DNA and the genetic code

Write out the following DNA organic bases and match them with their specific and complementary base pairs:

A

G

T

A

C

C

C

T

G

A

A

G

T

A

G

A

T

G

C

A

T

DNA organic bases matched with their specific and complementary base pairs:

A

G

T

A

C

C

C

T

G

A

A

G

T

A

G

A

T

G

C

A

T

A

A A

A

A

T

T

T

T

T

T

T

G

G

G

G

C

C

C

C C

See p9 of HB

Remember that:

• A gene is a sequence of organic bases on DNA that code for the order of amino acids to make a polypeptide chain (or protein)

Complete the top third of your handbook p11 now.

produceproteins

(including enzymes)

genes

Important points:See HB p11 and highlight these points in 3 different colours:

1)The order (or sequence) and number of amino acids differ from one protein to the next.

2)It is the order (or sequence) of organic bases on DNA that determines the order of amino acids in a protein.

3)Different sequences of bases on DNA code for different amino acids.

Do the task at the top of HB p12 starting from ’Only one of these strands actually codes for the protein….’

DNA code = GCG TGC GAA TCAAmino acids Arg – Thr – Leu – Ser Coded for =

So….

• DNA codes for proteins.

• The sequence of organic bases down the DNA controls the sequence of amino acids in a polypeptide.

• Remember, this is called the Primary structure of a protein.

• This genetic information is passed on to daughter cells during mitosis cell division, when the DNA replicates.

How many organic bases along DNA are needed to code for each individual amino acid?• Fact = There are 20 different types of amino

acids.• If one organic base on DNA coded for one amino

acid - only 4 amino acids could possibly be coded for. (This is not enough!)

• If 2 organic bases along DNA coded for each amino acid –

only 4X4 = 16 amino acids could possibly be coded for. (This is still not enough!)

• If 3 bases coded for each amino acid – 4X4X4 = 64 amino acids could possibly be coded

for. (This is now more than enough!)

The Triplet Code

Note: These codes are from mRNA where there is alwaysa U (Uracil) organic base put instead of a T (Thymine) base.

THE GENETIC CODE IS A TRIPLET CODE OF ORGANIC BASES

The Genetic code

• It is a Triplet Code – three organic bases code for one amino acid.• It is Universal – the same triplet, codes for the same amino acid in all

organisms on the planet!• It is Non Overlapping – an individual base does not occur in more than one

triplet. GCGTGCGAA GCGTGCGAA

• It is Degenerate – there is more than one code for some amino acids (ie It has spare coding power).

• Within a gene some parts are coding regions of DNA (called EXONS), some regions are non coding regions (called INTRONS). Other non-used DNA sections are called ‘Multiple repeats’.

• A ‘stop’ triplet codes for the end of a particular gene.• Often it is only the first two bases of the triplet that are specific for a

particular amino acid and any third base will do.

“A gene is the sequence of bases in DNA that codes for the order of amino acids in a polypeptide chain” note polypeptide is a better word than protein.

DNA protein/enzyme control cellFeatures of the code:

• EXON = coding region of DNA

• INTRON = non coding region of DNA

Now do diag of a single chromosome from JA’s pdf

Draw and label a homologous pair of chromosomes from JA’s pdf.

Diagram of a Chromosome

• EXON = coding region of DNA

• INTRON = non coding region of DNA

• Now complete bottom half of p11 in your handbook and the first question at the top of handbook p12.

• Now complete bottom second half of handbook p12.

ANSWERS TO QUESTIONSON BOTTOM HALF OF HB

p12

Diploid Number of Chromosomes

A term used for cells in which the nucleus contains 2 sets of chromosomes (ie in ‘body cells’ not gametes). (2n)

In Humans the Diploid number of chromosomes is 46.

• The diploid number of chromosomes in a human cheek cell is 46

Alleles• Alleles are different forms of the same gene

Eg The gene for eye colour will have several different alleles – an allele for blue eyes, an allele for brown eyes, an allele for green eyes etc.

The alleles of a particular gene are found at the SAME GENE LOCUS on a pair of homologous chromosomes (see diag on next slide)……….

Alleles (continued)

Different alleles of the same gene do have some different DNA organic base sequences along them and therefore

different alleles code for a different sequence of amino acids and hence a different protein!

The protein would fold differently due to different places that hydrogen, ionic and disulphide bonds have occurred. This would lead to a different tertiary structure.

1. If a different organic base appears in the sequence for a gene.

2. This could code for a different amino acid.

3. Leads to different primary structure of the polypeptide chain.

4. Polypeptide then folds differently into a different tertiary shape for the protein. (bonds occur in different places).

5. If the protein is an enzyme, the active site may be changed.

6. Now the substrate can’t bind to the active site/ no enzyme substrate complexes form.

7. Protein/enzyme is said to be non-functioning.

• Answer the ‘peach box’ summary questions 1,2 and 3 on the top of AS textbook p137.

• Complete the ‘green Application exercise’ Q1 just parts (a) and (b) page 137.

Now check your answers from p249

section 8.2.

The answer to Q3 is VERY IMPORTANT, ensure you have this written down in full.

HW (challenge!)Experimental evidence for DNA replication

being semi-conservative

Read textbook p166-167 extremely carefully.

Use this info to complete bottom p13 and p14 of handbook (part 3)

Do meiosis next.