The beginning of protein synthesis

OVERVIEW  Uses a strand of nuclear DNA to produce a single-stranded RNA molecule  Small section of DNA molecule (gene) unwinds and free-floating nucleotides bind to the strand  The strands are complimentary Eg. DNA - A T A T G C RNA - U A U A C G with U replacing T in RNA

1. The enzyme RNA POLYMERASE binds to the PROMOTER site on the DNA 2. The DNA being transcribed is separated by the RNA polymerase in the region where the gene is to be transcribed 3. RNA nucleotides pair with their complimentary bases on one strand of the DNA. REMEMBER: Uracil pairs with adenine on the RNA 4. RNA polymerase forms covalent bonds between the nucleotides 5. RNA separates from the DNA and the double helix reforms

 The section of DNA being read is called the ANTI- SENSE or TEMPLATE strand  The opposite side of the DNA is called the SENSE or COMPLIMENTARY strand  In eukaryotes, the immediate product of transcription is known as PRE-mRNA  it must go through several stages of modifications before becoming MATURE mRNA  One of these is called RNA SPLICING  throughout the mRNA are sequences that will not contribute to the formation of the polypeptide  these pieces of mRNA are called INTRONS  the introns must be removed and the remaining coding positions of mRNA are called EXONS  the exons are spliced (joined) together to form mature mRNA  the ends are capped (METHYLATED CAP) and tailed (POLY- A TAIL)

VIDEO: TRANSCRIPTION

THE FORMATION OF A PROTEIN TRANSLATION

Translation Overview Translation is the process of protein production using mRNA as a guide THE GENETIC CODE  Is the code used that enables the cells machinery to convert the base sequence into an amino acid  The sequence of 3 bases on the mRNA is called a CODON  Each codon (triplet) codes for a specific amino acid which will be added to the polypeptide  Each amino acid is carried by a specific transfer RNA (tRNA) which has a 3-base code complimentary to the mRNA called an ANTI-CODON  A tRNA with the correct anti-codon attaches to the codon on the mRNA  There are 64 possible codons  Different codons may code for the same amino acid  ‘STOP’ codons code for translation to stop RIBOSOMES  Translation takes place outside the nucleus at the RIBOSOMES  They consist of protein combined with RIBOSOMAL RNA (rRNA) which does not contain a genetic message  Ribosomes contain 2 sub-units: a small 40s sub- unit and a larger 60s sub-unit  Generally proteins secreted from cells are synthesised on ribosomes bound to the ER and those that remain in the cytosol are made on free ribosomes  May form POLYRIBOSOMES or POLYSOMES that all bind to a single mRNA moving along it like a production line. This allows more than one ribosome to translate the mRNA at the same time

RIBOSOME POLYSOME Translation Diagrams

Translation, the process 1. mRNA binds to the small subunit (40s) of the ribosome 2. tRNA molecules carry a specific amino acid corresponding to its anti-codon 3. The tRNA binds to the ribosome at the site where its anticodon matches the codon on the mRNA 4. The ribosome moves along the mRNA, adding amino acids to the polypeptide chain, until a stop codon is reached 5. The completed polypeptide is then released 6. Each tRNA molecule is recognised by a tRNA- activating enzyme that binds the specific amino acid to the tRNA, using ATP for energy 7. The amino acid attaches at the 3’ end of the tRNA which terminates with the sequence CCA 8. Each ribosome has 3 tRNA binding sites: E – Exit site P – Peptidyl site A – Aminoacyl site (see diagram previous) A tRNA molecule

Translation diagram VIDEO: CRASHCOURSE PROTEIN SYNTHESIS VIDEO: TRANSLATION

Every time a cell divides, the genetic information contained in the chromosomes needs to be COPIED and DISTRIBUTED to the DAUGHTER CELLS

The double stranded DNA molecule is acted upon by the enzyme DNA HELICASE. The enzyme breaks the hydrogen bonds between the base pairs. The strands move apart creating replication forks.

Each of the strands acts as a template for a new DNA strand. A short sequence, made of RNA called a PRIMER attach to the single DNA strands and initiate replication Individual nucleotides are added at the 3’ end of the DNA strand according to the base-pairing rules The process is catalysed by the enzyme DNA POLYMERASE by linking the phosphate of the newest nucleotide to the sugar before it

One DNA strand (THE LEADING STRAND) forms continuously in the direction of replication On the other DNA strand (travelling in the opposite direction) forms discontinuously. This strand is called the LAGGING STRAND. The fragments on the lagging strand are joined by the enzyme DNA LIGASE The lagging strand must wait for the DNA to unwind before the DNA polymerase can act on the template. This results in the new DNA strand forming in sections called OKAZAKI FRAGMENTS

Each of the double helix molecules is an EXACT copy of the original DNA The process is SEMI-CONSERVATIVE  each of the new double stranded molecules contains one of the ORIGINAL DNA strands and one NEW DNA strand