L16-21

• Flow of Central Dogma of molecular Biology:

  • DNA- information molecule- tells what need to be done.
  • mRNA- messenger of info from DNA to synthesise protein.
  • Protein- "doer"
• Transcription is the process by which mRNA is made.
• Gene is a defined region of the DNA that produces a type of RNA molecule that has a function.

  • contains sequences that are responsible for:

  1. regulation of the synthesis of RNA
  2. production of RNA- from template strand
  3. further processing of DNA.
• Three stages of transcription:

  1. Initiation
  2. Elongation
  3. Termination
• Transcription:

  • The macromolecule involved in transcription is the DNA molecule which helps in the synthesis of mRNA (nucleic acid). The mRNA is always synthesised in the 5' to 3' direction and uses the non-coding strand as the template.
• Transcription- synthesis of mRNA

  • only 1 strand of the DNA molecule contains genetic information also known as the coding strand.
  • To make mRNA, it must be synthesised using the non-coding strand to make a replica of the coding strand with the genetic info.
• Template or non-coding strand must run from the 3' to 5 ' direction given mRNA is synthesised in the 5' to 3' direction.
• Initiation process of Transcription:

  • occurs in the promoter region which is a region in the DNA where the proteins bind.
  • RNA polymerase cannot bind yet, therefore, transcription factors (proteins) bind first within the TATA box (specific region within the promoter region.
  • As a result RNA polymerase unwinds DNA apart (small region) and can start transcribing because it has an internal 3' OH group.
• Elongation process of Transcription:

  • RNA polymerase uses template strand to make RNA by adding complementary nucleotide but rather that having T as the complementary base for A, it uses Uracil (U).
  • As RNA Polymerase goes along, the DNA strand with the newly synthesised RNA snap back together.
  • In the process, Topoisomerase also releases tension.
• Parts of a gene that is transcribed from DNA to RNA:

  • 5' UTR ( Untranslated Regions) and 3' UTR
  • Coding Sequence
• Parts of a gene that is translated from mRNA to Protein:

  • Coding sequence
• Function of Gene Structure Elements:

  • Coding Sequence- portion of the gene that is translated into a protein.
  • Promoter: DNA segment recognised by RNA polymerase to inititae transcription.
• UTRs:

  • Transcribed but not translated
  • Contains sequences that influences gene expression at the transcriptional and translational level
  • 5' UTR facilitates addition of 5' G Cap
  • 3' UTR facilitates addition of poly-A tail
• • 5' G Cap prevents mRNA degradation and promote intron removal and provides binding site for small ribosomal subunits
  • Poly-A tail prevents mRNA degradation and facilitates export of mRNA from nucleus to cytoplasm.
• Splicing is where the introns are removed from the Pre-mRNA gene making it into a mRNA which is then translated into a protein.

  • mRNA must not have introns ( intervening sequences within the RNA transcript)
• Eukaryotic Gene Structure:

  • Contains non-coding region upstream and coding region downstream as well as the coding sequence.
  • non-coding region can be transcribed but not translated. It is also involved in regulating gene expression.
• Mutation within the non-coding strand (template) may disrupt normal gene expression.
• Triplet Codon Hypothesis:

  • 3 nucleotides= codon which code for a specific amino acid.
  • 61/64 codon specify an amino acid
  • most amino acid have more than 1 codon
• Codons:

  • 3 codons specify STOP ( UAA, UAG, UGA)
  • 1 codon specify START (AUG) which also specifies methionine.
• Amino acid is carried by an adapter molecule tRNA to the template mRNA. The Adapter is what fits with the RNA.
• Features of tRNA:

  • Anticodon which attaches to codon of the mRNA
  • Amino acid attachment side at the 3' end.
  • 3D structure- 'fits like a glove' when interacting with mRNA codon.
• Charging a tRNA:

  • an enzyme (aminoacyl tRNA synthase) recognises both a specific amino acid and correct tRNA and binds them together.
  • There are 20 different aminoacyl tRNA synthase enzymes; one for each amino acid.
• Translation:

  • synthesis of protein by ribosomes using mRNA as template.
  • Ribosomes contain both ribosomal RNA (rRNA) and proteins.
• Anatomy of a ribosome:

  • has an exit site (E), (P) site where peptide bond links with amino acid, and an (A) site where new amino acid attached to tRNA enters.
• • Free Ribosomes are synthesised in the cytoplasm and released within the cell
  • Bound Ribosomes are synthesised in the rER and are used within the plasma membrane or secreted via exocytosis.
• Translation: Initiation Stage

  • ribosomal subunit binds to charged tRNA which carries methionine. tRNA is positioned in the P site.
  • ribosomal subunit locates the 5' G Cap attached to 5' UTR and move along template until reaching START codon (AUG).
  • Anticodon of tRNA must be complementary to AUG on the mRNA.
• Translation: Elongation:

  • next codon (CUG) with amino acid Leu (L) then binds with tRNA which enters A site.
  • Methionine then moves to A site with Amino Acid L while the ribosomal subunit moves along one codon leaving tRNA initially attached to Meth in the E site where it exits.
  • process continues.
• Translation: Termination

  • when 1 of the stop Codon (UAA, UAG, UGA) is positioned in the A site, there will no longer be any tRNA entering A site.
  • A protein then comes in which releases the long chain of amino acid.
  • The long chain must then fold into a 3D shape with the 1st part (methionine) chopped off as a signal molecule for protein to know where to go.
• Eukaryotic cell VS Prokaryotic Cell

  1. Eukaryotes:

  • Transcription occurs in the nucleus.
  • Translation occurs in the cytoplasm

  2. Prokaryotes:

  • Both transcription and translation occurs in the cytoplasm.
• Monohybrid Cross:

  • when both parents are heterozygous
  • will result in a 3:1 ratio
• Dihybrid:

  • involves two genes (eg. yellow/green and smooth/wrinkled)
  • Phenotypic ratio is 9:3:3:1 (F2) involving heterozygous egg and sperm.
  • P generation is homozygous dominant and recessive for both traits= F1: Heterozygous
• Test Cross:

  • helps determine one allele of an individual by testing with a homozygous recessive partner.
  • If all offspring display the dominant trait, then parent must be homozygous dominant.
  • If there is a 1:1 ratio then parent must be heterozygous to produce an offspring with a recessive trait.
• Mendel's 1st Law:

  • Law of segregation
  • during meiosis, two alleles located at the gene locus segregate from each other.
  • Each gamete (one unreplicated copy of chromosomes after meiosis 2) has an equal probability of containing either allele.
• Mendel's Second Law: Independent Assortment

  • random orientation of homologous chromosome pairs during M1 (1-2 ways) allow gamete production with many different assortments of chromosomes.
• Even though an individual can only have 2 Alleles, some genes have many alleles (eg eye color in Drosophila). These genes are called Polymorphic.
• Co-Dominance:

  • when both phenotype exist side by side
  • both phenotype are expressed.
• The presence of A and B antigens on the surface of red blood cells is an example of co-dominance.
• You may observe a normal distribution around an average with polygenic traits as more people have intermediate phenotype and it is rare, but possible, to get extreme phenotypes.
• Characteristics such as height and intelligence are polygenic and are controlled by groups of genes.

  • eg height is influenced by a combination of genetic factors, environmental factors, and interactions between genes and the environment.
• Environment also influences phenotype.

  • eg. Two genetically identical lines of hydrangeas, planted in different areas can be blue or pink or somewhere in between depending on the pH of the soil.