Cellular Control(F215) How DNA codes for proteins Transcription of DNA

Cellular Control(F215)

How DNA codes for proteins

Transcription of DNA

Learning objectives(a) state that genes code for polypeptides, including enzymes;

(b) explain the meaning of the term genetic code;

(c) describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino acid sequence in a polypeptide;

What is a gene?

• Genes are lengths of DNA, made from a sequence of nucleotide bases.

• These code for one or more polypeptides.

• Genes are units of hereditary – they can be passed on.

• The majority of genes are found inside the nucleus of a cell, although some are found in the mitochondria.

• The specific space where a gene is found is known as the locus.

Histone proteins

One molecule of DNA

The genetic code• 4 different bases (A, T, C and G)

• Triplet codes make 64 different combinations (43) of amino acid (there are only 20 so this is plenty)

• A degenerate code (several codes with the same meaning) except methionine

• Some codes do not code for an amino acid, but for a “stop” or end of the polypeptide chain sequence.

• Some code is universal e.g. serine is the same in all organisms (TCT) – this is useful in genetic engineering as gene transfer will still result in production of the same protein

Degenerate code

RNA Structure

• This is a second type of nucleic acid called ribonucleic acid

• RNA is also made up of smaller subunits called nucleotides

1. Phosphate group2. Ribose Sugar3. Base

RNA Structure

• Unlike DNA, RNA only has one nucleotide strand.

• RNA has one base that is different from DNA – the base Uracil replaces Thymine and so Uracil now becomes the complementary base partner of Adenine.

Messenger RNA

• DNA is the “instruction manual” for making proteins, but protein synthesis itself occurs in the cytoplasm.

• Messenger RNA (mRNA) a type of RNA copies information from a particular strand of DNA and carries it out into the cytoplasm.

• This process is called transcription.

Transcription

• In order to copy the set of instructions needed to make a particular protein, the section of the DNA helix where the gene is located needs to unwind and unzip to expose the code (series of bases).

• The unwound and unzipped part of DNA that is to be copied is known as the DNA template.

(Note: bases in DNA easily break apart because the bonding here is weak)

Transcription• Free RNA nucleotides start to pair up with their complementary

base of DNA on the strand opposite them and form weak hydrogen bonds.

• The RNA nucleotides are activated – this means that they have two extra phosphoryl groups attached (ATP, UTP, GTP and CTP)

• The RNA nucleotides start to link together by forming strong chemical bonds between the phosphate of one nucleotide linking on to the sugar of another to make a single RNA strand. This is called messenger RNA or mRNA.

• The two extra phosphoryl groups are released, which releases energy for bonding between adjacent nucleotides

• The enzyme RNA polymerase controls this process.

Weak hydrogen bonds forming between bases

Strong chemical bonds forming between sugar and phosphate groups. Energy for this comes from the release of phosphoryl

groups

Bonding between ribose sugar and phosphate group of the next nucleotide

Transcription

• Once the set of instructions have been copied from the DNA strand onto mRNA, the weak hydrogen bonds binding the bases of DNA and mRNA together start to break.

Transcription

• The newly transcribed section of mRNA now leaves the nucleus via the nuclear pores and enters the cytoplasm.

• Hydrogen bonds between DNA bases re-form and DNA winds back up into a double helix.

Codons, starting and stopping

• A group of three adjacent bases in mRNA are known as a codon.

• Different sequences of bases code for different amino acids – this is the genetic code

Codons, starting and stopping

• Some of these codons tell the cell when to start and stop production of protein

• These are known as start and stop codons.

• They are found at the beginning and end of the gene e.g TAG is a stop codon

Transcription Summary•Occurs in the nucleus

•A copy of the DNA code (gene) for a protein is made – in the form of m-RNA

•To copy the DNA code, the DNA must unwind and then unzip to expose its base sequence

•A selection of RNA nucleotides must then be available to lock into the DNA strand

•These nucleotides pair up with the exposed bases of the DNA

•The base pairing is – A with U, T with A, C with G, G with C

•They then join to each other forming a single strand of mRNA

•3 nucleotides in mRNA make a codon. Some codons are found at the start and the end of a protein code to start and stop protein synthesis

•The mRNA leaves the nucleus via a nuclear pore to travel to a ribosome

G

A T

C

C

C

C

G

G

G

A

A

T

T

AT

A C G C T A A T C G T A G C A T G C A T

T G C G A T T A G C A T C G T A C G T A



the double stranded DNA starts to unwind...









exposing the sense and antisense (DNA) strands





antisense strand (DNA)

sense strand (mRNA)









RNA polymerase enzyme









U G

C A



U G

CA

free mRNA nucleotides



U G

C

A



U G

C

A



U G

C

A



UG

C

A



UG C

A



U G C

A

the free nucleotides bind with their complementary base pairs



U G C

A



U G C

A

G



U G C AG

the RNA polymerase moves along the sense strand of DNA



U G C AG U U A



U G C AG U U A G C A



U G C AG U U A G C A U C G



U G C AG U U A G C A U C G U A C



U G C AG U U A G C A U C G U A C G U A




when a “stop” sequence is reached the enzyme becomes detached



























U G C AG U U A G C A U C G U A C G U Athe double stranded DNA rewinds and the mRNA is released



U G C U U A G C A U C G U A C G U A


T G C G A T T A G C A T C G T A C G T A U U U A G C A U C G U A C G U A











U U A G C A U C G U A C G U A

A U U A G C A U C G U A C G U A




the mRNA leaves the nucleus via a pore in the nuclear membrane




U G C U U A G C A U C G U A C G U A

Animations

Animation 1

Animation 2

(3 minutes each)

http://www.youtube.com/watch?v=983lhh20rGY

http://www.youtube.com/watch?v=NJxobgkPEAo

Exam questions1. Describe the role of mRNA

and the role of tRNA (2 marks)

2. A piece of DNA has the following nucleotide sequence

AGAAGAATACACCGT

a) How many amino acids does this sequence code for?

b) Using the table opposite, write down the amino acid sequence it codes for.

Amino acid DNA sequence

Serine AGA

Leucine GAT

Tyrosine ATA

Valine CAC

Alaine CGT

arginine GCG

Exam answers1. mRNA carries the genetic code from the DNA in the nucleus to

the cytoplasm, where it is used to make a protein during translation (1 mark). tRNA carries the amino acids that are used to make protein to the ribosomes during translation

2. a)5 amino acids

b) AGA = serineATA = tyrosineCAC = valineCGT = alanine

Serine, serine, tyrosine, valine, alanine

Quiz!

1. What type of nucleic acid is shown in this diagram?

2. What type of bond does the arrow point to?

3. What base is missing from RNA that is present in DNA?

4. What base is it substituted with?

5. Where does the first stage of protein synthesis take place?

6. When does RNA polymerase stop making mRNA?

Answers

1. Ribonucleic acid

2. Covalent bond

3. Thymine

4. Uracil

5. Nucleus

6. mRNA will stop being made when RNA polymerase reaches a stop codon.

Documents

Cellular Control(F215) How DNA codes for proteins Transcription of DNA