High school biology worksheets

Biology 30—Lab III.5 Page 4

DNA and RNA

Deoxyribonucleic acid (DNA) is a complex molecule found in all living organisms. DNA is the chemical of which genes are composed. An understanding of the organization of this molecule has answered many questions. Scientists now know how chromosomes can duplicate during cell division and transfer their genetic information to new chromosomes. Scientists also understand how chromosomes in the cell nucleus can direct the formation of specific proteins outside the nucleus.

In this investigation, you will

1. learn the names of the molecules which make up DNA.

2. use models to construct a molecule of DNA and show how it replicates.

3. learn the names of the molecules which make up RNA.

4. use models to show how the base sequence code in DNA is transcribed exactly to RNA.

PART A: Structure of DNA Nucleotides

Two important molecules which make up DNA are deoxyribose and phosphoric acid. Their structural formulas are:

deoxyribose	phosphoric acid

1. Give the molecular formula for

a) deoxyribose C___ H___ O___

b) phosphoric acid H___ P___ O___

Deoxyribose is a carbohydrate. In addition, there are four different molecules called nitrogen bases. Their structural formulas are:

guanine	cytosine
adenine	thymine

2. Of the four bases, which other base does

a) adenine most resemble in shape?

b) thymine most resemble in shape?

A molecule of deoxyribose joins with phosphoric acid and any one of the four bases to form a chemical compound called a nucleotide. A nucleotide is named after the nitrogen base that joins with the deoxyribose. For example, if thymine attaches to deoxyribose, the molecule is called a thymine nucleotide.

3. List four different nucleotides that can be made (4 marks).

4. a) How is each nucleotide alike (2 Marks)?

b) How does each nucleotide differ?

PART B: Structure of a DNA Molecule

A DNA molecule is “ladder like” in shape. Deoxyribose and phosphoric acid molecules join to form the sides or uprights of the ladder. Base molecules join to form the rungs of the ladder.

Print two copies of BIOL200-28 Lab Sheet 1 and one copy of BIOL200-28 Lab Sheet 2. Cut out the 24 nucleotide models (cut only on solid lines).

Fit six nucleotides together in puzzle like fashion to form a row in the following sequence from top to bottom: cytosine, thymine, guanine, adenine, guanine and cytosine. Let this arrangement represent the left half of a ladder molecule. It should consist of one side plus six half rungs.

5. If DNA is “ladder like”, which two molecules of a nucleotide form the sides, or the upright portion of the ladder (2 Marks)?

6. To which molecule does each base attach?

7. Name the molecule of each nucleotide that form part of the ladder's rungs.

Complete the right side of the DNA ladder by matching the bases of other nucleotides to form complete rungs. It may be necessary to turn molecules upside down in order to join certain base combinations. NOTE: The ends of each base will allow only a specifically shaped matching new base to fit exactly.

Your completed model should look like a ladder with matched bases as the rungs. Besides being shaped like a ladder, a DNA molecules is twisted. It looks like a spiral staircase. However, your paper model cannot show this shape.

8. Is the order of half-rung bases exactly the same from top to bottom of each side of your model?

9. Only two combinations of base pairings are possible for the rungs. Name the molecule combinations or pairs (2 Marks).

10. If four guanine bases appear in a DNA model, how many cytosine bases should there be?

11. Your DNA model has four guanine bases.

a) Does the number of cytosine bases in your model agree with your prediction?

b) The following are the bases on the left side of a DNA molecule. List the bases that would make up the right side of a DNA molecule.

i) Thymine

ii) Adenine

iii) Guanine

iv) Guanine

v) Cytosine

PART C: DNA Replication

A chromosome contains DNA. Your DNA model represents only a short length of the DNA portion of a chromosome. An entire chromosome has thousands of rungs rather than only six. Although your model is only a small part of a chromosome, its replication is the same as that of an entire chromosome during mitosis and meiosis.

Open your DNA model along the point of attachment between base pairs (rungs) and separate the two ladder halves (A chromosome untwists and “unzips” in a similar way prior to replication).

Using the left half of your model as a pattern, add new nucleotide to form a new right side. Build a second DNA model by adding new nucleotides to the right half of the original model.

12. Do the two new molecules contain the same number of rungs?

13. Is the order from top to bottom of the base pairs (rungs) different or the same for each new DNA molecule?

14. How many molecules of adenine and thymine are in each DNA molecule?

15. Do the numbers agree?

16. Are the two DNA molecules exact copies of each other?

The specific order of bases in DNA serves as a code or language. When a chromosome replicates, the code (the order in which the bases occur) is carried over to the new chromosome.

17. What is the code of a chromosome?

PART D: RNA Structure

Besides ensuring the exact replication of chromosomes, the sequence (order) and pairings of bases are a genetic code of the instructions for the entire cell. How does a cell “read” the chemical message coded in its DNA in the form of specific base sequences? Part of the answer lies with a second molecule in the nucleus of cells called ribonucleic acid (RNA).

RNA is similar to DNA in that its molecules are also formed from nucleotides. However, deoxyribose and thymine are not found in RNA. Two other molecules, ribose and uracil, are present. Ribose replaces deoxyribose, and uracil replaces thymine. Looking at their structural formulas and models, you will see certain similarities between the molecules that they replace. Formulas and models are shown below:

ribose	uracil

18. a) Which base is replaced in RNA by uracil?

b) What chemical replaces deoxyribose in RNA?

19. To which base in DNA do the following RNA bases pair?

a) guanine

b) adenine

c) cytosine

d) uracil

Analysis

Complete the table by using “x” marks to indicate to which molecule each characteristic applies.

SIMILARITIES AND DIFFERENCES BETWEEN DNA AND RNA
	DNA	RNA
deoxyribonucleic acid
ribonucleic acid
deoxyribose present
ribose present
phosphoric acid present
adenine present
thymine present
uracil present
guanine present
cytosine present
contains a chemical message or code
formed from nucleotides
double stranded
single stranded
remains in nucleus
moves out of nucleus

DNA and RNA

DNA and RNA Lab Page 6 of 7