DNA and Protein Synthesis

• DNA Structure (Nucleotide)

  • A reminder that a nucleotide consists of a phosphate group, sugar, and organic nitrogenous base.
• Amino acids

  • 20 different amino acids.
  • Amino acids are building blocks of polypeptide chains and proteins.
  • A polypeptide chain is a string/chain of amino acids that are connected together by peptide bonds. These make up proteins.
• Polypeptide Chain Protein

  • Some proteins consist of one polypeptide chain into a 3D shape.
  • Some proteins contain multiple polypeptides linked together.
  • Poly = many. Peptide = proteins.
• Genes

  • A gene consists of a unique set of DNA nucleotides (bases) which code (provide instructions) for a polypeptide chain protein or an RNA molecule.
  • Genes control specific characteristics (e.g. eye and hair colour) and are located on specific parts of a chromosome.
  • The genes location on a chromosome is called a locus.
  • Chromosomes contain several hundred to a few thousand genes.
• Genome

  • Genome is the total collection of genes that an organism contains.
  • Between 1990 and 2001, scientists globally worked together to map the human genome of all 46 chromosomes.
  • This has led to use in biomedicine, forensics and other fields.
• DNA Structure (Nucleotide )

  • Contains a sugar-phosphate backbone.
  • Deoxyribose (5-carbon) sugar.
  • A phosphate group.
  • One of four organic bases:
  • Adenine
  • Thymine
  • Cytosine
  • Guanine
• RNA

  • RNA (Ribonucleic acid) are found in all living organisms.
  • Responsible for the expression and regulation of genes.
  • RNA is made up of repeating nucleotides, like DNA.
  • However, the structure of RNA is slightly different to DNA.
  • RNA is made up of:
  • A pentose (5 carbon) sugar called ribose.
  • A phosphate group.
  • Four nitrogenous bases: Uracil (U), Adenine (A), Cytosine (C), Guanine (G).
  • Uracil replaces Thymine.
• RNA

  • When RNA is synthesised, it will temporarily bond with their complementary strand on the DNA molecule.
  • Bonding's are A-U (two H bonds) and C-G (three H bonds).
• DNA

  • Deoxyribonucleic acid
  • Double stranded sugar phosphate
  • Thymine and Adenine
  • Cytosine and Guanine
  • Base pair
• RNA

  • Ribonucleic acid
  • Usually single stranded sugar phosphate
  • Cytosine and Guanine
  • Adenine and Uracil
  • Single nucleobase
• RNA

  • Four major types of RNA:
  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)
  • microRNA (miRNA)
  • All single stranded.
• Exons and Introns

  •Eukaryotic cells contain two types of DNA in genomes:

  Introns

  • Are non-coding sequences of DNA. Do not code for polypeptides/proteins, but can be transcribed into RNA molecules:
  • Ribosomal RNA (rRNA), transfer RNA (tRNA), microRNA (miRNA).
  • Roles of these will be explored later.

  Exons

  • Are coding sequences of DNA that synthesise (create) polypeptides/proteins
• Premature messenger RNA (pre-mRNA)

  • Genes are transcribed into pre-mRNA strands in the nucleus.
  • If a gene has introns and exons, the introns must be spliced out (removed).
  • This allows for the correct sequencing of amino acids and proteins.
  • Evidences suggests human DNA is 98% introns and 2% exons.
  • This allows for more conserved exons, allowing less changes to genes/translated proteins
  • Most prokaryotic cells do not contain introns, but exons, as exons are required for protein synthesis (and thus survival)
• Messenger RNA (mRNA)

  • mRNA is synthesised using DNA as a template through transcription.
  • This occurs in the nucleus and once modified will move into the cytosol.
  • mRNA is single stranded, consists of RNA nucleotides (hundred to thousands).
  • mRNA sequence codes for specific sequence of amino acids.
  • Every three bases = RNA codon (codes for specific amino acid).
• Transfer RNA (tRNA)

  • tRNA is about 80 nucleotides long.
  • 3D shape like a clover.
  • Contain 3 nucleotides (bases) which are anticodons.
  • Opposite end carries a specific amino acid.
  • tRNA’s function is to place the amino acid into the polypeptide being synthesised.
• Transfer RNA (tRNA)

  • Once tRNA has transferred to polypeptide chain, it collects another corresponding amino acid in the cytosol.
  • Attachment of an amino acid to the tRNA involves enzymes.
  • Each enzyme have a specific shape which ensures each 20 amino acids only attach to their specific tRNA(s).
• Ribosomal RNA (rRNA)

  • Ribosomes are comprised of rRNA and proteins.
  • Ribosomes are site of translation during protein synthesis.
  • Found in cytoplasm and rough endoplasmic reticulum.
  • Ribosomes move along mRNA strand, translating the code into a sequence of amino acids (polypeptide molecules).
• Micro RNA (miRNA)

  • miRNA are small non-coding RNA molecules.
  • Consist of 22 nucleotides.
  • Role is to help regulate gene expression after transcription.
  • Bind to specific mRNA molecule, preventing it from being translated (explored later).
• Protein Synthesis

  •Is the process of synthesising (creating) proteins that cells use around the body.

