12.2 The Structure of DNAName:

Biology Date: Period:

Objectives

  • Identify the chemical components of DNA.
  • Discuss the experiments leading to the identification of DNA as the molecule that carries the genetic code.
  • Describe the steps leading to the development of the double-helix model of DNA.

The Components of DNA - DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds. Nucleotides may be joined in any order.

  • A DNA nucleotide is a unit made of a nitrogenous base, a 5-carbon sugar called deoxyribose, and a phosphate group.
  • DNA has four kinds of nitrogenous bases: adenine, guanine, cytosine, and thymine.

Solving the Structure of DNA

  • Erwin Chargaff showed that the percentages of adenine and thymine are almost always equal in DNA. The percentages of guanine and cytosine are also almost equal.
  • Rosalind Franklin’s X-ray diffraction studies revealed the double-helix structure of DNA.
  • James Watson and Francis Crick built a model that explained the structure of DNA.

The Double-Helix Model - The double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held together. The model showed the following:

  • The two strands in the double helix run in opposite directions, with the nitrogenous bases in the center.
  • Each strand carries a sequence of nucleotides, arranged almost like the letters in a four letter alphabet for recording genetic information.
  • Hydrogen bonds hold the strands together. The bonds are easily broken allowing DNA strands to separate.
  • Hydrogen bonds form only between certain base pairs–adenine with thymine, and cytosine with guanine. This is called base pairing.

The Components of DNA

For Questions 1–5, complete each statement by writing in the correct word or words.

  1. The building blocks of DNA are .
  2. Nucleotides in DNA are made of three basic components: a sugar called , a , and a nitrogenous .
  3. DNA contains four kinds of nitrogenous bases: , , , and .
  4. In DNA, can be joined in any order.
  5. The nucleotides in DNA are joined by bonds.
  6. The nitrogenous bases are joined by bonds.

Complete the table to describe each scientist’s contribution to solving the structure of DNA.

Scientist / Contribution
Erwin Chargaff
Rosalind Franklin
James Watson and Francis Crick

Complete the table by estimating the percentages of each based on Chargaff’s rules.

DNA sample / Percent of adenine / Percent of thymine / Percent of guanine / Percent of cytosine
1 / 31.5
2 / 30 / 20
3 / 17

The Double-Helix Model For Questions 8–13, on the lines provided, label the parts of the DNA molecule that correspond to the numbers in the diagram.

The drawing below shows half of a DNA molecule. Fill in the appropriate letters for the other half.

Function / Structure of the Molecule
Store information
Copy information
Transmit information

Complete this table to show how the structure of the DNA molecule allows it to perform each essential function.

Chargaff’s DNA DataName: ______

BiologyDate: Period:

Introduction – DNA was first discovered in 1869, but not much was known about the molecule until the 1920’s. Early researchers discovered that DNA was comprised of repeated units called nucleotides.

  1. Each nucleotide contains three parts. Label the three parts.

  1. There are four different nitrogenous bases found in DNA. List the 4 bases:
  2. In the 1920’s, it was believed that these nitrogen bases occurred in all living things in the same repeated pattern, such as ATGC ATGC ATGC ATGC. If this were true, then DNA could not be the hereditary molecule. Explain why?

After World War II, the biochemist Erwin Chargaff made some major discoveries about the nitrogenous bases in DNA. His research revealed the percentage of each base (A, T, G, and C) found in an organism’s DNA. Table 1 below includes some of Chargaff’s data and some more recent additions.

Table 1. Nitrogen Base Make-Up of Different Organisms’ DNA (in Percentages)
Organism / A / T / G / C
Mycobacterium tuberculosis / 15.1 / 14.6 / 34.9 / 35.4
Yeast / 31.3 / 32.9 / 18.7 / 17.1
Wheat / 27.3 / 27.1 / 22.7 / 22.8
Sea Urchin / 32.8 / 32.1 / 17.7 / 17.3
Marine crab / 47.3 / 47.3 / 2.7 / 2.7
Turtle / 29.7 / 27.9 / 22.0 / 21.3
Rat / 28.6 / 28.4 / 21.4 / 21.5
Human / 30.9 / 29.4 / 19.9 / 19.8

Data Analysis – Refer to Table 1 to answer the questions.

