A tour through the Cell cont…

The framework of a cell - the Cytoskeleton

Organelles in action - Protein Synthesis

CytoplasmCytoplasm• semi-fluid-like jelly within the cell• division into three subdivisions: cytosol, cytoskeleton & organelles

Cytoskeleton:•internal framework of the cell•gives the cytoplasm flexibility and strength•provides the cell with mechanical support•gives the cell its shape

•can be rapidly disassembled in one area of the cell and reassembled in another

•anchorage points for organelles and cytoplasmic enzymes•also plays a role in cell migration and movement by the cell

•three major components1. microfilaments2. intermediate filaments3. microtubules

Cytoskeleton:

microfilaments = thin filaments (7 nm) made up of a protein called actin-twisted double chain of actin subunits-forms a dense network immediately under the PM (called the cortex)-also found scattered throughout the cytoplasm -function: 1. anchor to membrane proteins

2. interaction with myosin = interacts with larger microfilaments made up of myosin - results in active movements within a cell (e.g. muscle cell contraction)3. provide much of the mechanical strength of the cell 4. give the cell its shape5. also provide support for cellular extensions called microvilli (small intestines)

intermediate filaments = range from 8 to 12 nm in diameter-function: 1. impart strength to the cytoskeleton (like microfilaments)

2. support cell shape3. anchors & stabilize organelles4. transport materials within a cell

microtubules = hollow rods or “straws” of 25 nm in diameter- made of repeating units of proteins called tubulin

- function: 1. cell shape & strength2. organelles: anchorage & movement3. mitosis - form the spindle (chromosome

movement)4. form many of the non-membranous

organelles - cilia, flagella, centrioles

Organelles in Action• organelles attach to the cytoskeleton – held in place• each organelle has a distinct function• organelle of protein synthesis = Endoplasmic Reticulum

– large organelle surrounded by a phospholipid bilayer and attached to the nucleus

– can be found studded with ribosomes = Rough ER (protein synthesis)– parts found without ribosomes and make lipids = Smooth ER

• organelle of protein modification and packaging = Golgi apparatus

Protein synthesis

• known as translation– translating the message found in DNA/RNA into a

polypeptide chain protein• requires three things– 1. mRNA – messenger RNA transcribed from the

DNA template– 2. tRNA – transfer RNA that carries the amino

acids of the future protein– 3. ribosome – the “machine” of translation

• How are the instructions for assembling amino acids into proteins encoded in your DNA?

• first the DNA gets transcribed into a message = mRNA• the mRNA gets exported out into the cytoplasm• the mRNA gets bound by a ribosome• tRNA molecules bring the correct amino acid into the ribosome• amino acids are linked together

mRNA

• the mRNA nucleotide sequence is “read” by the ribosome in groups of 3 nucleotides = “codon”

• each codon codes for 1 of the 20 amino acids that make up proteins in eukaryotes

• all of these codons grouped together is called the “genetic code”

• the code is redundant - each amino acid can be coded for by more than one codon

• e.g. alanine – GCU, GCC, GCA and GCG• the GC defines the amino acid as alanine

• in many cases the 3rd codon is important in defining the amino acid– serine – codons are: AGU, AGC– BUT arginine codons are: AGA and AGG

Protein Translation: The Genetic Code

Second mRNA base

Firs

t mR

NA

bas

e (5

end

of c

odon

)

Third

mR

NA

bas

e (3

end

of c

odon

)

UUU

UUC

UUA

CUU

CUC

CUA

CUG

Phe

Leu

Leu

Ile

UCU

UCC

UCA

UCG

Ser

CCU

CCC

CCA

CCG

UAU

UACTyr

Pro

Thr

UAA Stop

UAG Stop

UGA Stop

UGU

UGCCys

UGG Trp

GC

U

U

C

A

U

U

C

C

CA

U

A

A

A

G

G

His

Gln

Asn

Lys

Asp

CAU CGU

CAC

CAA

CAG

CGC

CGA

CGG

G

AUU

AUC

AUA

ACU

ACC

ACA

AAU

AAC

AAA

AGU

AGC

AGA

Arg

Ser

Arg

Gly

ACGAUG AAG AGG

GUU

GUC

GUA

GUG

GCU

GCC

GCA

GCG

GAU

GAC

GAA

GAG

Val Ala

GGU

GGC

GGA

GGGGlu

Gly

G

U

C

A

Met orstart

UUG

G

Amino acidattachmentsite

3

5

Hydrogenbonds

Anticodon(a) Two-dimensional structure (b) Three-dimensional structure(c) Symbol used

in books

Anticodon Anticodon3 5

Hydrogenbonds

Amino acidattachmentsite5

3

A A G

• where do the amino acids come from• they are brought into the ribosome bound to tRNA molecules• tRNA molecule consists of a single strand of RNA - about 80 RNA nucleotides

long• at one end – anticodon site for binding with the mRNA template• at the other end – attachment site for the amino acid that corresponds to the

mRNA codon

Building a protein: tRNA

Building a Protein: Ribosomes• machine of translation• made in the nucleolus in eukaryotic cells• comprised of two subunits of proteins (large and small) linked

together – eukaryotes: small subunit = ~33 proteins + large subunit = ~50 proteins– subunits are exported out via nuclear pores

Ribosomes• within the large subunit are two sites for the binding of tRNAs

– P-site or Peptidyl-tRNA site – “old” AA– A-site or aminoacyl-tRNA site – incoming AA

• and one E site/Exit site for the exit of the old tRNA off the ribosome

Amino end

mRNAE

(c) Schematic model with mRNA and tRNA

5 Codons

3

tRNA

Growing polypeptide

Next aminoacid to beadded topolypeptidechain

Translation

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__how_translation_works.html

Lysosomes = “garbage disposals”-dismantle debris, eat foreign invaders/viruses taken in by endocytosis or phagocytosis-also destroy worn cellular parts from the cell itself and recycles the usable components = autophagy-form by the budding of vesicles off the Golgi and their fusion-acidic interior-1. contain enzymes that breakdown DNA, RNA (nucleases) and proteins (proteases)-2. contains enzymes for the breakdown of lipids and phospholipids

Organelles in Disease: The lysosome

Tay Sachs and lysosomes: human genetic disease-severe mental degradation-lysosomes lack one of the 40 required enzymes-results in a build up of fatty material on neurons-failure of nervous system communication -infantile form of the disease = death by 4 yrs-juvenile form = death from 5 to 15 yrs-adult onset – not fatal; progressive loss of nervous function-most common in Ashkenazi Jews, FrenchCanadians and Cajun populations in Louisiana (same mutation as Jews)

-only identified in 1954-found in all cells – abundant in liver and kidney cells-major function is breakdown of long chain fatty acids-other functions:

1. synthesis of bile acids2. breakdown of alcohol by liver cells3. anti-oxidant function - contains enzymes to break down dangerous

chemicals made by the cell during metabolism

F-actin and peroxisomes

Organelles in Disease: The Peroxisome

Adrenoleukodystrophy and peroxisomes: -X linked disorder -1:20,000 to 1:50,000 births-peroxisomes can’t break down fatty acids properly-leads to a build up of big, saturated fatty acids on cells of throughout the body-can result in neuron death – not known why-lethargy, skin darkens, blood sugar drops, altered heart rhythm due to imbalanced electrolytes, paralysis, death

*** slowed by a certain triglyceride found in rapeseed oilLorenzo Odone = “Lorenzo’s Oil” (mixture of unsaturated fatty acids that slows the

development of these saturated FAs)