Chapter 1.2 DNA and Protein Synthesis PDF

DNA & PROTEINS CHAPTER 1.2 DNA & PROTEIN SYNTHESIS MOLECULES INVOLVED IN PROTEIN SYNTHESIS 1. DNA  DNA is a double helix molecule consisting of 2 strands of nucleotide building blocks.  Has 2 strands that are complementary to each other and joined by weak hydrogen bonds.  Has 4 org...

DNA & PROTEINS CHAPTER 1.2 DNA & PROTEIN SYNTHESIS MOLECULES INVOLVED IN PROTEIN SYNTHESIS 1. DNA  DNA is a double helix molecule consisting of 2 strands of nucleotide building blocks.  Has 2 strands that are complementary to each other and joined by weak hydrogen bonds.  Has 4 organic bases: adenine (A), thymine (T), guanine (G) and cytosine (C). These bases bond through complementary base-pairing: A-T, G-C  Contains genes that are hereditary units.  Has one strand that can act as a template to synthesise a molecule of mRNA.  MOLECULES INVOLVED IN PROTEIN SYNTHESIS  Gene: a unique sequence of DNA nucleotides that code for a protein or an RNA molecule.  Genes are heritable factors that control specific characteristics (e.g. eye colour) and are located at specific sites on a particular chromosome (i.e. a locus).  Genome: the total number of genes of an organism. The scientific study of genomes is genomics.  In 1990 an international effort was directed at mapping the entire human genome on the 46 human chromosomes. This data is used worldwide in a range of biomedical, forensic and other related The disease listed above occur when the fields. particular gene malfunctions MOLECULES INVOLVED IN PROTEIN SYNTHESIS  Eukaryotic DNA can be divided into 2 types of DNA – ‘coding’ (exons) and ‘non- coding’(introns).  98% of human DNA are introns and 2% are exons  Exons are the DNA in the gene that is transcribed into messenger RNA (mRNA) and then translated into a polypeptide (protein).  Introns are DNA in a gene that is only transcribed into an RNA molecule: -ribosomal RNA (rRNA) involved in translation. -transfer RNA (tRNA) involved in translation -micro RNA (miRNA) involved in regulation of gene expression.  Introns do not code for proteins.  Prokaryotic DNA do not contain introns  MOLECULES INVOLVED IN PROTEIN SYNTHESIS 2. Messenger RNA (mRNA)  mRNA is made in the nucleus using one strand of DNA as a template before it moves to the cytosol.  Single-stranded molecule made up of mRNA nucleotides. DNA is a double-stranded molecule made up of DNA nucleotides.  mRNA has a ribose sugar; DNA has a deoxyribose sugar  mRNA bases: A, uracil (U), G, and C DNA bases: A, thymine (T), G and C  A group of 3 bases on a mRNA molecule = codon. Each codon codes for 1 amino acid  MOLECULES INVOLVED IN PROTEIN SYNTHESIS 3. Transfer RNA (tRNA)  A tRNA molecule is about 80 nucleotides long & has a 3-D clover-leaf shape.  Each tRNA molecule has 1) an amino acid binding site and an anticodon consisting of 3 bases that can complementary base-pair to the mRNA codon.  Each tRNA molecule can carry one specific amino acid molecule (there are 20 naturally occurring amino acids altogether).  The function of tRNA is to place amino acids into its correct sequence in the polypeptide being synthesised as specified by the bases on the mRNA. MOLECULES INVOLVED IN PROTEIN SYNTHESIS 4. Ribosomal RNA (rRNA)  Made of a large subunit and a small subunit. Ribosomes are made up of ribosomal RNA (rRNA) and proteins.  Can be free-floating or attached to endoplasmic reticulum to form rough endoplasmic reticulum (RER).  Site of protein synthesis: Ribosomes move along the mRNA molecule translating the mRNA code into a sequence of amino acids in a polypeptide molecule. MOLECULES INVOLVED IN PROTEIN SYNTHESIS 5. Micro RNA (miRNA)  Small RNA molecules consisting of about 22 nucleotides.  Regulate gene expression after transcription. They achieve this by binding to mRNA molecules and preventing the translation of these sections.  MOLECULES INVOLVED IN PROTEIN SYNTHESIS 6. Amino acids (aa)  Are the building blocks of polypeptides and protein molecules  20 naturally occurring amino acids  Proteins differ in their number, type and sequence of amino acids. THE GENETIC CODE  3 bases (codon) on an mRNA molecule code for 1 amino acid  Amino acids are the building blocks of polypeptides (proteins).  Protein molecules can consist of one polypeptide chain folded into a precise shape or more than one linked together to form a functional protein. THE GENETIC CODE  64 possible combinations of 3 bases.  AUG is both a ‘start’ codon and a codon for methionine.  Some triplet codons are ’stop’ codons that terminate the production of a polypeptide sequence. OVERVIEW OF PROTEIN SYNTHESIS  Protein synthesis is a unidirectional process mRNA Transcription splicing Translation DNA pre-mRNA mRNA Protein TRANSCRIPTION  Occurs in the nucleus  Where mRNA is synthesised from a gene on DNA  Genetic code is transcribed/re-written from DNA to mRNA  One strand of DNA is termed the template strand and the other is the complementary strand.  During transcription, an enzyme, RNA polymerase separates the 2 DNA strands and & links together RNA nucleotides as they base pair along the DNA template.  After transcription the 2 DNA strands will re-form the double helix.  The mRNA travels out of the nuclear pore of the nucleus and to the cytoplasm. TRANSCRIPTION  After transcription mRNA splicing occurs.  The introns (non-coding regions) are removed from the ‘pre- mRNA’. TRANSLATION  Translation: A polypeptide chain is built using a sequence of codons in a mRNA molecule  Occurs in the cytoplasm at ribosomes Initiation The use of the start codon AUG Chain Building of the amino acid sequence from elongation the codons Termination The completion of the mRNA sequence with a stop codon TRANSLATION STEPS 1. An initiating transfer RNA (tRNA) molecule arrives at the start codon, AUG and carries the amino acid methionine. It’s anticodon complementary base-pairs to the mRNA’s codon 2. The second tRNA molecule arrives and it’s anticodon complementary base pairs to the mRNA codon that is adjacent to the initiating tRNA molecule. 3. A peptide bond forms between the two adjacent amino acids to begin forming a polypeptide chain. 4. The ribosome shifts by one codon. 5. The initiator tRNA leaves the ribosome. 6. A new tRNA binds to the newly exposed codon. 7. Steps 2-6 repeat until the ribosome reaches the ’stop’ codon. At this stage the polypeptide chain breaks off from the tRNA molecule. SUMMARY OF PROTEIN SYNTHESIS DNA mRNA tRNA Amino acid Triplet Codon anti- transcribed codon AAA UUU AAA Phenylalanine GCG CGC GCG arginine TAT AUA UAU isoleucine