of DNA's double helix structure?

• Watson and Crick

• Rosalind Franklin

• Chargaff and Franklin

• Watson and Chargaff

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Common Misconceptions About DNA

Misconception	Correction
DNA strands are held together by very strong bonds.	They are held by weak hydrogen bonds, allowing them to separate easily.
The two DNA strands in a double helix are identical.	Strands are complementary (A with T, C with G), not identical.
Genes are different from DNA.	Genes are specific segments of the DNA molecule that code for traits.

Multiple Choice

Why is the sequence of the nitrogenous bases in a DNA molecule so important for living things?

• It determines the traits an organism will have.

• It provides structural support to the cell nucleus.

• It determines the strength of the DNA backbone.

• It gives the DNA molecule its double helix shape.

Multiple Choice

If one strand of DNA has the sequence TAC-GGC-ATA, what is the sequence of the complementary strand?

• ATG-CCG-TAT

• AUG-CCG-UAU

• GCA-TTA-CGC

• ATC-GGC-ATA

Multiple Choice

How did Rosalind Franklin's X-ray image, Photo 51, provide crucial evidence for Watson and Crick's DNA model?

• It showed that the DNA structure was a helix with two strands.

• It identified the four nitrogenous bases in DNA.

• It proved that adenine pairs only with thymine.

• It measured the exact length of a DNA molecule.

Multiple Choice

What key contribution did James Watson and Francis Crick make to DNA research?

• They discovered the chemical components of DNA.

• They developed the first accurate 3D model of the DNA double helix.

• They extracted DNA from cells for the first time.

• They identified the genetic code within DNA sequences.

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Summary

• DNA is the blueprint of life, carrying the genetic code for all organisms.

• It has a double helix structure and is found in the cell's nucleus.

• DNA is made of nucleotides, which follow the base-pairing rule: A with T, and C with G.

• The work of Franklin, Watson, and Crick was key to discovering DNA's structure.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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