Protein Synthesis- Transcription

What does DNA contain?

the genetic code to make proteins.

What is needed to translate the genetic code into a protein?

mRNA & tRNA

Describe DNA:

This molecule contains the genetic code e.g the base sequence that determines the amino acid sequence in a polypeptide. The antisense strand acts as a template during transcription.

Describe mRNA:

This molecule is made during transcription in the nucleus and is copy of the DNA antisense strand. It is made up of codons (triplet of bases) which code for particular amino acids. It is free to leave the nucleus and attaches to the ribosome.

Describe tRNA:

This molecule attaches to specific amino acids and carries them into the ribosome. Two of these molecules can enter the ribosome at one time. They have anticodons which complementary base pair with mRNA codons allowing them to hold the amino acids in place when condensation reactions occur to form peptide bonds.

What is RNA?

• a single stranded molecule

• It is made up of RNA mono nucleotides.

What does RNA mono nucleotide consist of?

phosphate group, ribose sugar and nitrogenous bases.

What are the 4 bases in RNA?

1. Adenine

2. Uracil

3. Cytosine

4. Guanine

Describe the structure of an RNA mononucleotide

A phosphate group covalently bonded to a ribose sugar and a ribose sugar covalently bonded to a nitrogenous base (either A, U, C, G

Give the definition of a gene

A specific sequence of bases in a section of DNA that codes for a protein

How are proteins produced?

• The genetic code is a sequence of base triplets (codons) in DNA or mRNA which codes for a specific amino acid.

• Each triplet base is read in sequence. The triplet code is discrete and code is non-overlapping.

• Protein synthesis is split into two stages called transcription and translation.

Explain the stages of transcription:

1. DNA Helicase breaks hydrogen bonds between the bases on the two stands. The DNA molecule unwinds One strand acts as a template strand (antisense strand).

2. RNA polymerase catalyses condensation reactions leading to the formation of phosphodiester bonds between adjacent RNA mono-nucleotides, creating a strand of mRNA known as the sense strand.

3. Free RNA nucleotides line up next to the template stand according to complementary base pair rules.

4. mRNA detaches from the DNA and leaves the nucleus via a nuclear pore.