genetic information

What is the purpose of transcription?

To create an mRNA copy of a gene from the DNA template

Steps of transcription? (9)

1. The RNA polymerase enzyme binds to DNA at the beginning of a gene

2. The hydrogen bonds between the DNA bases are broken by DNA helicase and the two strands of the double helix uncoil and separate exposing some of the bases

3. The template strand acts as a template for mRNA synthesis that runs 3 to 5 as RNA polymerase can only build RNA in the 5 to 3 direction

4. Free RNA nucleotides are lined up by RNA polymerase along the DNA template through complementary base pairing. The free nucleotides are attracted to the exposed bases

5. The nucleotides bind to complementary bases uracil to adenine, adenine to thymine, cytosine to guanine

6. RNA polymerase catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides

7. A complementary mRNA strand is formed carrying the same base sequence as the DNA non template strand

8. The process ends when RNA polymerase reaches a stop codon detaches from DNA and terminates transcription

9. mRNA detaches from DNA and moves out of the nucleus through nuclear pores and DNA rewinds into its double helix structure

2 differences between transcription in prokaryotes and eukaryotes

mRNA is directly produced from transcription without any splicing in prokaryotes, pre-mrna is spliced after transcription to remove introns in eukaryotes, it takes place in the cytoplasm in prokaryotes and in the nucleus in eukaryotes

Where does translation occur?

It occurs in the cytoplasm

What is translation?

The process of decoding the information in mRNA to synthesise a polypeptide chain with the help of tRNA

Explain translation (9)?

1. The ribosome attaches to the mRNA strand at a start codon

2. A trna molecule carrying a specific amino acid with an anticodon that is complementary to the start codon, binds to the mRNA (ATP provides the energy needed for the bond between amino acids and trna to form

3. A second trna molecule with an anticodon complementary to the next mrna codon and also carrying a specific amino acids attaches to mrna

4. The amino acids carried by the first two trna molecules are linked together via a peptide bond using ATP

5. The first trna molecules detaches from mrna and is free to collect another amino acid for future use

6. The ribosome moves along mRNA allowing another tRNA molecule which carries the next amino acid to bind to the next codon on mRNA

7. Steps 4-6 repeat making the polypeptide chain longer and longer

8. Process continues until the ribosome reaches a stop codon on mRNA

9. The completed polypeptide chain detaches from the ribosome

What is a gene mutation?

A change in the DNA base sequence

What increases the rates of mutations?

Ultraviolet, ionising radiation, chemicals and viruses

What is substitution?

One base is replaced by another

What is deletion

One base is removed from the sequence

What is insertion?

One base is added into the sequence

What is degenerate?

Many amino acids are coded for by more than one triplet

What is polyploidy

When organisms have more than two sets of chromosomes

What is non disjunction

This occurs when homologous chromosomes fail to seperate in meiosis resulting in gametes with extra or missing chromosomes so individuals with extra or missing chromosomes in all cells

What is non overlapping

Each base triplet is only read once in a dna base sequence

What is universal

The same base triplets code for the same amino acids in all living things

What does deletion cause?

Frame shift