1. Cell protein production
Text: Chapter 3
Human Biology Stage 3

2. Keywords DNA Protein synthesis mRNA Transcription Ribosomes tRNA
RNA polymerase
Template strand
Coding strand
Codon
Intron
Exon
Ribosomes
tRNA
Anticodon
Translation
Start codon methionine
Protein structure
Endoplasmic reticulum (ER)
Golgi bodies

3. Protein synthesis
Synthesis is the combining of small molecules to make larger molecules
Proteins are long chains of amino acids. They are very large and consist of several hundred amino acids
Proteins have a coiled shape

4. Functions of proteins Proteins have many different functions:
Structure: the protein keratin gives strength to hair, nails etc.
Transport across membranes: some proteins form channels in cell membranes to aid the transport of molecules
Communication: many hormones are proteins.
Cell metabolism: enzymes are proteins . Enzymes help speed up the chemical reactions in a cell.
Recognition: proteins in the cell membrane are unique to a particular person
Movement: protein molecules are able to change shape and this is the basis for the movement of structures within the cell.

5. The genetic code
The blue print for the body’s proteins are contained within DNA
The genes which form DNA determine the types of proteins a cell is able to make
Genes are made up of a combination of 4 chemical ‘letters’
These chemical ‘letters’ are:
Adenine & thymine
Guanine & cytosine
They always occur in pairs
Each amino acid is coded for in a sequence of 3 bases. This is called a codon

6. Transcription
The DNA molecule is too large to leave the nucleus, so the code for each amino acid is taken from the DNA to the ribosomes by a special molecule called RNA (ribonucleic acid)
This process is called transcription
RNA is slightly different to DNA. Instead of the chemical ‘letter’ thymine, it has the chemical ‘letter’ uracil

7. Transcription There are 2 types of RNA
mRNA
tRNA
The RNA that takes the code from the DNA to the ribosomes is called messenger RNA (mRNA)
mRNA is small enough to pass through the pores in the membrane around the nucleus

8. Transcription
Transcription is the process by which the mRNA molecule is formed using the code in the DNA molecule
The genetic instructions are copied from the DNA to the mRNA molecule
Transcription is triggered by chemical messengers that enter the nucleus from the cytoplasm and bind to the DNA at the relevant gene

9. Transcription
This causes an enzyme called RNA polymerase to begin the process of making mRNA
RNA polymerase makes the double stranded DNA molecule come apart, about 17 base pairs at a time

10. Transcription
RNA polymerase then transcribes (copies) the bases from one strand of the DNA to make a complementary molecule of mRNA
At the end of the gene there is a sequence of bases that tells the RNA polymerase to stop copying and the mRNA molecule is released

11. Transcription
Only one of the strands of DNA is copied during transcription
This strand is called the template strand
The other strand is called the coding strand
Because the bases always form complementary pairs, the order of bases in the mRNA molecule will be the same as in the coding strand, but opposite to the template strand

12. Transcription
Not all the bases in the DNA molecule are code for an amino acid
The non-coding sequences are called introns
The base sequences that code for amino acids are called exons
The mRNA that has been copied, contains both introns and exons
Introns must be removed before the mRNA can be used to assemble amino acids into a protein

13. Translation
Translation is the process whereby a protein is synthesised from the information contained within a molecule of mRNA
Translation occurs at the ribosome
The starting sequence of bases (AUG) code for the amino acid methionine
This is the start codon that every protein begins with (it may be removed later)
This ensures that the ribosome attaches to the correct end of the mRNA

14. Translation
The ribosome then moves along the mRNA three bases at a time
The ribosome pulls the mRNA through like a ribbon, reading the bases as it goes

15. Translation
Small molecules of RNA called transfer RNA (tRNA) bring the individual amino acids to the ribosome to be joined together as proteins
There is at least one kind of tRNA for each of the 20 amino acids
Each tRNA molecule has a section that binds to its corresponding amino acid

16. Translation
Half way along the molecule, the tRNA forms a tight loop. The loop has 3 nitrogen bases that form an anticodon
These 3 bases of the anticodon can bind with the complementary bases of a codon in the mRNA molecule
The anticodon determines the type of amino acids carried by the tRNA
Example: the tRNA with the anticodon CGG always carries the amino acid alanine

17. Translation
As the ribosome reads the codon on the mRNA, the tRNA molecules with the matching anticodon are brought in
The amino acids carried by the tRNA are joined together so the protein is assembled with the amino acids in the right order
Each bond formed between the amino acid requires 1 ATP
Once the tRNA has delivered its amino acid it detaches from the ribosome, it can then pick up another amino acid from the cytoplasm

18. After a protein has been synthesized, it then must be organised in a particular shape
Protein shape is very important for the correct functioning of the protein
Proteins have 3 or 4 levels of complexity in their structure

19. Protein Structure
Primary structure: is the sequence of amino acids in the chain that makes up the protein
Secondary structure: this is either a coiled or folded shape that is brought by bonds between different parts of the amino acid chain
Tertiary structure: this is formed by further bending and folding of the protein into globular or fibrous shapes
Quaternary structure: when 2 or more chains of amino acids interact

20. Packaging of proteins
If proteins are to be used outside the cell, they must be packaged for secretion
After proteins are made at the ribosome, they travel through the ER into the golgi bodies where they are modified and packaged via exocytosis

21. Lipid and carbohydrate synthesis
DNA only contains code for the manufacture of proteins
However, it contains code for the production of enzymes (which are proteins) and therefore DNA indirectly controls the synthesis of lipids and carbohydrates

22. PROTEIN SYNTHESIS – TRANSLATION & TRANSCRIPTION
Class resources
Bozeman Science
Transcription and Translation: 12 mins
Student resources