Essay: The stages of protein synthesis from DNA

A gene is a segment of DNA which is used for an amino acid sequence which then creates polypeptides. Protein is synthesised through a process in which the DNA is decoded and then the amino acids are then generated from this. This is called gene expression.
There are some components needed in order for DNA replication and transcription to occur, these are:
• A gene (DNA) to act as the template
• Supply free RNA nucleotides
• Enzymes
• ATP (Adenosine triphosphate)
The genetic code is made up of four letters, A T C and G. The letter A will always go with T and the letter G will always go with C. However, in RNA the A will always be paired with U instead. The code determined which amino acids are needed and what order they need to be joined together in in order for them to make the protein required.
In DNA the code is read in threes which is called a triplet code and each triplet codes for an amino acid. The triplet bases of DNA in mRNA are known as a codon and the DNA in tRNA are known as anti-codons.
Transcription
Figure 1. (BBC, 2011) Figure 1 above is showing the process of transcription.
The process of transcription takes place in the nucleus where the DNA is unzipped by the helicase via breaking down the hydrogen bonds that hold it together. Then DNA polymerase reads the DNA and ribonucleotides are added to create a mRNA strand which compliments the template strand. The DNA is then zipped back up and the newly made mRNA is now small enough to fit through the nuclear pores.
The process of translation
Figure 2. (BBC, 2011) The process of translation.
The process of translation is where cellular ribosomes are used to create proteins. The mRNA created from transcription is decoded by ribosomes and used to produce an amino acid chain or a polypeptide. The ribosome decodes the mRNA with the binding of complementary tRNA anticodon with mRNA codons. Translation occurs in three stages:
• Initiation: A small fraction of ribosome used together with tRNA and an amino acid interacts with mRNA which then attach and start to scan for a start signal. When the start sequence is found the large fraction of ribosome joins together with the smaller one and complete the ribosome and then protein synthesis begins.
• Elongation: New tRNA and an amino acid enter the ribosome. If the anticodon matches with mRNA they will base pair and the ribosome will link the two amino acids together. The ribosome will move the triplet forward to where the process will be repeated
• Termination: This is where the ribosome reaches a stop codon. The termination proteins will bind to a ribosome and begin to release the polypeptide chain from the ribosome.
Processing the proteins
After translation the protein pass through the channels of the rough endoplasmic reticulum (ER) and are then passed from the rough ER to the Golgi apparatus. The Golgi apparatus modifies, processes and packages the proteins and may even add a carbohydrate to them to form a glycoprotein. The protein is then released from the cell.
The stages of meiotic cell division
Mitosis is used to divide cells in your body. It also adds new cells and replaces old used cells. Mitosis main goal is to create daughter cells; these are cells which are genetically identical to their parent cells.
Meiosis however, is used for the production of gametes (sex cells such as sperm or eggs). The main goal of meiosis is to create daughter cells with exactly half as many chromosomes as that of the starting cell.
The phases of meiosis
Phase 1: Pre-Meiotic Interphase
The chromosomes are duplicated before meiosis. Outside of the nucleus are two centrosomes which contain a pair of centrioles. The two centrosomes are created from the duplication of one centrosome whilst in the pre-meiotic interphase. The centrosomes act as microtubules and extend from the centrosomes and form an aster. The centrosomes are found in animal cells but are not found in plant cells. The pre-meiotic interphase only occurs once.
Phase 2: Prophase
Homologous chromosomes start to cross over. This is the process where the chromosomes begin to pair up and exchange parts of their genetic material with each other to form recombinant chromosomes. The nuclear envelope disappears at the end of prophase and allows the meiotic spindle to enter the nucleus.
Phase 3: Metaphase
The centrioles move to opposite sides of the cell and the homologous chromosomes begin to arrange themselves between the metaphase plate. Spindle fibres from the two poles begin to attach to one of the chromosomes in the pair with one beings attached to the chromosome and one being attached to the homologous chromosome
Phase 4: Anaphase
The two chromosomes start to separate from each other and move to opposite sides of the poles in the cell. The sister chromatids remain attached to their centromeres.
Phase 5: Telophase
The homologous chromosomes complete their migration to the poles which leaves there being a haploid set of chromosomes at each pole. A cleavage furrow appears and now the cell has divided into two daughter cells. This separation is called cytokinesis.
The whole process is then restarted and done once again with the newly divided daughter cells but the pre-meiotic interphase is not apparent.
The principles of Mendelian genetics and the laws of inheritance
Mendel’s two laws of inheritance are:
• Two alleles of the same gene separate (segregate) so that each gamete only receives one allele
• All alleles segregate independently from the alleles any other genes
A monohybrid cross is the inheritance of one characteristic. The genotypes of a monohybrid cross can be explained by the phenotype and genotype of the parents, the gametes which can be produced and the way the gametes combine. This can be done by using a punnet square which will show the possible genotypes that the offspring will show. An example of a punnet square is:
Figure 1: Punnet square
The punnet square above is determining the colouration of what the gerbil could potentially be. The colour black here is known as being a dominant allele whereas the white colour is known as being a recessive allele. In the punnet square above the chances of the gerbil being the colour black are 75% and the chances of it being white are 25%. The ratio of this is 3:1
A dominant allele will always show even if there is only one of the alleles, whereas a recessive allele will only show if there are two copies of the allele.
A dihybrid cross represents the inheritance of two characteristics from two homologous chromosomes.
Figure 1: Punnet square of dihybrid cross in mammals
The mice in the diagram above show the odds of what the characteristics their offspring’s body colouration could potentially be. The overall chance of the offspring having some sort of brown colouration is 3:1. The dihybrid punnet square shows more of what the offspring could potentially look like and gives out a greater odd percentage of its chances.