Chapter 7- The Blueprint of Life PDF

Chapter 7 The Blueprint of Life, from DNA to protein Genetics – the science of heredity Molecular Biology – the science dealing with DNA and protein synthesis The total DNA contained in the cell – genome Consists of the chromosome(s) and any plasmids Chromosomes contain the genes Gene...

Chapter 7 The Blueprint of Life, from DNA to protein Genetics – the science of heredity Molecular Biology – the science dealing with DNA and protein synthesis The total DNA contained in the cell – genome Consists of the chromosome(s) and any plasmids Chromosomes contain the genes Genes – sections of DNA that code for a functional product DNA – macromolecule made of nucleotides Each nucleotide in DNA has: Nitrogenous base (A, T, G, C) 3 Sugar (deoxyribose) Numbered 1ʹ to 5ʹ A phosphate. DNA forms a double helix Two strands are held together by hydrogen bonds between bases The base pairing rule: A always pairs with T (A – T) G always pairs with C (G – C) Strands of DNA are complementary Sequence of one strand determines the sequence of the other.  Nucleotides are linked together by covalent phosphodiester bonds 5ʹ carbon of one nucleotide is joined to 3ʹ carbon of the next nucleotide, with a phosphate between them We usually consider DNA in the 5' to 3' direction Starting at 5' end Finishing at 3' end Two strands of DNA run antiparallel. The flow of genetic information 1. DNA is copied before cell division - Replication 2. DNA is used to make proteins - Gene expression 3. DNA can flow between two different bacterial cells – Recombination. DNA replication One parental double stranded DNA molecule is used to make 2 identical double stranded DNA molecules Because the strands are complementary: One strand can serve as a template for synthesis of the other strand DNA polymerase reads the order of nucleotides in the template strand to make a complementary new strand. 1. A small segment of the dsDNA unwinds and the strands are separated Forms the replication fork Each separated strand serves as template for synthesis of a complementary strand A Short RNA primer is produced by the enzyme: Primase Serves as starting site for nucleotides to form new strand of DNA. 2. Synthesis of the Leading strand DNA pol can only synthesize DNA in one direction = 5′ → 3′ Template must be read in the 3' → 5' direction Follows the replication fork Synthesis of the leading strand is continuous in the 5' to 3' direction. 3. Synthesis of the Lagging strand DNA polymerase can only make DNA in 5' to 3' direction But the second strand must be made in the opposite direction DNA polymerase synthesizes small fragments of DNA: Okazaki fragments Made in the 5' to 3' direction Afterwards, the RNA primers are removed and the fragments are joined together by enzyme DNA ligase. Gene expression Two parts: Transcription – information stored in DNA is copied into RNA Translation – information in RNA is decoded to make protein. Transcription Synthesis of RNA from a DNA template Ex. If Gene is: 3'-ATGCAT-5' Sequence is complementary to a gene mRNA will be: Except: it contains U instead of T 5'-UACGUA-3' Three types of RNA Messenger RNA (mRNA) – carries information for making specific protein Ribosomal RNA (rRNA) – forms part of the ribosome Transfer RNA (tRNA) – transports specific amino acids for protein synthesis Steps in Transcription 1. Initiation RNA polymerase binds to the gene at specific site called the promoter Separates (melts) the two strands Only one DNA strand is copied – the template The template is read in the 3' → 5' direction so that RNA can be made in the 5' → 3' direction. Steps in Transcription 2. Elongation RNA polymerase moves along the template synthesizing new RNA Allows the DNA to rewind behind it 3. Termination When RNA polymerase encounters the terminator (end of the gene) it falls off the template and releases the newly synthesized RNA. The genetic code Information in mRNA must be translated to make proteins Organized into sets of 3 nucleotides – codons Each codon specifies an amino acid to be added during protein synthesis ex. GGC specifies the amino acid glycine Sequence of codons in an mRNA determines sequence of amino acids in the protein Three codons specify no amino acid: UAA, UAG, UGA “Stop codons” Signal the end of protein synthesis. Translation 1. Initiation A ribosome assembles on the mRNA a tRNA carrying the amino acid formyl- methionine enters the P site a tRNA carrying a second amino acid enters the ribosome Specified by the codon in the A site The ribosome joins the amino acids together by a peptide bond. 2. Elongation The ribosome moves a distance of one codon down the mRNA The next codon is now in place in the A site The correct tRNA enters the A site, bringing with it the next amino acid to be added The amino acid is joined to the chain Forms a polypeptide Elongation continues until a “stop codon” is reached. 3. Termination When a stop codon enters the A site, the ribosome disassembles and releases the polypeptide The polypeptide is folded into the correct shape and becomes a protein The ribosome can initiate translation of another mRNA.