Essential Biology 3.3 & 7.1: DNA Structure (HL Only)

Resources: http://sciencevideos.wordpress.com/bis-ib-diploma-programme-biology/02-the-double-helix/dna-structure-and-replication-inc-ahl-71-72/

1. Draw and label the structure of a simplified single nucleotide, including sugar, phosphate and base.

Phosphate

Sugar

Base

2. Complete the table below to show the pairings of the bases in DNA:

Purine Pyramidine

Adenosine (A) Thymine (T)

Guanine (G) Cytosine (C)

3. Where would one find the base uracil?

It is found in RNA and replaces adenosine which bonds with thymine.

4. In the space below, draw a single strand of three nucleotides, naming the bonds between them and showing the correct relative position of these bonds.

The red lines represent covalent bonds which joins the nu-

cleotides.

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Bases are read in groups of 3 called triplets

Essential Biology 3.3 & 7.1: DNA Structure (HL Only)

5. Define the term double helix.

A double helix is a pair or parallel helices about a common axis which is the shape the DNA takes when two single strands of nucleotides are bonded together.

6. In the space below, draw a section of DNA, showing two anti-parallel strands of three nucleotides. Label the bonds which hold the bases together as well as the correct complementary base pairs. Also include the 3’ and 5’ linkages (and ends), and the distinction between purines and pyramidines.

A and G are purines and T and C are pyramidines.

7. Explain the relevance of the following in the double-helix structure of DNA:

a. Complementary base pairing

Ensures that mistakes are not made when copying or transcribing DNA

b. Hydrogen bonds

Hold adjacent sections and complementary base pairs together.

c. Relative positioning of the sugar-phosphate backbone and the bases

Sugar-phosphate backbone is hydrophillic and posi-tioned on the outside because nitrogenous bases are very reactive and are protected on the inside.

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A T

C G

5’ (5-prime) the end nearest carbon number 5

3’ (3-prime) the end nearest carbon number 3

Essential Biology 3.3 & 7.1: DNA Structure (HL Only)

8. In the space below, draw the structure of a simplified nucleosome, including the H1 linker and histone proteins.

9. Nucleosomes allow the DNA to be supercoiled.

a. What is the approximate length of the DNA strand in one chromosome?

2 meters

b. During which phase of the cell cycle is DNA most likely to be supercoiled?

In metaphase

c. Outline how nucleosomes help regulate transcription.

Histones are important for gene expression

10.Distinguish between unique or single-copy genes and highly repetitive se-quences:

Single-copy genes Highly-repetitive sequences1.5% of genome makes polypeptides3% codes for ‘on/off’ gene switchesContains EXONS (coding regions) and INTRONS (non-coding regions which are edited out)

Makes up about 5-45% of the genomeOnce called ‘junk’ DNAAlso known as satellite DNA, each repeated sequence can be 5-300 base pairsHRSs are used in genetic finger-printing.

11.Distinguish between introns and exons in eukaryotic genes.

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Essential Biology 3.3 & 7.1: DNA Structure (HL Only)

Introns consist of the functional part of the genome and exons consit o the non functional part of the genome or ‘junk DNA’

12.The discovery of the structure of DNA earned a Nobel Prize for Watson, Crick and Wilson.

Read the resources at the Nobel Prize website: http://nobelprize.org/educational_games/medicine/dna_double_helix/readmore.html

How is it a good example of the following:

a. Internationalism in science?

It shows internationalism in science because it speaks of many different scientists from around the world who were contributing to the observations.

b. Cooperation in science?

It shows cooperation in science because it took a group of scientist to come to-gether to come up with a final conclusion. With only one or two sets of eyes the same results would have not been achieved, they need fresh eyes to get other opin-ions and need to cooperate and be open minded and work as a team.

c. Competition in science?

It shows competition because each scientist was working to be the most successful and solve the puzzle of DNA and get the recognition.

13.What was the role of Rosalind Franklin in the process of the discovery of the structure of DNA and why was she not included in the Nobel Prize?

Rosalind Franklin discovered the ‘photograph of 51’ that revealed the helical structure of DNA however it was Watson and Crick that were able to recog-nize it as she was too familiar with the data. She was not included in the No-bel Prize because it was Watson and Crick who saw the discovery although she had done all the work. She then died before they could nominate her in ’62 for the next prize.

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