Transcription & Translation 5.3

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Inquiry Question 3: Why is polypeptide synthesis important?

Content Descriptor: Construct appropriate representations to model and compare the forms in which DNA exists in prokaryotes and eukaryotes: (5.3.1)

Prokaryotes DNA:

  • Foundin the nucleoid (non-membrane bound)
  • Circularstructure
  • Non-compact
  • Can also befound in plasmids

Eukaryote DNA:

  • Foundin the nucleus
  • Linearstructure
  • Compact(compacted in histone proteins)
  • Alsofound in mitochondria and chloroplasts


Content Descriptor: Model the process of polypeptide synthesis including:

Transcription and translation: (5.3.2)


  1. Initiation – RNA polymerase moves along the double stranded DNA chain, separating the
  2. Elongation– RNA polymerase adds the complementary RNA nucleotides reading 5’ è 3’
  3. Termination– Sequences known as terminators signal RNA transcript is 
  4. mRNAstrand moves through the nuclear envelope towards the 


  1. Freefloating tRNA binds to specific amino acids in the 
  2. tRNA is attracted into the ribosome, its anticodon binding with the corresponding codon tothe nucleic bases read on the mRNA.
  3. tRNAdeposits the amino 
  4. Chainlengthens as more amino acids are deposited, joining together to make a polypeptide.




Assessing the importance of mRNA and tRNA in transcription and translation: (5.3.3)

tRNA and mRNA are vital in the transcription translation process. In transcription, the mRNA strand forms from the template strand as read by RNA polymerase. This is then sent out through the nucleic envelope towards the ribosomes. Without mRNA, the DNA code specifying the sequence of amino acids would not form. Thus, mRNA is of vital importance to the transcription process, and for polypeptide synthesis. In translation, tRNA is attracted into the ribosome to deposit their corresponding amino acids onto the polypeptide chain. Without tRNA, the mRNA sequence could not be converted into a distinct sequence of amino acids. Thus, tRNA is of vital importance to the translation process and for polypeptide synthesis.

Analysing the function and importance of polypeptide synthesis: (5.3.4)

Polypeptide synthesis is the product of the transcription translation process. The deposited amino acids form long chains; the specific charges of each amino acid dictating the folding and thus shape of the polypeptide. This will affect its structure and therefore function of the polypeptide. Multiple polypeptides join together to form proteins.

Assessing how genes and environment affect phenotypic expression: (5.3.5)


The actual appearance of an organism, including all aspects of internal and external anatomy, behaviour and physiology.


The genetic constitution of an organism, this can refer to a particular set of alleles in an organism. The genome is the complete complement of an organism’s genetic information.

Genotype affects phenotype through the production of polypeptides. A gene is a section of DNA that codes for a particular polypeptide, and when that gene is expressed, that section of the DNA molecule is switched on, causing the polypeptide to be produced in the cell. These polypeptides are produced through the processes of transcription and translation. These polypeptides which join together to form proteins are essential in dictating cell function and thus the physiological and physical appearance of an organism.


Environment is the sum of all living and non-living surroundings. These conditions will affect that appearance of an organism and the appearance of individual cells within the organism. This may be due to the quality of nutrients and resources available to the organism or the amount of sunlight it receives. These factors limit or accelerate the growth of certain aspects of the organism, affecting phenotype. Phenotype may also be affected through the environment affecting genotype. Exposure to certain environmental factors may cause mutations or cause certain genes to be switched on or off, thus altering the phenotype of the organism. An example of this is a hydrangea.

Content descriptor: Investigate the structure and function of proteins in living things: (5.3.6)


The structure of protein can be divided into four sections. Primary, secondary, tertiary and quaternary. Each section is ultimately caused by the coiling and compacting of the previous, with primary being the specific bonds and sequence of amino acids.

Proteins have many roles. These include:

  • Enzymes – Catalyse reactions allowing them to occur at a lower activation energy. Theseinclude 
  • Structural– Fibrous protein such as collagen and keratin
  • Transport– Channel proteins
  • Storage– Ferritin
  • Receptor– Guanine nucleotide-binding protein-coupled receptors (GPCRs)
  • Antibodies– Immunoglobulins

Globular proteins are spherical in shape and form colloids with water. Fibrous proteins are elongated and form rods or wires.


Inquiry Question Review: Why is polypeptide synthesis important? (5.3.7)

You should be able to:

  • Describethe different forms of DNA in prokaryotes and 
  • Explaintranscription and translation and its biological 
  • Describethe formation, structure and function of 

Inquiry Question 4: How can the genetic similarities and differences within and between species be compared?

Content Descriptor: Conduct practical investigation to predict the variation in the genotype of offspring by modelling meiosis, including the crossing over of homologous chromosomes, fertilisation and mutations: (5.4.1)


  1. Prepare one or multiple pairs of chromosomes in homologous pairs. Note that thesechromosomes should be of similar size. Genes are denoted by a sentence, or alternatively through the use of upper and lower case letters (showing dominant and recessive). Since this is in prophase, each chromosome should be double-stranded.
  2. Cut and remove one gene from one chromatid and exchange it with a gene from itschromosome  This can happen independently across the two chromatid strands.
  3. Continue process using the remainder of meiosis. Note that the daughter cells should notcontain the same genetic arrangement as the parent cell.


Genetic variation:

Fertilisation – The mechanism by which meiosis and mutation act. The uniting of gametes allows for the new assembly of alleles producing genetic variation. Mate selection can also impact which genes are selected to be passed on.

Meiosis – Division into homologous pairs and crossing over re-arrange existing alleles into new combinations creating genetic variation.

Mutation – Alterations in the genetic code causes changes to the amino acids which they code for. This changes the amino acids produced, altering protein shape and therefore cell functioning. If this mutation is in gametic cells, it may be passed onto offspring causing new alleles and thus genetic variation from parents.