Bio 1010 Dr. Bonnie A. Bain

DNA Structure and Function Part 2

Chemical Languages: The best way to understand the next section is to think of there being a DNA language, an RNA language, and a polypeptide language

DNA Language: DNA has 4 kinds of monomers (nucleotides): A,T, C, G Monomers are the letters in this language The language of DNA is written as a linear sequence of nucleotides: AAACCGGCAAAA

DNA Language: Gene: A specific sequence of bases, each with a beginning and an end A typical gene can consist of thousands of nucleotides One DNA molecule can contain thousands of genes

RNA Language: RNA has 4 kinds of monomers (nucleotides): A,U, C, G The language of RNA is written as a linear sequence of nucleotides: UUUGGCCGUUUU

Transcription When the language of DNA is transcribed into the language of RNA This is how a mRNA molecule is built: DNA sequence is AAACCGGCAAAA mRNA sequence isUUUGGCCGUUUU

Figure 10.10

Transcription The nucleotide bases of the RNA molecule are complementary to the ones on the DNA strand The DNA molecule was used as a template to make mRNA

Translation Conversion of the nucleic acid language to the polypeptide language The monomers or letters of the polypeptide language are the 20 different amino acids

Translation The sequence of nucleotides in a mRNA molecule dictates the sequence of amino acids in the polypeptide mRNA UUU GGC CGU UUU Polypeptide AA1 AA2 AA3 AA4

Figure 10.10

The Triplet Code Every 3 bases codes for one amino acid Each triplet in the mRNA is called a Codon mRNA UUU GGC CGU UUU Polypeptide AA1 AA2 AA3 AA4

Why a triplet code? This is the only way to get enough “words” to specify all of the required amino acids 4 bases (in mRNA): A, G, C, U 20 different amino acids 4 3 = 64 different possible codons

With 64 possible code words, can have redundant coding for each amino acid Example: Glutamic Acid codons: GAA, GAG Glycine codons: GGU, GGC, GGA, GGG REDUNDANCY in the code, but not AMBIGUITY

Also, can have START and STOP codons START codon AUG Note: also codes for an amino acid (Met) STOP codons UAA UAG UGA

First Base Second Base Third Base In a codon, first base is the first letter, second base is the second letter, third base is the third letter AUG

Question: What are these codons or triplets? Answer: The Genetic Code It is shared by all organisms, although there is a tiny bit of variation

Figure 10.11

Figure 10.00d Should say most, not all share the same genetic code—there are a few variations Tobacco plant + a Firefly gene

Transcription (more details) A mRNA molecule is transcribed from the DNA molecule in the nucleus of the cell Enzyme required: RNA Polymerase

Figure 10.13a

Transcription (con't) Similar to DNA Replication, but not quite the same 1. DNA molecule is split apart by RNA Polymerase 2. One of the DNA strands serves as a template

Transcription (con't) 3. Initiation of transcription 4. Elongation of mRNA strand 5. Termination of transcription

Figure 10.13b

3. Initiation of Transcription Promoter region: A specific site on the DNA where the RNA Polymerase attaches Located at the beginning of a gene Dictates which DNA strand is to be transcribed The RNA Polymerase “knows” that it starts transcription at the promoter

Figure 10.13b

3. Initiation of Transcription First, the RNA Polymerase binds to promoter Second, RNA synthesis begins During transcription, an entire gene is transcribed into a mRNA molecule

Figure 10.13a

Transcription (con't) 4. Elongation of mRNA strand During this phase, the mRNA grows longer Behind it, the DNA strands come back together

Figure 10.13b

Transcription (con't) 5. Termination of transcription Eventually the RNA Polymerase reaches the Terminator region of the gene (end of the gene) The terminator region says, “Stop transcribing” At this point, the RNA Polymerase detaches and the mRNA leaves the nucleus

Prokaryotic cells: Lack a nucleus, after mRNA is transcribed, it immediately takes part in translation Eukaryotic cells Have a nucleus, after mRNA is transcribed, it has to be processed before leaving the nucleus

Processing of Eukaryotic RNA 1. Addition of extra nucleotides Cap and Tail are added to the RNA strand Protect the mRNA from cellular enzymes Help ribosomes recognize the mRNA

Figure 10.14

Processing of Eukaryotic RNA 2. RNA Splicing Introns are removed from the mRNA Introns are non-coding stretches of the mRNA Intron functions: ?? (still unknown)

Figure 10.14

2. RNA Splicing (con't) Exons: The coding regions of the mRNA After the Introns are removed, only the Exons are left

2. RNA Splicing (con't) In humans, the Exons in the mRNA can be selectively removed, making different polypeptide “recipes” Allows 25,000 genes (in humans) to make lots more than 25,000 different polypeptides EXAM GOES TO HERE Note: this lecture continues on Part 4—this file is too big!!

Figure 10.10

Translation (p ) (translate the mRNA code into a polypeptide) The Players mRNA tRNA Ribosomes (rRNA + protein) The Process Initiation Elongation Termination

Figure mRNA Carries the information from the DNA to the Ribosome

Figure tRNA

Transfer RNA or tRNA The “Interpreter” Converts the 3-letter codon to an amino acid At the ribosome (protein factory), the tRNA molecules match the amino acids with the appropriate codons in the mRNA

Transfer RNA or tRNA Source of amino acids: Cytoplasm of the cell tRNA tasks: 1. Pick up the appropriate amino acids 2. Bring them to the ribosome and place them on the right spot in the mRNA

tRNA Structure Single strand of RNA consists of approx. 80 nucleotides The single strand is twisted and folded into a particular shape

Figure 10.15

tRNA Structure Has two distinct ends: Anticodon: A triplet (3 bases) which will pair with a specific codon in the mRNA Amino Acid attachment site A triplet which pairs with a specific amino acid 20 different tRNA molecules One for each of the 20 amino acids

Figure Ribosomes: Protein Synthesis occurs here

Ribosomes Site where polypeptides are made (protein factory) Ribosome structure Small subunit has binding site for mRNA Large subunit 2 binding sites for tRNA

Large subunit 2 binding sites for tRNA: P site Holds the tRNA carrying the growing polypeptide chain A site Holds a tRNA carrying the next amino acid to be added to the growing polypeptide

Figure 10.16a

Figure 10.16

Translation (translate the mRNA code into a polypeptide) The Players *mRNA *tRNA *Ribosomes (rRNA + protein) The Process Initiation Elongation Termination

Initiation of Translation 1. An mRNA molecule binds to a small ribosomal subunit A special “initiator” tRNA then binds to the Start codon on the mRNA The “initiator” tRNA carries the amino acid methionine on its amino acid attachment site

Translation Initiation Elongation Termination

Translation Initiation Elongation Termination

Translation Initiation Elongation Termination