  1. Create a graph to show the nitrogen base make-up for yeast, wheat and humans.

Title:

YeastWheatHuman

  1. What observations can you make about the data in Table 1? What patterns do you notice? Use evidence to support your answer.
  2. What does the data in Table 1 show about the make-up of DNA for different species?
  3. Does this support the idea that DNA could be the genetic material? Explain.
  4. For at least 3 species in the data table, calculate the ratio of purines to pyrimidines and organize your results in a table.

Species / A + G = Total % Purine / T + C = Total % Pyrimidine
  1. What can you conclude about the purine: pyrimidine relationship within DNA?
  2. A scientist is analyzing the DNA of a frog. He discovers that about 6% of the frog’s DNA contains the base adenine. Estimate the percentage of the DNA that contains thymine, the percentage of guanine and finally the percentage of cytosine.
  1. How do you think Chargaff’s data helped Watson and Crick to build their model of DNA?
  2. What characteristic of life could be supported by the information gained from this?

Piecing it Together - Chargaff’s data was a central piece of evidence used by James Watson and Francis Crick in 1953 to successfully describe the structure of DNA. Look at the drawing of DNA below.

  1. What do you notice about the arrangement of the nitrogen bases? Record as many observations as you can.
  1. How do you think Chargaff’s data helped Watson & Crick to predict that DNA looks like this?

DNA 5’ and 3’ Practice QuestionsName:

BiologyDate: Period:

  1. In the diagram to the right, number the carbons in the top two deoxyribose molecules. (Remember the bases attach to the 1’ carbon)
  1. In the diagram to the right label the ends of each strand as 3’ or 5’.
  1. Draw a circle around one nucleotide.
  1. How many nucleotides are shown in the diagram to the right?
  1. Write the complimentary DNA strand for the sequences of bases below.
  1. What type of bond is formed between the nitrogenous bases?
  1. What two molecules form the backbone of a DNA molecule?
  1. Why are the two strands of a DNA molecule considered antiparallel?

Use the diagram to answer the remaining questions.

  1. Label the phosphate, deoxyribose, and nitrogenous base.
  1. Number carbons in the deoxyribose molecule.
  1. Does the nucleotide represent a purine or pyrimidine? Explain.


  1. Which molecules are not considered organic compounds? Explain why they are not considered organic.
  1. Which molecule is known as the energy currency of the cell? How does this molecule provide energy?
  1. Which molecule would be classified as a purine nucleotide? How can you tell?
  1. Which molecule would be classified as a pyrimidine nucleotide? How can you tell?
  1. Which molecule is released every time two nucleotides are joined together? What is the name of this process?
  1. Which molecule represents a protein monomer? What functional groups are found in this molecule
  1. Which diagram represents a carbohydrate? Is this carb a monosaccharide, disaccharide, or polysaccharide?
  1. In the phospholipid diagram, label the polar and nonpolar regions of the molecule.
  1. Which diagram represents a monomer that can be found in a molecule of DNA?
  1. Which molecule makes up all the membranes of a cell? Name this molecule.

DNA Replication Web ActivityName:

BiologyDate: Period:

Use the following website to help you answer the questions

1.Watch each section/animation and answer the review questions after each step.

2.With the help of a diagram, explain what is meant by “semi-conservative” replication.

3.What is a replication fork?

4.What does antiparallel mean?

5.Describe the role the following molecules play in DNA replication:

a)DNA polymerase:

b)RNA primer:

c)Okazaki fragments:

d)DNA ligase:

e)Helicase:

6.How are the leading and lagging strands different?

7.Explain how the leading and lagging strands of DNA are replicated. Why are each replicated in a different manner?

