Bio 1010

Dr. Bonnie A. Bain

DNA Structure and FunctionPart 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 is UUUGGCCGUUUU

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 thepolypeptide 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

43 = 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 codonAUGNote: also codes for an amino acid

(Met)

STOP codons UAAUAGUGA

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 strandDuring 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 cellsHave a nucleus, after mRNA is

transcribed, it has to be processed before leaving the nucleus

Processing of Eukaryotic RNA

1. Addition of extra nucleotidesCap 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. 183-186)(translate the mRNA code into a polypeptide)

The PlayersmRNAtRNARibosomes (rRNA + protein)

The ProcessInitiationElongationTermination

Figure 10.17

mRNA

Carries the information from the DNA to the Ribosome

Figure 10.15

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 siteA triplet which pairs with a specific

amino acid

20 different tRNA moleculesOne for each of the 20 amino acids

Figure 10.16

Ribosomes: Protein Synthesis occurs here

Ribosomes

Site where polypeptides are made(protein factory)

Ribosome structureSmall subunit

has binding site for mRNA

Large subunit2 binding sites for tRNA

Large subunit2 binding sites for tRNA:

P siteHolds the tRNA carrying the

growingpolypeptide chain

A siteHolds 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 ProcessInitiationElongationTermination

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 acidmethionine on its amino acid attachment site

Translation

InitiationElongationTermination

Translation

InitiationElongationTermination

Translation

InitiationElongationTermination