Learning Intentions

• To describe the structure of DNA

• To extract DNA from plant matter

• To explain semi-conservative replication

Genes

• Genes contain the chemical code which controls the production of proteins.

• The code is contained in deoxyribonucleic acid (DNA) molecules.

• The structure of DNA enables the correct sequence of amino acids to make a particular protein.

Structure of DNA

• A DNA molecule is made up is 2 chains of nucleotides twisted into a double helix

Structure of a nucleotide

phosphate

sugar base

There are four different nucleotide bases

• Nucleotides are linked together by bonds between phosphate and sugar molecules

• Adenine

• Guanine

• Thymine

• Cytosine

Structure of DNA

• Two nucleotide chains are joined by

hydrogen bonds between the bases.

• Adenine on one chain always bonds to

thymine on the other

• Guanine bonds with cytosine

• The two linked

nucleotides

chains are

twisted into a

coil called a

double helix.

Starter Question Without looking at your notes answer the following questions

1. What shape is DNA?

2. What does a DNA nucleotide look like? Draw a fully labelled nucleotide in the box below.

3. What are the names of the four bases found in DNA?

4. How do these bases pair up in a molecule of DNA? Write the bases in the boxes below.

DNA Structure to function

• Information storage • Long molecules

• Replication • Base-paring rules

• Hydrogen bonds

• Stable

DNA REPLICATION

Learning Intentions

• To explain semi-conservative replication

DNA Replication

• DNA can reproduce itself exactly, this process is called replication.

• Replication requires • DNA – to act as a template

• A supply of the four types of nucleotide

• The appropriate enzymes

• ATP – for energy

DNA Replication

• When DNA replicate the molecule untwists, the two strands separate and unattached nucleotides join on to the parent chain (A with T, G with C).

• Neighbouring nucleotides then form sugar phosphate bonds and a new half of the DNA molecule is created, complementary to the parent half

• Each strand acts as a template for making a new strand • This is known as semi-conservative replication

Experimental Evidence for the semi-conservative replication of DNA • Three ways were suggested for DNA

replication • Conservative replication

• Semi-conservative replication

• Dispersive replication

• Scientists thought that semi-conservative replication was most likely but there was no evidence to support this theory.

• 1958 Matthew Meselsohn and Franklin Stahl demonstrated that DNA replication was semi-conservative following experiments with E. Coli.

Stage 1

• E. Coli were grown in a medium containing a heavy isotope nitrogen (15N).

• The bacteria used 15N to make the purine and pyrimidine bases in its DNA.

Stage 2

• After many generations, they were then transferred to light isotope nitrogen (14N)

Stage 3

• Bacteria were taken from the new medium after one generation, two generations and later generations.

• DNA was extracted from each group of bacteria,

• samples were placed in a solution of caesium chloride and spun in a centrifuge.

Results

Generation 1 2 3

Conclusions

• Explain why the band of DNA in the first generation is higher than that in the parental generation.

• If replication were conservative what results would you expect in the first generation?

• If the DNA had replicated dispersively what results would you expect in the first generation?

• Explain how the second generation provides evidence that the DNA has reproduced semi-conservatively and not dispersively

• What results would you expect to see from a third generation, draw a diagram of the results?

Explanation of results

• Parental generation - both strands made with 15N

• First generation – DNA made of one strand 15N and one strand 14N

• Second generation – some DNA made of 2 strands of 14N and some made of 15N and 14N.

DNA Replication

• Double helix unwinds and the DNA “unzips” as hydrogen bonds break

• Existing strand acts as a template for assembly of nucleotides

• Free nucleotides move towards exposed bases of DNA

• Base pairing occurs between free nucleotides and exposed bases

• Enzyme DNA polymerase forms covalent bonds between free nucleotides

• Two daughter DNA molecules form separate double helices.

Semi-conservative replication • Each new DNA molecule receives one

strand from the original parent molecule.