  Two stages:

  •Transcription: DNA is converted in messenger RNA (RNA)

  Translation: mRNA is read and decoded into amino acid chain (proteins).
• DNA and Codons/Anti-Codons

  • DNA consists of:
  • Coding (non-template strands. Codes genes of interest and determines correct mRNA sequence. Does not take part in transcription.
  • Template stand, complementary to coding strand. Provides genetic code template for a mRNA strand to form against
• DNA and Codons/Anti-Codons

  DNA consists of:

  • Coding (non-template strands). Consists of triplets of bases (DNA codons).
  • Template stand. Consists of triplets of bases (DNA anticodons).
  • Due to being complementary to coding strand.
• DNA and Codons/Anti-Codons

  • During transcription, mRNA molecules are synthesised against the template strand in a complementary fashion.
  • Therefore consist of mRNA codons.
  • During translation tRNA molecules with complementary anticodons to the mRNA codons.
• Genetic Code

  • Triplets of bases (codons) are the smallest unit necessary to code for all 20 amino acids.
  • Make up both the coding DNA strand and mRNA strands.
  • Codon usually generally refers to mRNA codons.
  • Due to mRNA molecules transmit copy of a gene from the coding strand in the nucleus to ribosomes.
  • Some codons code for the start of a polypeptide chain (AUG), while others code to stop (UAA, UAG, UGA).
• Transcription

  • Occurs in the nucleus of eukaryotic cells.
  • mRNA is synthesised from a gene of DNA.
  • the genetic code is re-written from DNA into mRNA.
  • Enzyme RNA polymerase separates two strands of DNA.
  • RNA nucleotides are made in a 5’ to 3’ direction.
  • RNA nucleotides are added to the 3’ end of the DNA template strand.
• RNA Splicing

  • Reminder that introns are spliced out during the pre-mRNA process.
  • This occurs in the nucleus, producing mature mRNA molecules.
  • These are then translated into polypeptides by ribosomes in the cytoplasm.
• Translation

  • During translation, polypeptide chains are built using a sequence of codons in a mature mRNA molecule.
  • Occurs in the ribosomes in cytoplasm and RER.
  • Ribosomes read the mRNA strand 5’ to 3’ direction at the start codon (AUG).
  • Two tRNA molecules move to the ribosomes and bond with the mRNA strand based on complementary codons and anti-codons.
• Translation

  • Ribosome facilitates formation of a peptide bond between two amino acids.
  • As ribosome moves along the mRNA, it results in a growing polypeptide chain.
  • tRNA molecules continue to bring amino acids to the ribosome until a stop codon is reached.
  • Once reached, the ribosome will release the polypeptide chain into the RER.
  • Many ribosomes can move along the same mRNA molecule, creating many copies of the same polypeptide chain, creating many proteins quickly.
• Translation

  • The mature mRNA strand will continue to be translated until a miRNA (microRNA) strand is sent and binds to the mRNA (in a complementary fashion).
  • This deactivates the mRNA, by promoting its destruction, or by blocking translation.
  • This prevents the expression of that gene.
• Explain what it means when it is stated that a particular gene is linked toa a specific chromosome

  Genes are unique sequences of DNA nucleotides that code for a polypeptide. Each gene is located in a particular position (locus) on a specific chromosome, hence the term 'linked'
• Name three different types of RNA that genes can code for and state the function of each type of RNA

  • messenger RNA (mRNA) - carries gene message to ribosomes for translation
  • transfer RNA (tRNA) - transports specific amino acids to codon
  • ribosomal RNA (rRNA) - important constituent of ribosomes which are the site of protein synthesis
  • microRNA (miRNA) - regulates gene expression after transcription by binding to mRNA and deactivating it
• Describe what it means when a gene is termed the 'unit of heredity'

  Genes are sections of DNA that are inherited from previous generations hence the term 'unit of heredity'
• State the approximate percentage of each found in the human genome

  About 98% are introns and 2% are exons in humans
• Introns are often terms "junk DNA". Explain why it is naïve to disregard introns as useless DNA.

  It is now known that introns can code for rRNA, tRNA, and miRNA. These molecules have essential roles in protein synthesis and gene expression,
• A particular protein was found to contain 12,300 amino acids


  12,300 codons needed for this protein

  36,900 mRNA nucleotides required to code for this protein (times by three)
• Explain how miRNA molecules silence mRNA translation

  miRNA binds to the mature mRNA in a complementary fashion. This deactivates the mature mRNA by either promoting its destruction or by blocking the translation of it. As a result, the synthesis of the respective polypeptide is stopped preventing the expression of the gene.
• Outline why the coding strand is most similar to the mRNA molecule made during transcription

  • The mRNA molecules are made during transcription when mRNA nucleotides that are complementary to the DNA template nucleotides bind to these exposed bases. The double helix also consists of complementary binding of nucleotides i.e. of the template to the coding strand.
• State two ways that the mRNA molecule would differ from the coding strand

  mRNA has uracil (U) instead of thymine (T). mRNA has a ribose sugar compared to DNA's deoxyribose sugar
• Compare the processes of transcription and DNA replication

  Transcription occurs so that cells can produce the protein molecules are that required for range of needs. This is in comparison to DNA replication which makes an exact copy of the DNA that is necessary for cell division. Transcription involves making a mRNA molecule for a gene, whereas DNA replication involved copying 2 full strands' the template and the coding strand.
• Explain why it is necessary to remove the intron sections of pre-mRNA molecules

  Introns sections do not contain code for polypeptides and therefore need to be removed to ensure only exons are transcribed
• 'Alternative splicing' refers to the way that different combinations of exons can be joined together. Describe how different combinations of exons from one gene could code for different proteins.

  The exons are the coding sections of DNA transcribed into polypeptides. The alternative splicing of exons means that it is possible to produce a range of different polypeptides/proteins just from one gene.