From Dna To Protein

From DNA to Protein Chapter 14

RNA transcribed from DNA • Messenger RNA (mRNA)- genes that specify proteins that are transcribed. It is the only class of RNA that carries the protein-building codes. • Ribosomal RNA (rRNA)- is a component of ribosomes, the structure in which polypeptide chains are assembled. • Transfer RNA (tRNA)- delivers amino acids one by one to ribosomes in the order specified by mRNA. • RNA has four kinds of ribonucleotides each with the five-carbon sugar ribose, one phosphate group, and one base. • The four bases in RNA are adenine, guanine, cytosine, and uracil.

Cont. • Transcription differs from DNA replication in 3 aspects. – Only part of one DNA strand is unwound and used as a template. – The enzyme RNA polymerase adds ribonucleotides one at a time to the end of a growing strand of RNA. – Transcription results in one free RNA strand, not a hydrogen-bonded double helix. • Promoter- is a start signal, which is a base sequence in DNA to which RNA polymerase bind and start transcription. • RNA polymerase recognizes a gene region and moves along it.

Cont. • In eukaryotic cells the mRNA is modified before leaving the nucleus. – Which is considered pre-mRNA. – Some are modified by adding a guanine cap to the start of the pre-mRNA – Some add 100-300 adenine ribonucleotides as a tail, which we call a poly A tail. • Exons- protein-coding base sequences. • Introns- noncoding protein sequences. • Both exons and introns are transcribed on the same sequence but the introns are snipped out, which produces mature mRNA. • Alternative splicing- one gene can specify 2 or more proteins that differ slightly in form and function.

Genetic Code • The mature mRNa must be read by the ribosome in transcription and it does this by a three letter code. • The 3 letter code is the bases, they are read three bases at a time. • Codon- is the base triplets in mRNA. • There are 64 different codons even though there are only twenty amino acids. • Certain codons can specify for more than one amino acid. • Start signal for transcription is AUG (methionine). It is the first amino acid in all polypeptide chains.

Other RNAs • All cells have pools of tRNAs and amino acids in their cytoplasm. • The tRNA has a attachment site for an amino acid which is anticodon. • Anticodon- a ribonucleotide base triplet that can basepair complementary codon in a mRNA transcript. • When the tRNA binds to the mRNA on a ribosome the amino acid attached to each becomes positioned in the order that the codon specifies. • Ribosome have 2 subunits made of rRNA and structural proteins. – They are built in the nucleus and moved to the cytoplasm. – There is a large subunit and a small subunit. – They only fucntion when a mRNA is attached to them. • Adenine pairs to uracil and cytosine pairs with

Three Stages of Translation • Initiation stage can only be started by one tRNA and that is UAC, which is complementary to the start codon of mRNA. • The anticodon and the codon meet at the small ribosomal subunit. • Then the large ribosomal subunit joins to the small subunit. • Elongation stage a polypeptide chain is synthesized while the mRNA passes between the two ribosomal subunits. • One region of the rRNA molecule located at the center of the large ribosomal subunit is highly acidic and it functions as an enzyme. • It catalyzes the formation of peptide bonds between amino acids.

Cont. • Termination-the last stage of translation the stop codon from mRNA enters the ribosome. • Proteins called release factors bind to the ribosome, which triggers enzyme activity that detaches the mRNA and the polypeptide chain from the ribosome. • Many new polypeptide chains carry out their functions in the cytoplasm. • Some have a special sequence that tell

Mutated Genes and their Protein Products

• Gene mutation- is when a base is either lost or inserted in a DNA strand. This change may alter the message that becomes encoded in mRNA. • Base-pair substitution- when the wrong nucleotide becomes paired with an exposed base on the DNA template. This may produce the wrong amino acid or a premature stop codon. • Frameshiftmutation- shifts the three base at a time reading frame. There are two different types. – Insertions- is when one or more bases are inserted into the DNA strand. – Deletion is when one or more bases are deleted from a DNA strand. Transposons- can jump around a genome. They are

Cont. • Mutations can happen spontaneously in DNA especially while its being replicated. • DNA polymerase and ligase can fix most of the mistakes. • Mutations can also occur when DNA are exposed to mutation-causing agents. – Ionizing radiation (x-rays)- break chromosomes into pieces. It can indirectly effect DNA also by leaving behind free radicals. – Nonionizing radiation- excites electrons to a higher energy level. DNA absorbs UV light, which forms thymine dimers ( a string of thymines bonded together it is very bulky). – Thymine dimers are the original mutation that lead to cancer. – Alkylating Agents- can transfer charged methyl or ethyl groups to reactive sites in DNA. It makes DNA more vulnerable to mistakes. – If a mutation arises in somatic cells its effects will not be passed on to offspring.