RNA and Protein Synthesis

RNA Nucleotide

• Nucleotides are made up • base

• Ribose sugar

• Phosphate

Phosphate Ribose sugar

base

RNA Bases

• There are 4 bases: • A Adenine

• U Uracil

• G Guanine

• C Cytosine

• Complementary base pairing • A U

• G C

Comparing DNA and RNA

RNA DNA

Number of strands one two

Complementary base

pair of adenine uracil thymine

sugar Ribose Deoxyribose

DNA and Protein Synthesis

• All chemical reactions are controlled by enzymes, all enzymes are proteins, DNA codes for proteins, therefore DNA controls all the activities of a cell.

• The shape and behaviour of a protein depends on the exact sequence of amino acids in the primary structure (polypeptide).

The Genetic Code

• DNA determines the exact order in which amino acids join together.

• The genetic code • sequence of bases along the DNA molecule, • There are 20 different amino acids, only 4 bases, • a sequence of 3 bases codes for an amino acid. • This is called the triplet code.

• A gene is the part of a DNA molecule, which codes for just one polypeptide.

RNA

• mRNA • Messenger RNA • Carries the genetic information from the

nucleus into the cytoplasm • Triplet of bases is a codon

• tRNA • Transfer RNA • Only a triplet of bases is exposed – anti-codon • Transfers amino acids from cytoplasm to the

ribosome

Transcription

• This is the process by which mRNA is built up against one side of an opened up piece of DNA.

• The relevant section of DNA unwinds, the hydrogen bonds between base pairs are broken and the two strands split apart. Free nucleotides then assemble against one strand of DNA.

• The enzyme RNA polymerase moves along the DNA adding on RNA nucleotide at a time.

• Each triplet of bases is called a codon.

Transcription

Transcription

1. DNA strand unwinds

2. Breaking of hydrogen bonds between two bases causing the DNA strand to separate

3. Free RNA nucleotides find the complementary base on the DNA strand being transcribed

4. Weak hydrogen bonds form between complementary bases

5. RNA polymerase links RNA nucleotides to form the sugar-phosphate backbone

6. Weak H bonds between DNA and RNA bases break the transcribed mRNA separates from the DNA template

7. mRNA moves out of nucleus and into the cytoplasm

8. DNA strands reunite to form double helix

Movement of mRNA to ribosomes • mRNA leaves the nucleus through a

nuclear pore into the cytoplasm, and attaches to a ribosome.

Amino Acid Activation

• Enzymes attach amino acids to their specific tRNA molecule.

• This needs energy supplied by ATP.

Ribosomes

• Small spherical structures

• occur freely in cytoplasm • Proteins made are for cell use

• Occur attached to endoplasmic reticulum • Proteins made are for export

• The site of protein synthesis.

Translation

• Amino acid attaches to the ribosome

• Adjacent amino acids are joined together by peptide bonds and a polypeptide chain is built up.

• This carries on until the ribosome reaches a stop codon, the completed polypeptide is released into the cytoplasm

• Summary of protein synthesis

• Read the information provided on the reverse of the sheet and colour in as instructed

• You will face a short “protein synthesis” test after 25 minutes

• Describe the steps in the synthesis of the enzyme pepsin under the following headings:

1. Copying genetic information as mRNA

2. Translating the genetic information into protein

Copying genetic information as mRNA

• Transcription • DNA unzips, • Short length / one gene for one protein • Enzymes / ATP required • Free RNA nucleotides line up on one strand of DNA • Complementary base pairing

• G-C, A–T, U-A

• Nucleotides mRNA join / sugar phosphate backbone

• Sequence DNA bases species sequence mRNA bases

• mRNA migrates to cytoplasm • Through pores in nuclear membrane

Translating the genetic information into protein • mRNA attaches to ribosome

• 3 bases on mRNA code for an amino acid

• Triplet is known as codon

• tRNA has anti-codon / base triplet

• tRNA anticodon matches complementary codon

• tRNA brings amino acid

• Each tRNA specific for an amino acid

• Peptide bonds join amino acids

• Chain of amino acids forms polypeptide

• Sequence bases on mRNA determines amino acid order

What happens to the RNA?

