The Basics of DNA, Transcription, Translation, and RNA

chromosomes

• the nucleus contains genetic information in the form of chromosomes.

• Chromosomes are long lengths of DNA coiled up.

• human body cells are diploid. They have 2 copies of each chromosome, arranged in pairs.

• a cell contains 46 chromosomes in total.

genes

• a gene is a short section of DNA.

• each separate gene in a DNA molecule is a chemical instruction that codes for a particular protein.

• genes control are inherited characteristics.

genome

the entire DNA of an organism.

DNA is a double Helix…

• a DNA molecule has 2 strands coiled together in the shape of a double helix (two spirals.)

• the 2 strands are held together by chemicals called bases.

name the 4 different bases in a double helix.

1. adenine (A)

2. Cytosine (C)

3. Guanine (G)

4. Thymine (T)

• The bases are paired, and always pair up in the same way:

  • A-T

  • C-G

• This is called complementary base-pairing.

nucleotides

the molecules DNA is composed of.

RNA

RNA is a type of nucleic acid called ribonucleic acid:

• RNA molecules are single stranded and contain uracil (U) instead of thymine (T.)

• RNA is a shorter molecule and can leave the nucleus in order to take the information on a gene → a ribosome.

  • this enables a protein to be synthesised.

Name 3 differences of DNA vs RNA

• RNA is single stranded while DNA is double stranded.

• RNA has a sugar called ribose while DNA has a sugar called deoxyribose.

• RNA has the base uracil while DNA has the base thymine.

Name the 3 types of RNA

1. mRNA

2. tRNA

3. rRNA

mRNA

• Messenger RNA.

• a molecule which carries a copy of the gene, in order to be expressed from the nucleus.

• Every group of three bases (a 'triplet') is described as a codon because each codon codes for an amino acid.

tRNA

• Transfer RNA.

• made from a single strand of RNA.

• it folds due to the base pairing.

• It has a triplet anti-codon site and an attachment site for a specific amino acid.

rRNA

• Ribosomal RNA and proteins that form the ribosome.

• The ribosome is the structure where protein synthesis takes place.

codon

• a DNA or RNA sequence of 3 nucleotides.

• each amino acid is coded for by a sequence of three bases in the gene - this is called a codon!

anti codon

• sequences of nucleotides that are complementary to codons.

• they are found in tRNA, and enable the tRNA to bring the correct amino acid in line with an mRNA during protein production.

why do DNA molecules contain a genetic code?

• it determines which proteins are built.

• the proteins determine how all the cells in the body function.

Transcription and Translation

the process of turning a gene into a specific sequence of amino acids is split into 2 stages:

1. Transcription.

2. Translation.

Where does Transcription take place, and what does it do?

• happens in the nucleus of the cell.

• produces a copy of a section of DNA, in the form of a strand of mRNA.

describe the process of Transcription

1. a section of the DNA molecule unwinds, when hydrogen bonds between the complementary base pairs break.

2. This exposes the template strand of the gene.

   • this gene codes for the protein that is being synthesised.

3. the free mRNA nucleotides (that are in the nucleus) bind to complementary nucleotides on the template strand.

4. the mRNA nucleotides are joined to neighboring nucleotides.

   • this forms a single strand of mRNA.

5. the mRNA molecule leaves the nucleus via a pore in the nuclear envelope.

Where does Translation take place, and what does it do?

• occurs in the cytoplasm in the nucleus.

• results in the production of a chain of amino acids (which goes on to form a protein.)

Describe the process of Translation

1. after leaving the nucleus, the mRNA molecule attaches to a ribosome.

2. in the cytoplasm, there are free molecules of tRNA:

   • tRNA molecules have a triplet of unpaired bases at one end, the anticodon, and an amino acid at the other end.

   • each specific anticodon corresponds to a specific amino acid.

3. the anticodon on each tRNA molecule pairs with a complementary codon triplet on the mRNA molecule.

   • the mRNA molecule brings its specific amino acid with it.

4. a second tRNA molecule attaches to its complementary codon on the mRNA.

5. a peptide bond is formed between 2 neighboring amino acids.

6. the process continues until a ‘stop’ codon on the mRNA molecule is reached.

   • this is the signal for Translation to stop.

7. the amino acid chain is complete.

what happens to the amino acid once translation is complete?

the amino acid chain is folded and modifies to form the final protein molecule.

eg. an enzyme or antibody.