A1.2 Nucleic acids | What the Cell

Guiding questions

How do nucleic acids store information that can be inherited?
Which features of DNA allow it to be copied accurately?

Slides

1 of 16

Notes

Written for this site from the class slides. The chapter is on pages 14–24 of the course textbook. Image credits are on slide 16.

Building blocks

A1.2.1

DNA is the genetic material of life

pp. 14–24

All living organisms use DNA as their genetic material. Some viruses use RNA, but viruses are not considered to be living.

A1.2.2

The three parts of a nucleotide

pp. 14–24

A nucleotide has a phosphate, a pentose sugar and a nitrogenous base. In diagrams, use a circle, a pentagon and a rectangle, in these positions.

Slide: a nucleotide drawn as a circle, a pentagon and a rectangle

A1.2.3

The sugar-phosphate backbone

pp. 14–24

Each sugar bonds to the phosphate of the next nucleotide. This makes a continuous chain of covalently bonded atoms, so that the backbone is strong. Any base can follow any other.

A1.2.4

Bases carry the code

pp. 14–24

DNA has adenine, cytosine, guanine and thymine. RNA has uracil in place of thymine. Only the base varies along a strand, so the base sequence carries the information.

A1.2.5

RNA is a polymer formed by condensation

pp. 14–24

RNA nucleotides are monomers that join by condensation, releasing water for each new bond. RNA is a single strand with a sugar-phosphate backbone.

DNA

A1.2.6

The DNA double helix

pp. 14–24

Two antiparallel strands, running in opposite directions, are linked by hydrogen bonds between complementary bases: A with T, and G with C.

Exam tip: draw the two strands antiparallel. You do not need to draw the helix, or to remember the numbers of hydrogen bonds.

A1.2.7

Differences between DNA and RNA

pp. 14–24

	DNA	RNA
Strands	Two	One
Sugar	Deoxyribose	Ribose
Bases	A, C, G, T	A, C, G, U

Ribose has an -OH group on the carbon where deoxyribose has only -H, so deoxyribose has one oxygen atom fewer.

Using the structure

A1.2.8

Why base pairing matters

pp. 14–24

Pairing depends on hydrogen bonds, which form only between the correct bases. Each strand can therefore act as a template in replication. In expression, DNA is a template for mRNA, and anticodons on tRNA pair with codons on mRNA.

A1.2.9

Limitless capacity to store information

pp. 14–24

Any length of DNA molecule and any base sequence is possible. With four bases, there are 4 possible sequences for one position, 16 for two and 64 for three, and the number rises very quickly. So DNA stores data with great economy.

A1.2.10

The code as evidence of common ancestry

pp. 14–24

Almost all organisms use the same genetic code. The chance of this arising separately many times is extremely small, so the best explanation is universal common ancestry.

Videos

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Note-taking sheet

Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.

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Practice questions

A printable worksheet covering all ten statements, from quick recall to drawings, data and a longer answer. It has 36 marks and takes about 35 minutes, or you can do one section at a time.

Worksheet PDF

Your teacher has the mark scheme.

Practical

Extracting DNA from strawberries

Lab35 minA1.2.1

You will need: Strawberries, zip-lock bag, washing-up liquid, table salt, filter or coffee paper, ice-cold ethanol, test tube

Mash a strawberry in a bag with water, salt and washing-up liquid
Filter the mixture into a test tube
Slowly pour cold ethanol down the side to form a layer on top
Spool the white strands that appear, and explain what each ingredient did

Safety: Ethanol is highly flammable, so keep it away from flames. Eye protection. Do not eat anything.

Carry this out with your teacher's supervision and follow your school's lab rules.

Nature of science and TOK

Nature of science

Models

Watson and Crick built their double helix model using Rosalind Franklin's X-ray data, which she had not given them directly.

Theory of knowledge

Who gets the credit in collaborative knowledge?

Franklin's contribution was long under-recognised. Consider how credit is assigned, and who may be left out.

Syllabus statements

Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.

A1.2.1DNA carries the genetic information in every living organismp. 14
A1.2.2The three parts of a nucleotidep. 15
A1.2.3How sugars and phosphates bond to form the backbone of DNA and RNAp. 16
A1.2.4The four bases in each nucleic acid and how they form a codep. 17
A1.2.5RNA: a polymer built when nucleotides join by condensationp. 17
A1.2.6The DNA double helix: two antiparallel strands held by hydrogen bonds between complementary basesp. 19
A1.2.7How DNA and RNA differp. 22
A1.2.8How complementary base pairing allows genetic information to be copied and usedp. 23
A1.2.9Why DNA can hold an almost unlimited amount of information in its many possible base sequencesp. 23
A1.2.10The shared genetic code as evidence that all life has a common ancestorp. 24

HL extension

AHL · Biomolecules