Learning Outcomes
- Proteins, including enzymes and structural proteins, are essential to cell structure and functioning
- Protein synthesis involves transcription of a gene into messenger RNA in the nucleus, and translation into an amino acid sequence at the ribosome
- The genetic code is a base triplet code; genes include ‘coding’ and ‘non-coding’ DNA, and many genes contain information for protein production
- Select, construct and use appropriate representations, including models of DNA replication, transcription and translation, Punnett squares and allele frequencies in gene pools, to communicate conceptual understanding, solve problems and make predictions
1. The purpose and structure of proteins...
Proteins are essential to the structure and functioning of cells.
Proteins fall into a few broad categories
In all cases the structure of the protein determines the function.
Functions:
Enzymes: Accelerate biochemical reactions
Structural: Form biological structures
Transport: Carry biochemically important substances
Defence: Protect the body from foreign invaders
Structures:
Globular: Complex folds, irregularly shaped tertiary structures
Fibrous: Extended, simple folds -- generally structural proteins
Cellular Locations:
Membrane: In direct physical contact with a membrane; generally water insoluble.
Soluble: Water soluble; can be anywhere in the cell.
Proteins fall into a few broad categories
In all cases the structure of the protein determines the function.
Functions:
Enzymes: Accelerate biochemical reactions
Structural: Form biological structures
Transport: Carry biochemically important substances
Defence: Protect the body from foreign invaders
Structures:
Globular: Complex folds, irregularly shaped tertiary structures
Fibrous: Extended, simple folds -- generally structural proteins
Cellular Locations:
Membrane: In direct physical contact with a membrane; generally water insoluble.
Soluble: Water soluble; can be anywhere in the cell.
What are Proteins made of?
The components of proteins are called Amino Acids.
There are 20 essential amino acids for living things.
Amino Acids are linked together by PEPTIDE bonds.
Proteins have many peptide bonds and so are called POLYPEPTIDES.
There are 20 essential amino acids for living things.
Amino Acids are linked together by PEPTIDE bonds.
Proteins have many peptide bonds and so are called POLYPEPTIDES.
1. Primary Structure:
This is the sequence of amino acids in a protein molecule. Linear and precisely ordered. Start at the amino end and finish at the carboxyl end. |
3. Tertiary Structure:
Tertiary structure is the result of further folding caused by more distant interactions. As the protein folds groups more distant in the chain are brought closer together and interact with each other. It affects the whole molecule but is restricted to that molecule. |
2. Secondary Structure:
The folding of the chain due to electrostatic interactions between different side chains. These folds/changes happen as they are energetically favourable (energy is released). |
4. Quaternary Structure:
Many proteins are made of more than one subunit. This means that more than one folded polypeptide chain associates with others to make the overall protein. For example Haemoglobin has 4 subunits. |
2. Protein Synthesis
- Genes contain the information needed to make functional molecules called proteins.
- The central dogma of genetics: DNA codes for RNA and RNA codes for Proteins.
- The journey from gene to protein is complex but consists of two major steps: transcription and translation.
- Together, transcription and translation are known as gene expression.
Transcription:
What?
- The information stored in DNA is transferred to RNA (ribonucleic acid) in the cell nucleus.
- Both RNA and DNA are made up of a chain of nucleotide bases, but RNA is single stranded and has Uracil in place of Thymine.
- Type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm.
- DNA unwinds and unzips in a particular region exposing nucleotide bases. One strand is used for synthesis, this is called the template strand.
- Proteins position the enzyme RNA polymerase to the Promoter region, which is a short region that signal the start of a gene. It is at the 5' end.
- The DNA code on the template strand is the template for free nucleotides to attach creating mRNA.
- Transcription stops at terminator sequence on DNA strand
- RNA polymerase detaches & releases the new mRNA
- mRNA moves out of nucleus and into cytoplasm
Translation:
What?
- The amino acids that the mRNA codes for are stacked together to form a protein.
- The mRNA interacts with a ribosome, which "reads" the sequence of mRNA bases.
- Another type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time by carrying proteins to the polypeptide chain.
- This step, takes place in the cytoplasm.
- mRNA binds to a ribosome
- There are many tRNA molecules in the cytoplasm, these recognise the start codon on mRNA (The start codon is AUG).
- tRNAs each contain a anti-codons which enables them to to bind to the mRNA
- tRNA's carry and amino acid with them, the tRNA binds to the mRNA and its amino acid attaches to the amino acid chain by forming a peptide bind with the previous amino acid in the chain.
- the tRNA is then released and the process continues until the entire mRNA is read (stop codon is reached)
- Remember,Protein assembly continues until the ribosome encounters a “stop” codon (a sequence of three bases that does not code for an amino acid).
The complete genetic code
![Picture](/uploads/1/2/5/9/125983434/published/amino-acid-chart-codons.png?1644371805)
In 1966 the complete genetic code was determined!
Note the start and stop codons
Page 43 of your text book gives a list of the full amino acid names noted above.
The enzymes...
- RNA polymerase I - synthesises ribosomal RNA (rRNA) which codes for the ribosomes used in translation
- RNA polymerase II - used to transcribe mRNA
- RNA polymerase III - synthesises tRNAs
Activity 2.1 p. 44 of textbook
Now you should...
Read Chapter 2 up to page 45
Complete Question Sets 2.1 and 2.1
Complete Activity Sheets 2.2 and 2.3 (on Seqta)
Complete Question Sets 2.1 and 2.1
Complete Activity Sheets 2.2 and 2.3 (on Seqta)