Transcription Transcription factors bind to the TATA box on the DNA strand RNA polymerase pries the two strands of DNA apart RNA polymerase joins RNA nucleotides together that are complementary to the DNA template RNA polymerase works only in the 5' to 3' direction - just like DNA polymerase
Translation mRNA transcript is brought to the ribosome Initiation = the rRNA, mRNA transcript, and tRNA carrying methionine bind together Elongation = amino acids are added one by one to create the polypeptide Termination = when a stop codon is reached on the mRNA
What two processes ensure that the correct amino acid is added to a growing polypeptide chain?
There are 64 codons. Three are stop codons, so we can say there are 61 amino acid coding codons. However, there are only 45 tRNA molecules that exist. How is this possible?
The need for molecular recognition... Pairing of the tRNA anticodon and mRNA codon, but wobble allows for some mistakes Pairing of the tRNA with the amino acid, accomplished by aminoacyl-tRNA synthetases
The Genetic Code mRNA is read three bases at a time 3 mRNA bases = a codon
Ribosomes Bring mRNA and tRNAs together 3 sites = A, P, and E P= holds the tRNA that is attached to the growing polypeptide chain A = holds the tRNA carrying the next amino acid to be added to the chain E = is where discharged tRNAs leave the ribosome
5' Cap and Poly A tail Facilitate the export on mRNA from the nucleus Help protect the mRNA from degradation from hydrolysis enzymes in the cytoplasm Help ribosomes attach to the 5' end of the mRNA prior to translation
RNA Splicing The average transcription unit is 27,000 bases long. The average protein is 400 amino acids long, requiring only 1,200 RNA bases. This means that most mRNA transcripts are initially too long and contain a lot of non-coding segments.
RNA Splicing continued... Non-coding segments (introns) of the mRNA transcript are dispersed among the coding segments (exons) Introns are removed from the mRNA transcript prior to it leaving the nucleus. This forms a mRNA transcript with a continuous coding sequence
RNA Splicing continued... The signal for RNA splicing is a short nucleotide sequence at each end of an intron snRNPs (small nuclear ribonucleoproteins) recognize these sequences
Splicesomes Includes multiple snRNPs and proteins Cuts out introns and pastes together exons
A Note... A single gene can code for more than one type of polypeptide This is because of alternative RNA splicing Whether mRNA segments are treated as introns or exons depends on the polypeptide product being expressed For this reason the number of protein products an organism produces can be much greater than its number of genes
Mutations! The ultimate source of new genes and, therefore, genetic diversity.
Mutations are caused by... Physical mutagens (e.g. radiation) Chemical mutagens (e.g. tobacco) Chemical mutagens that cause cancer are called carcinogens Exposure to mutagens increases over an organism's lifetime
So, what is a gene? A discrete unit of inheritance that affects phenotype A specific segment of a chromosome A specific nucleotide sequence on a DNA molecule A DNA sequence that codes for a specific polypeptide product
Let's use... Gene = a region of DNA that can be expressed to produce a final functional product that is either a polypeptide or an RNA molecule
Mutations Silent mutations do not change what the gene expresses Frameshift mutations can change what, if anything, the gene expresses
Exit Slip How are transcription and translation similar? How are they different?