Translation

mRNA Codons and the Amino Acids

Protein SynthesisName:

Biology Date: Period:

Use the genetic code below to complete the table

A / B / C /

D

DNA Sequence / mRNA Codon / TRNA Anticodon /

Amino Acid

1 / TAC / AUG / UAC / Methionine
2 / TTA
3 / CCG
4 / AAA
5 / CCG
6 / CCU
7 / GTC
8 / GUA
9 / AUA
10 / Tryptophan
11 / TGT
12 / UAG
13 / ATT

Questions:

  1. Moving from column A to column B is equivalent to what process?
  1. Moving from column B to column D is equivalent to what process?
  1. Where does transcription take place?
  1. Where does translation take place?
  1. How many bases are needed to code for one amino acid?
  1. How many different codon combinations are possible?
  1. How many amino acids are there?

8. List all of the codons that code for Proline?

9. What has been created when all of the substances listed in column D have been bonded together?

Read each term or phrase in the left-hand column of the following table. If the term or phrase applies to DNA, place a check mark in the column labeled DNA. If it applies to RNA, place a check mark in the column labeled RNA. If it applies to both nucleic acids, place a check mark in both columns.

Term or Phrase: / DNA / RNA
Nucleotides
Deoxyribose
Ribose
Single-stranded
Double-stranded
Nitrogen bases
Thymine
Uracil
Double Helix
Replication
Transcription
Messenger
More than one form
Found in the nucleus
Leaves the nucleus
Does not leave the nucleus

Given the sequence A T G G C G A A T C A C G T C A C T T G A

a)Write the sequence of nucleotides for the complementary strand of DNA.

b)Write the mRNA sequence transcribed from the complementary strand.

c)What is the tRNA sequence that would be used to translate this sequence?

d)Convert the message into an amino acid sequence.

Protein Synthesis WebquestName______

BiologyDate: Period:

Go to Please complete each section listed below and/or write a brief summary in that section of the website. If there is a video, please watch the video.

WHAT ARE DNA AND GENES?
DNA molecules have 2 strands that complementary base pair. ____ pairs with ____ and ____ pairs with ____. These building blocks function together in larger units called ______, which is a set of instructions for building a specific ______. A complete set of genes is called a ______, which is a set of instructions for building an ______.
The order of building blocks in a strand of DNA make up a ______. Contained within the 3 billion letters of the human genome are about ______genes. All humans have the ______genes arranged in the ______order, but few differences in the sequence of the baes in our genes make each person ______. These differences can change the ______and ______of a protein, or they can change ______protein is made, ______it's made, or ______it's made. / BUILD A DNA MOLECULE interactive explore
What did you do as you were building a DNA molecule?
How long does it take for your cells to copy genetic information?
How does the cell speed up the copying process?
THINGS YOU MAY NOT KNOW ABOUT DNA watch the video and list at least 5 facts that you learned / ANATOMY OF A GENE
By putting different combinations of ______together, our cells can make different mRNAs from the same gene. This process, known as ______. Our cells can make many more proteins than we have ______. More-complex organisms don't typically have more ______than simpler organisms. Our genomes have ______that allow our genes to be used in more ways, leading to ______. ______give cells the flexibility to react to signals from the outside world. They are also central to ______, the process by which a cell takes on an ______. Each cell type has a different combination of ______and ______genes. Whether a gene is turned "on" or "off" is regulated in part by ______.
How does a limited number of switch proteins regulate such a large number of genes?
TYPES OF PROTEINS - list the types of proteins, click on each one to give examples / RNA’S ROLE IN THE CENTRAL DOGMA
Click on each molecule. List each molecule and describe the function of each.
WHAT MAKES A FIREFLY GLOW?
Luciferase is an enzyme that catalyzes a chemical reaction between ______and ______. This reaction uses energy from ATP and it releases light.
Why do fireflies glow? / BEYOND THE CENTRAL DOGMA - what are some important jobs for RNA besides building proteins
TRANSCRIBE AND TRANSLATE A GENE
Type the mRNA sequence that is complementary to the DNA sequence. Record your answers below:

Describe the process of transcription (look at the bottom of the web page if you need help).
After transcription is complete, the strand of mRNA must be translated. Describe the process of translation. Make sure to use terms such as codon, anticodon and ribosome.
Each protein made during translation must begin with the “Met” amino acid. What is the combination of mRNA nucleotides that corresponds to “Met?’
To translate the mRNA sequence, drag the amino acid from the chart to the flashing box. List the order of amino acids for the entire mRNA molecule.

