Aqa A2 Biology Unit 5 - Genetic Control

Front 1

What are the main features of the genetic code?

Back 1

1. The triplet code, 3 bases to one amino acid.

2. The code is degenerate.

3. The code is non-overlapping.

4. The code is universal.

Front 2

Describe the structure of RNA

Back 2

1. Ribose pentose sugar.

2. Bases are A G C U.

3. A phosphate group.

Front 3

Briefly describe mRNA and how is it suited to its structure.

Back 3

It is manufactured when DN forms a copy of itself in the nucleus. It then leaves via the nuclear pores and enters the cytoplasm where it associates with the ribosomes.

Its suited to its structure because it possesses the correct sequence of organic bases and is easily broken down so only exists while it is needed to manufacture a protein.

Front 4

Describe the structure of tRNA.

Back 4

A single stranded polynucleotide chain folded into a clover shape. Three bases form an anticodon at one end. There is also a point of attachment for an amino acid. For each amino acid that can be attached there is a different sequence of organic bases at the anticodon.

Front 5

Describe transcription.

Back 5

1. DNA helicase unwinds DNA strand at the specific region to be copied by breaking the Hydrogen bonds. It starts at the promotor region.

2. RNA polymerase uses template strand to create a new polynucleotide using free organic nitrogenous bases that are complimentary to the template strand until the stop codon.

3. Hydrogen bonds reform on the DNA to make a double helix.

4. This forms premRNA.

Front 6

Describe splicing.

Back 6

Non coding introns are cut out so that mRNA only contains exons. This is done by a spliceosome complex.

Front 7

What are the evolutionary advantages of not having introns?

Back 7

1. Protein synthesis is faster so organisms can grow and colonise an area more quickly.

2. More rapid division as there's less DNA, this means there is a shorter reproductive period.

Front 8

What are the evolutionary advantages of having introns?

Back 8

1. Mutation to intron that is disadvantageous won't affect the phenotype.

2. Alternative splicing can produce different proteins from the same gene.

Front 9

Describe translation.

Back 9

1. Ribosome attaches to one end of the mRNA with the start codon.

2. tRNA with complimentary bases to the first triplet pairs up with the mRNA.

3. the tRNA continues to match up a sequence of amino acids which have complimentary anticodons. These are joined with peptide bonds.

4. The tRNA detaches from the amino acid it has paired up with to be used again.

5. This continues until the stop codon.

6. The polypeptide then folds into a secondary, tertiary, quaternary structure.

Front 10

What are the basic types of gene mutation?

Back 10

Addition, deletion, substitution.

Front 11

What is a nonsense mutation?

Back 11

This occurs if one of the bases are changed resulting in a stop codon. The polypeptide production is stopped prematurely and the resultant protein will have a very different structure which is unlikely to be suited to its function.

Front 12

What is a mis-sense mutation?

Back 12

This occurs when a base change results in a different amino acid being coded for. This means that the polypeptide differs by one amino acid. The protein could then have a different tertiary structure, which could prevent it from functioning properly.

Front 13

What is a silent mutation?

Back 13

This occurs when a substituted bas, although different codes for the same amino acid due to the degeneracy of the code. The mutation therefore has no effect.

Front 14

Why does deletion or addition of one base nearly always produce a non functional protein?

Back 14

Because it causes a frame shift. Therefore the gene is read in the wrong three base groups. A deletion/addition near the end of the gene will have a smaller effect than at the start of the gene.

Front 15

What can cause mutations?

Back 15

Mutations happen naturally but can be stimulated by:

1. High energy radiation that can disrupt the DNA molecule.

2. Chemicals that alter the DNA structure of interfere with transcription.

Front 16

What is the rate of cell division controlled by?

Back 16

Protooncogenes - which stimulate cell division.

Tumour Suppressor genes - which slow cell division.

Front 17

In a healthy person how do growth factors switch on cell division?

Back 17

Growth factors attach to a specific receptor on the cell surface membrane.

Relay proteins travel through the cytoplasm and nuclear proteins switch on the genes needed for DNA replication.

Front 18

What can cause uncontrolled cell division?

Back 18

Proto-oncogenes can mutate into oncogenes. These can affect cell division by:

1. The receptor protein on the cell surface membrane being permanently activated so that cell division is switched on even in the absence of growth factors.

2. Oncogene may cause growth factors to be produced in excessive amounts, causing excessive cell division.

Front 19

What is a totipotent cell?

Back 19

A cell which has the potential to differentiate into any type of specialised cell. Eg. a stem cell.

Front 20

How are genes prevented from being expressed in a totipotent cell?

Back 20

1. By preventing transcription hence the production of mRNA.

2. By breaking down the mRNA before it can be translated.

Front 21

What is a blastocyst?

Back 21

A few days after fertilisation the cells form a blastocyst. The inner cells are pluripotent meaning they can differentiate into a few cells.

Front 22

What do transcription factors do?

Back 22

For transcription to begin, the gene needs to be stimulated by a specific transcription factor that moves from the cytoplasm to the nucleus. Each transcription factor has a site that binds to a specific region of DNA in the nucleus. When it binds, it stimulates this region of DNA to begin the process of transcription. mRNA is produced and the genetic code it carries is then translated into a polypeptide. When a gene isn't being expressed, the site on the transcription factor that binds to DNA is blocked by an inhibitor molecule. Therefore transcription cannot be stimulated.

Front 23

How can hormones like oestrogen 'switch on' genes?

Back 23

1. Oestrogen is lipid soluble so can diffuse through the phospholipid bilayer.

2. Once inside the cytoplasm, oestrogen combines with a site on a receptor molecule of the transcription factor as they have complimentary shapes.

3. By combining with the site, the oestrogen changes the shape of the receptor molecule which releases the inhibitor molecule from the DNA binding site on the transcription factor.

4. The transcription factor can now enter the nucleus through a nuclear pore and combine with DNA to stimulate transcription of the specific gene.

Front 24

How do siRNA work?

Back 24

They can prevent a gene from being expressed.

Double stranded RNA is put in a liposome so it can pass through the cell surface membrane. A dicer enzyme cuts up the RNA into smaller pieces of double stranded RNA called siRNA. The siRNA are recognised by a protein complex called RISC. It separates the two strands to produce single stranded siRNA. One of the strands is discarded but the one complimentary to the mRNA is kept. The siRNA and complimentary mRNA are bought together by RISC. This causes cleavage of the mRNA so this gene cannot be translated into a protein.

Front 25

What are the medical and scientific roles of siRNA?

Back 25

1. Using it to identify the role of genes in a biological pathway. Some siRNA that blocks a particular gene can be added to cells. By observing the cells afterwards we can determine the role of the blocked gene.

2. As some diseases are caused by genes it may be possible to use siRNA to block these genes and so prevent the disease.