• mRNA is reused to produce more of the same polypeptide

• tRNA attaches to another amino acid ready to go

Rough Endoplasmic Reticulum (RER) • Bears ribosomes on outer surface of membrane

• Continuous with the outer nuclear membrane

• Provides a large surface area for chemical reactions to occur.

• Folding of proteins

Golgi Apparatus

• Flattened fluid filled sacs

• Vesicles from RER pinch off an fuse with Golgi Apparatus

• Golgi modifies the proteins • E.g. attaching a carbohydrate to make a

glycoprotein

• Finished products are pinched off in a vesicle which moves towards and fuses with the cell membrane • This is secretion by exocytosis

• Describe the steps in the synthesis of the enzyme pepsin under the following heading: • Secretion of the enzyme

LESSON STARTER

• A gene codes for the production of a protein • Name as many different types of protein in the

human body and state the function of each

Functional Variety of Proteins

Learning Intentions

• To examine the primary, secondary and tertiary structure of proteins

• To revise and expand the role of proteins

Protein Structure

• Proteins are organic compounds containing C, H, O and N, some contain S.

Primary structure

• The sub-unit is the amino acid, which are joined together by a peptide bond to form polypeptides.

• The sequence of amino acids in the polypeptide determines the structure and function of the protein.

Polypeptide Chain

Bonds

• Peptide bonds join the amino acids together in a protein

• Hydrogen bonds shape the protein chain. This affects the protein shape

Secondary structure

• Hydrogen bonds form between amino acids coiling the polypeptide into a helix.

Tertiary Structure

• Several polypeptide chains become linked by bridges between sulphur atoms (di-sulphide bridge) and further hydrogen bonds.

• This determines the final structure of the protein to enable it to carry out its specific function.

Tertiary Structure

• Proteins can either be: • Fibrous e.g. collagen and keratin

• Globular protein e.g. insulin

• Conjugated e.g. haemoglobin

Fibrous proteins

• Polypeptides linked in parallel strands

• Structural properties

• Examples • Elastin – walls of large arteries

• Collagen – bone, tendons and ligaments

• Keratin – hair

• Actin and myosin – muscle contraction

Globular Proteins

• Polypeptides folded into a spherical shape

• Enzymes are folded to expose and active site which is specific for a substrate.

• Structural proteins e.g. cell membrane

• Hormones are chemical messengers e.g. insulin and thyroxine

• Antibodies are used in defence against disease.

Enzymes are globular proteins

Antibodies are also globular proteins

Conjugated proteins

• Globular protein with a chemical

• Examples include • Glycoprotein e.g. mucus

• Haemoglobin has a haem group (contains iron)#

• cytochrome

Conjugated Proteins

haemoglobin

Summary of Protein structure

• Primary structure • Sequence of amino acids

• Secondary structure • H Bonds – twist into helix

• Tertiary structure • Further folding or coiling

• Disulphide bonds

• Hydrogen bonds

• Quaternary Structure • Adding of non-protein groups • More than 1 polypeptide

chain

Pupil Activity – 15 minutes

• Torrance, J. pg 61 • Give three examples of a fibrous protein and

for each one state its function

• Name 3 types of globular protein and briefly summarise the role played by each one.

• Give two examples of a conjugated protein and explain their function.

Applying your knowledge time allowed 5 minutes

Name of protein Type of protein

(F, G or C)

Role of this protein

Antibody

Catalase

Cytochrome

Haemoglobin

Insulin

keratin

ERQ Protein Synthesis

• Give an account of the structure of DNA (deoxyribonucleic acid) and its control of protein synthesis within a cell.

• Give an account of the structure of protein and its synthesis and functions within a cell.