Read the animation page by page – just click the “next” button when you are ready to move on.

  1. Describe the role of DNA in the synthesis of mRNA.
  2. Explain the interaction between the ribosome and the mRNA molecule in protein synthesis.
  3. How does the mRNA leave the nucleus? ______
  4. Is just one mRNA molecule made? Explain.
  5. How many amino acids does each codon code for? ______
  6. Describe the structure and function of a tRNA molecule.
  7. In the diagram above label the ribosome, mRNA, tRNA, amino acids, anticodon, peptide bond.
  8. Why is ATP necessary for the ribosome to function?
  9. What has been created when all of the amino acids have been brought to the ribosome?
  10. After the amino acid chain leaves the ribosome, what happens to the shape of the chain?
  11. Can a single mRNA be read more than once? Explain.

Chapter 13: RNA and ProteinsName:

BiologyDate: Period:

The Role of RNA

Complete the table to contrast the structures of DNA and RNA.

Sugar / Number of Strands / Bases
DNA
RNA

On the lines provided, identify each kind of RNA.

  1. ______b. c.

RNA Synthesis

In transcription, RNA polymerase separates the two DNA strands. RNA then uses one strand as a template to make a complementary strand of RNA.

  • Label the RNA
  • Label the DNA.
  • Use the key to label the missing nucleotides marked on the diagram.
  1. In DNA, is always paired with cytosine.
  1. In RNA, replaces thymine.
  1. In DNA, is paired with adenine.

For Questions 4-10, complete each statement by writing the correct word or words.

  1. The process of using DNA to produce complementary RNA molecules is called
  2. The sequence of in mRNA complements the sequence in the DNA template.
  3. In eukaryotes, RNA is formed in the and then travels to the
  4. The enzyme binds to DNA during transcription.
  5. RNA polymerase binds to regions of DNA called , which are “start” signals for transcription.
  6. are portions of RNA that are cut out and discarded.
  7. are spliced together to make the final mRNA.

RNA Synthesis - In DNA replication a cell copies its DNA. Both strands of the double helix are used as templates to make complementary, or matching, strands of DNA. In DNA transcription a single strand of DNA is used as a template to generate a strand of mRNA.

  1. Fill in the missing information. One row has been completed for you.

Template / Complementary DNA / Messenger RNA (mRNA)
TTACGG / AATGCC / AAUGCC
GGCGGC
ACGUAG
AGACTC
GATAAG
CUGGCUACA
  1. What is the mRNA if the complementary DNA is TCTGAG?
  2. What does a cell copy in DNA replication?
  3. How many strands of DNA are used to make complementary strands of DNA?
  4. How does the cell make RNA?
  5. What are introns?
  6. What are exons?

Create your own example of DNA. Fill in the chart.

Template / Complementary DNA / Messenger RNA (mRNA)

The Genetic Code

Use the diagram to answer Questions 1–7.

  1. What are the words along the outside of the circle?
  1. What can you find by reading this diagram from the inside out?
  1. For which amino acid is AAA a codon?
  1. What is the codon for tryptophan?
  1. For which amino acid is GGA a codon?
  1. What is a codon for alanine?
  1. What are three other codons for alanine?

Translation

Use the diagram to answer Questions 8–10.

  1. What is the anticodon for leucine?
  1. What is the codon for leucine?
  1. List the amino acids in the order they would appear in the polypeptide coded for by this mRNA.
  1. What is the difference between transcription and translation?
  1. Complete the table to describe the steps in protein synthesis

Step / Description
Beginning of translation
Assembly of polypeptide
Completing the polypeptide
  1. Describe the role of rRNA during translation.

The Molecular Basis of Heredity

For Questions 14–18, write the letter of the correct answer on the line at the left.