DNA as the genetic code

What is a gene?

Sequence of nucleotide bases that determines the order of amino acids in polypeptides produced

What are the main bases of the genetic code?

1. Three base (triplet) code - sequence of 3 nucleotide bases codes for 1 amino acid. 4 bases arranged in groups of 3 - 64 sequences. 20 amino acids used for protein synthesis.

2. Degenerate code - since there are more triplet codes (64) than amino acids (20), most amino acids have more than 1 triplet code

3. Non-overlapping code - triplet codes read only once

Why isn't the genetic code doublet?

As a doublet code would only produce 16 codes, insufficient for 20 amino acids

How do you find how many amino acids are in a given number of bases?

Divide by 3

Why is DNA transcribed into RNA and doesn't act directly in protein synthesis?

1. It's a very long molecule so it's too long to move out of nucleus into cytoplasm where protein synthesis takes place

2. It is retained in nucleus where it is better protected from damaging changes

3. Used to produce multiple copies of messenger RNA so many polypeptides can be synthesised simultaneously

What are the 2 stages of protein synthesis?

1. Transcription - genetic code in DNA in nucleus is copied into molecule of mRNA

2. Translation - mRNA molecule then moved out of nucleus onto a ribosome where it's code is used to direct the synthesis of a polypeptide

What is the process of transcription?

1. Two strands of DNA are separated by helicase enzyme using energy from ATP

2. RNA polymerase enzyme binds to region of DNA near beginning of gene that is going to be transcribed

3. The exposed template strand is used as a template for mRNA production. As RNA polymerase moves along the template strand, ribonucleotides are taken from the nucleoplasm and matched up by a process of complementary base pairing

4. Complementary base pairing involved the bases in ribonucleotides forming hydrogen bonds with the exposed bases on DNA template strand and follows base pairing rules - ribonucleotide with Uracil binds with Adenine in DNA strand ect. RNA polymerase binds the newly arrived ribonucleotides along their sugar-phosphate backbone.

5. As RNA polymerase moves along, DNA double helix forms. When it reaches end of gene it releases fully formed mRNA. The mRNA produced is complementary to the nucleotide base sequence on template strand of DNA

What are the similarities and differences between transcription and DNA replication?

Similarities:

Both involve breaking hydrogen bonds to separate DNA strands

Both involve a DNA strand acting as a template

Both occur in nucleus

Differences:

Transcription involved only 1 DNA strand as a template where DNA replication involves both strands

Transcription involves RNA polymerase whereas DNA replication involves DNA polymerase

Transcription involves ribonucleotides where DNA replication involves deoxyribonucleotides

What is pre-mRNA splicing?

In eukaryotes, after transcription, pre-mRNA is made, which is mRNA that still contains the introns (Non-coding sequences of bases).

The introns are spliced out by a protein called a splicesome.

This leaves behind just the exons, (the coding regions) the finished mRNA that is ready to leave the neuclues.

In prokaryotes, transcription directly creates mRNA, as prokaryotes do not contain introns in their DNA.

How do genes code for multiple different polypeptides?

Exons are spliced in a range of combinations

What is translation?

Protein synthesis

What are the components of translation?

mRNA - contains triplets called codons

ribosomes - has 2 sites for mRNA attachment - peptidyl and aminoacyl (P) (A)

tRNA - carry anticodon at one end of triplet base code

What is the process of translation?

1. First 2 codons of mRNA enter a ribosome

2. tRNA molecules with complementary anticodons to the first 2 codons of mRNA bind together

3. Specific amino acids carried by 2 tRNA molecules carried onto ribosomes are joined by peptide bonds with energy supplied from ATP

4. Ribosomes move along mRNA by one codon length, bringing 3rd codon onto aminoacyl site on the ribosome. The freed tRNA returns to cytoplasm.

5. A tRNA with complementary anticodon binds to 3rd mRNA codon which brings its amino acid into position next to second amino acid held at peptidyl site, both amino acids bond and chain is lengthened

6. Ribosome moves along mRNA by one codon, bringing a further codon onto the aminoacyl site. Amino acid carrying tRNA with complementary anticodon binds codon and amino acid bonds existing in chain of amino acids at the peptidyl site

7. Process is repeated until termination code is in position, causing polypeptide to be released and translation of polypeptide is complete.

Where are polypeptides processed to produce final functional proteins?

Golgi apparatus

Newly synthesised polypeptides enter ER and vesicles are nipped off and moved to Golgi apparatus

What is a gene mutation?

Spontaneous change in the DNA molecule

What is nucleotide base substitution?

One base is replaced by another

What is nucleotide base deletion?

One base is omitted all together

What happens during base substitution?

E.g. - thymine on the DNA template strand is replaced by e.g. - adenine (base replaced by base) Resulting in change of DNA code, change in mRNA codon for the amino acid brought onto the ribosome and so a change in the amino acid sequence of polypeptide being synthesised

What causes sickle cell anemia?

Mutation resulting in chsnge in the beta chain of haemoglobin

Why is sickle cell anemia bad?

Changes shape of red blood cells which can clog blood vessels and deprive vital organs of full oxygen supply

What happens during base deletion?

E.g. - guanine is lost from template strand so all other bases move up by one - called frameshift

Change affects all triplets thereafter

Multiple changes in amino acid sequence occurs and final amino acid is missing

How may a mutation lead to a change in an encoded protein?

Change in base sequence in DNA Leads to a change in mRNA which leads to a change in the amino acids sequence of the polypeptide which leads to different globular conformation of the protein

What is epigenetics?

The study of changes in organisms caused by modification of gene expression rather than alteration of the genetic code itself.

What does DNA methylation do?

Prevent attachment of RNA polymerase , inhibiting transcription and preventing gene to be expressed

What does histone acetylation do?

loosens chromatin structure, thereby promoting the initiation of transcription

How does histone modification work?

The binding of epigenetic factors to histone "tails" alters the extent to which DNA is wrapped around histones and the availability of genes in the DNA to be activated

When do epigenetic changes occur?

Oogenesis, spermatogenesis, or early embryonic development

How are epigenetic modifications passed on?

During mitotic devision and from parent to offspring

How do cancer drugs work?

Reverse epigenetic changes by removing acetyl tags on Histones (silence genes) or removing methyl groups (activating genes)