Biological molecules

DNA (deoxyribonucleic acid)

RNA (ribonucleic acid)

Name the two types of nucleic acid

Double helix

What structure does DNA have?

Nucleotides

Name the monomers that make up nucleic acids

Pentose sugar

Phosphate group

A nitrogen containing base

Name the three components of a nucleotide

deoxyribose

Name the pentose sugar in DNA

ribose

Name the pentose sugar in RNA

Cytosine, adenine, guanine, thymine and uracil

Name the five types of nitrogenous base

Condensation reaction

What type of reaction joins the components of nucleotides?

A condensation reaction

A phosphodiester bond forms between the phosphate group of one mononucleotide and the pentose sugar of the other

A molecule of water is removed

A dipeptide is formed

How do two mononucleotides join together?

Cytosine, adenine, guanine and thymine

Name the four bases found in DNA

Cytosine, adenine, guanine and uracil

Name the four bases found in RNA

A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs.

Describe the structure of DNA

Hydrogen bonds between complementary base pairs

How are the two strands of DNA joined together?

Adenine always pairs with thymine (or uracil in RNA)

Guanine always pairs with cytosine

What are complementary base pairing rules?

• The phosphodiester backbone protects the more chemically reactive organic bases inside the double helix

• Hydrogen bonds between bases on different strands (each hydrogen bond it weak but lots of hydrogen bonds in the DNA molecule provide the strength)

• Interactive forces between bases on the same strand (stacking interactions)

Why is DNA stable?

3

How many hydrogen bonds form between cytosine and guanine?

2

How many hydrogen bonds form between adenine and thymine?

• It is very stable so normally passes from generation to generation without a change

• It is capable of self replication

• It is an extremely large molecule and so can carry a large amount of genetic material

• The sugar-phosphate backbone protects the bases

• Base pairing leads to DNA being able to replicate and transfer information as mRNA

-It has two separate strands, only joined with hydrogen bonds. This allows the strands to separate during DNA replication and protein synthesis

• Helix structure makes it compact

-Double stranded so replication can occur semi-conservatively

Why is DNA suitable as genetic material?

Different combinations of bases in the nucleotides

What makes DNA different in different people?

Messenger RNA (mRNA), transfer RNA (tRNA) and Ribosomal RNA (rRNA)

Name the three types of RNA

• The monomers for DNA are deoxyribonucleotides, the monomers for RNA are ribonucleotides

• DNA is much longer than RNA

• There is only one type of DNA, there are three types of RNA

• DNA is double stranded, RNA is single stranded

• In DNA complementary base pairing occurs, RNA is single stranded so no complementary base pairing occurs

Give differences between RNA and DNA

DNA

Which is longer- DNA or RNA?

Single stranded

Is RNA double or single stranded?

Deoxyribonucleotides

Name the monomers that make up DNA

Ribonucleotides

Name the monomers that make up RNA

Semi-conservative replication

Name the process by which DNA replicates

• DNA helicase separates the two DNA strands by breaking the hydrogen bonds between them as a result the helix unwinds

• Free nucleotides in the cytoplasm are attracted to and bind with the exposed polynucleotide strands by complementary base pairing

• DNA polymerase moves along each strand and catalyses the joining of adjacent nucleotides in condensation reactions by forming phosphodiester bonds

• The process continues along the entire length of the DNA until two new DNA molecules are synthesised

• Each new molecule contains one old and one new strand of DNA

Describe the process of semiconservative replication

The original DNA molecule remained intact and that a separate DNA copy was built up from new molecules of deoxyribose, phosphate and bases. Of the two DNA molecules produced. one would be made of entirely new material and one would be made entirely of old material.

What is the conservative model of DNA replication?

The original DNA molecule splits into two separate strands, each of which serves as a template to synthesise a new DNA strand. As a result, each of the two new DNA molecules produced contain one old and one new strand of DNA

What is the semi conservative model of DNA replication?

DNA helicase

Name the enzyme that unwinds the DNA double helix

DNA polymerase

Name the enzyme that catalyses the joining of adjacent nucleotides in condensation reactions

• Grew bacteria and supplied them with nitrogen in the form of the heavy, but non radioactive isotope ¹⁵N

• As a result the DNA of the bacteria became entirely heavy

• The bacteria were then transferred to a medium containing the normal, light isotope ¹⁴N

• Immediately before changing the medium and then at intervals corresponding to successive generations, samples of bacteria were removed and the DNA was extracted

-The extracted DNA showed that immediately before changing the medium all the DNA was heavy

• After one generation all the DNA was of intermediate density (this is to be expected as each new DNA molecule contains one heavy and one light strand)

• After two generations 50% of the DNA was intermediate and 50% was light (again this fits with the semi-conservative model)

How did Meselson and Stahl's experiment provide evidence for the semi conservative mechanism of DNA replication

What is the structure of a deoxynucleotide?

What is the structure of DNA?

• Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip

-Two strands, so both can act as templates

-Complementary base pairing allows accurate replication

Give features of DNA that are important in DNA replication

Pentose sugars contain 5' and 3' carbons

Antiparallel since one strand runs 3' to 5' + the other 5' to 3'

Explain in chemical terms how the polynucleotide chains in DNA are antiparallel.

5' to 3'

In what direction is DNA synthesised?

-DNA polymerase can only join nucleotides in a 5' to 3' direction

• DNA strands are antiparallel

Why does DNA polymerase move in different directions along each template strand?

The smaller units from which larger molecules are made

What are monomers?

Molecules made from a large number of monomers joined together

What are polymers?

A reaction which joins two molecules together with the formation of a chemical bond and involves the elimination of a molecule of water

What is a condensation reaction?

A reaction which uses water to break a chemical bond between two molecules

What is a hydrolysis reaction?

A shared pair of electrons between two atoms

What are covalent bonds?

Electrostatic forces of attraction between oppositely charged ions

What are ionic bonds?

Bonds formed between the slightly negatively charged atom on one molecule and the slightly positively charged hydrogen atom in another molecule

What are hydrogen bonds?

All the chemical reactions taking place in a living organism

What is metabolism?

Carbon, hydrogen and oxygen

Name the elements found in a carbohydrate

A single sugar monomer

What is a monosaccharide?

Two monosaccharides joined together by a glycosidic bond

What is a disaccharide?

From the condensation reaction between two monosaccharides

How are disaccharides formed?

Large sugars made up of many monosaccharides joined by condensation reactions and linked by glycosidic bonds

What are polysaccharides?

(CH₂O)ₙ

n can be any number from 3-7

What is the general formula for monosaccharides?

6

How many carbon atoms does a hexose sugar have?

C₆H₁₂O₆

What is the formula for glucose?

Substances with the same molecular formulae but which have a different arrangement of atoms (structural formulae)

What are isomers?

α- glucose and β-glucose

Name the two isomers of glucose

The gain of electrons or hydrogen

What is reduction?

A sugar that can reduce another chemical

What is a reducing sugar?

All monosaccharides and some disaccharides (e.g matlose)

What sugars are reducing sugars?

Benedict's reagent

What reagent is used to test for reducing sugars?

Blue

What colour is Benedict's reagent?

Orange-brown

(It may turn green, yellow, orange or red)

What colour will Benedict's reagent turn when heated with reducing sugars?

Copper (I) oxide

Name the precipitate formed with reducing sugars are heated with Benedict's solution

1. Add 2cm³ of the food sample to be tested to a test tube. (If the sample is not already in liquid form, first grind it up in water).