Proteins.ppt

(1)

Nucleic Acids and

Proteins

(2)

Every somatic cell…

• Has full genetic set

• BUT, specialized cells exist in multicellular

organisms

– brain, skin, bone, muscle, eye, gonads, etc...

How does DNA ‘know’ what kind of cell to make?

– Environmental cues (time, location within organism, chemical exposure, etc) tell the cell which genes to turn on/off

(3)

Remember, DNA…

• Codes for EVERYTHING in the cell

• Many nucleotide sequences repeat in one DNA

strand

– may indicate evolutionary relationships between organisms

– some ‘leap’ into/out of sequence to control gene expression

• Nucleotide sequences also support helical

structure

– _telomeres_{(= end caps) control degradation of DNA}

• Nucleotide sequences are specific for coding

proteins

– exons vs. introns

(4)

Is every base pair essential?

• Still under debate…

• Exon – RNA that codes for proteins

• Intron – RNA sequence that doesn’t code for amino acids • If cells determine a need for a specific protein

– nucleus use enzymes to cut out introns

– other enzymes fuse together the necessary exons

– edited sequence = gene, which is ready for specific protein synthesis

• Some genes can translocate to create new varieties of protein expression = Transposons and Barbara McClintock’s Indian corn

(5)

Gene expression

• 1 gene – 1 polypeptide ‘rule’ is often the

case

• BUT, when transposons jump from one

DNA strand to another, they change the

protein being formed, so more than one

n-peptide is formed from a set of genes

Red = Exon

Blue/black line = possible intron region

(6)

• “Central dogma” of transcription:

DNA → RNA → proteins

• how can nucleus get info to the

protein-making organelle (= __________)?

SEM of nuclear area

(7)

First, there are 3 types of RNA

• rRNA

: ribosomal RNA – this and proteins make

up a ribosome

– Made of 2 subunits – small and large – Bacterial rRNA = 70S (S = Svedburg units)

– Eukaryote rRNA = 80S

• mRNA

: messenger RNA

– provides a complementary (opposite), mobile strand to DNA

• tRNA

: transfer RNA

– Attached to amino acids

– Pairs with (opposes) mRNA

Ribosome from E. coli

(8)

Making proteins

Step 1. TRANSCRIPTION

• Section of DNA gets transcribed into mRNA

1) Sense (~newly made mRNA) vs. Antisense

(complement to sense, one that gets transcribed)

2) Antisense strands of DNA are identified by gene promoter regions (=UTR) of DNA

3) RNA polymerase unwinds, unzips DNA base pairs

4) RNA polymerase reads antisense (AKA

coding) strand in 3’ to 5’ direction (pairs RNA nucleotides in 5’-3’) to create single

messenger strand (=mRNA)

5) RNA polymerase recognizes gene

(9)

• In prokaryotes, mRNA doesn’t need to

be modified before exiting nucleus

• Eukaryotes need

introns

removed from

very long mRNA

• _{Finally mRNA moves out of nucleus and}

into cytoplasm or ER to be

translated

into proteins

(10)

Animations

Transcription

• http://www-class.unl.edu/biochem/gp2/m_biology/animation /gene/gene_a2.html

• http://bcs.whfreeman.com/thelifewire/content/chp12/12020 01.html

• http://www.johnkyrk.com/DNAtranscription.html

• http://learn.genetics.utah.edu/content/begin/dna/transcribe/

Translation

• http://www-class.unl.edu/biochem/gp2/m_biology/animation /gene/gene_a3.html

• http://www.stolaf.edu/people/giannini/flashanimat/molgeneti cs/translation.swf

• http://vcell.ndsu.edu/animations/translation/movie.htm

• http://highered.mcgraw-hill.com/olc/dl/120077/micro06.swf

(11)

Making proteins

Step 2. TRANSLATION₁₎ _mRNA_{attaches to}_ribosome,

read in 5’→3’ direction

2) Initiates enzymatic

attachment of amino acid to corresponding tRNA at their

3’ ends

3) Ribosome reads mRNA in

3-nucleotide codons. AUG = start codon for every protein sequence, at 5’ end of mRNA

4) Codons H-bond at A site with complementary anticodons

on transfer RNA (=tRNA) and specific amino acid

5) Amino acids attach, turn into

peptide (pre-protein) chain at P site

6) After amino acid

condensation, ribosome

moves empty tRNA to E site

for release

A site (amino acid) P site

(peptide) E site (empty tRNA exit)

(12)

Translation has 4 phases

A. Initiation: ribosome reads “start” codon (=AUG)

on mRNA

- tRNAs change shape, attach to amino acids using enzymes and energy

B. Elongation: condensation of amino acids

- Polypeptide chain made through translation

C. Translocation: tRNAs moved from A→P→E

sites

- Ribosome starts at mRNA’s 5’ end, moves toward 3’

D. Termination: complete peptide chain

(13)

Making proteins

Step 3. FOLDING

1) Depending on location of ribosome,

peptide chain folds in various ways

2) In cytoplasm, peptide chain can’t make

disulfide bonds, so there are less

complex protein structures to be used in

cytoplasm/few organelles

3) In ER, protected peptide chain is able to

make complex protein structures to be

used in cell membrane/outside

(14)

Step 3. FOLDING (continued)

1. Primary (1°)

– single chain of

covalently bonded amino acids

2. Secondary (2°)

– 1° chain being

folded by hydrogen bonds

3. Tertiary (3°)

– van der Waals

forces between R groups folds

2° structure into a 3-D

polypeptide

-

some are functional (lysozyme)

4. Quaternary (4°)

– multiple 3°

subunits joined into a functioning

protein complex (insulin,

(15)

Globular vs. Fibrous proteins

Globular

• compact, rounded

shape

• usually water soluble

• enzymes, antibodies

(=functional proteins)

Fibrous

• elongated, usually

insoluble in water

• collagen, keratin

(=structural proteins)

(16)

Antibodies

• =

immunoglobulins

• consists of 2 light (short)

amino + 2 heavy (long)

amino chains

• active site varies based on

amino acid combination

• active site binds with

(17)

Remember,

• Proteins are necessary for every life

function.

– No proteins = no enzymes (so no ETC, ATP

synthesis, carbon fixation/oxidation, etc…)

– No proteins = no structure (no keratin,

collagen, actin, myosin, microtubules in

cytoskeleton, etc…)

– No proteins = no protection (antibodies,

hemoglobin, membrane-bound carrier

proteins, receptor proteins)

(18)

mRNA codons and their amino

acids

There’s a lot of repetition. Except for methionine, more than one codon

codes for each amino acid.

(19)

Show this sequence on a piece

of paper,

1) Make a chart

2) Start with DNA sense strand of AATACGACCTAGGAATTGA. What is the antisense strand? 3) Build the mRNA. Where do

these steps occur?

4) Identify the codons, including start and stop.

5) What are the complimentary tRNA anticodons?

6) Which amino acids make up your protein (p.64)?

(20)