http://www.youtube.com/watch?v=Q6ucKWIIFmg&feature=related

ΚΥΤΤΑΡΙΚΟΣ ΚΥΚΛΟΣ

CELL CYCLE

G1: Gap1Growth and preparation of the chromosomes for replication

S: DNA synthesis (DNA replication)Synthesis of DNA and duplication of the centrosome

G2: Gap2Preparation for mitosis

M: Mitosis (nuclear/chromosome separateand cytoplasm division/cytokinesis)

Cyclins: major control switches of cell cycle

Cdk: cyclin dependent kinase adds phosphate to a protein

MPF: maturation promoting factorTriggers progression through the cell cycle

P53: blocks cell cycle if DNA is damagedmay lead to apoptosis (cell death)

P27: blocks entry into S phaseby binding to cyclin and cdk

REGULATION OF CELL CYCLE

The anaphase-promoting complex (APC) (also called the cyclosome) The APC/C: triggers the events that allow the sister chromatids to separate;

degrades the mitotic cyclin B.

REGULATION OF CELL CYCLE

Cyclins G1 cyclin (cyclin D) S-phase cyclins (cyclins E and A) mitotic cyclins (cyclins B and A)

Their levels in the cell rise and fall with the stages of the cell cycle.

Cyclin-dependent kinases (Cdks) G1 Cdk (Cdk4) S-phase Cdk ((Cdk2) M-phase Cdk (Cdk1)

Their levels in the cell remain fairly stable, but each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated. They add phosphate groups to a variety of protein substrates that control processes in the cell cycle.

Rising level of G1-cyclins bind to their Cdks and signal the cell

to prepare the chromosomes for replication. Rising level of S-phase promoting factor (SPF) — which includes cyclin A bound to Cdk2 — enters the nucleus and prepares the cell to duplicate its DNA (and its centrosomes). As DNA replication continues, cyclin E is destroyed, and the level of mitotic cyclins begins to rise (in G2). M-phase promoting factor (the complex of mitotic cyclins with the M-phase Cdk) initiates assembly of the mitotic spindle breakdown of the nuclear envelope condensation of the chromosomes These events take the cell to metaphase of mitosis.

STEPS IN THE CYCLE

At this point, the M-phase promoting factor activates the anaphase-promoting complex (APC/C) which

allows the sister chromatids at the metaphase plate to separate and move to the poles (= anaphase), completing mitosis; destroys cyclin B. It does this by attaching it to the protein ubiquitin which targets it for destruction by proteasomes.turns on synthesis of G1 cyclin for the next turn of the cycle; degrades geminin, a protein that has kept the freshly-synthesized DNA in S phase from being re-replicated before mitosis.

Λάθη κατά την αντιγραφή του DNA

Θαύση του DNA

Απώλεια τελομερών

Τα ελεύθερα άκρα του DNAεπάγουν την δημιουργία αναδιατάξεων

Ενεργοποίηση σημείων ελέγχου

Παύση της κυτταρικής ανάπτυξης

Επιδιόρθωση της βλάβης Κυτταρικής ανάπτυξης Κυτταρικός θάνατος

Αποσταθεροποίηση του γονιδιώματος από κακή λειτουργία των σημείων ελέγχου

ΜΙΤΩΣΗ

INTERPHASE

The cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division).

Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible.

The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules

PROPHASE

Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes.

The nucleolus disappears.

Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres.

Some fibers cross the cell to form the mitotic spindle.

PROMETAPHASE

The nuclear membrane dissolves, marking the beginning of prometaphase.

Proteins attach to the centromeres creating the kinetochores.

Microtubules attach at the kinetochores and the chromosomes begin moving.

Spindle fibers align the chromosomes along the middle of the cell nucleus.

This line is referred to as the metaphase plate.

This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.

METAPHASE

ANAPHASE

The paired chromosomes separate at the kinetochores and move to opposite sides of the cell.

Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.

TELOPHASE

Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei.

The chromosomes disperse and are no longer visible under the light microscope.

The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.

CYTOKINESIS

In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus.

In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

Prometaphase Prophase Metaphase

Anaphase Telophase Cytokinesis

MITOSIS

Many times a cell will leave the cell cycle, temporarily or permanently. It exits the cycle at G1 and enters a stage designated G0 (G zero). A G0 cell is often called "quiescent", but that is probably more a reflection of the interests of the scientists studying the cell cycle than the cell itself. Many G0 cells are anything but quiescent. They are busy carrying out their functions in the organism. e.g., secretion, attacking pathogens. Often G0 cells are terminally differentiated: they will never reenter the cell cycle but instead will carry out their function in the organism until they die. For other cells, G0 can be followed by reentry into the cell cycle. Most of the lymphocytes in human blood are in G0. However, with proper stimulation, such as encountering the appropriate antigen, they can be stimulated to reenter the cell cycle (at G1) and proceed on to new rounds of alternating S phases and mitosis.G0 represents not simply the absence of signals for mitosis but an active repression of the genes needed for mitosis. Cancer cells cannot enter G0 and are destined to repeat the cell cycle indefinitely.

GO

ΑΝΙΜΑΤΙΟΝ

http://www.youtube.com/watch?v=s1ylUTbXyWU&feature=player_embedded#

http://www.youtube.com/watch?v=lf9rcqifx34&feature=related

http://www.youtube.com/watch?v=2WwIKdyBN_s&feature=related

ΜΕΙΩΣΗ

Meiosis I as in mitosis

Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid.

MEIOSIS

Animation:http://www.biology.arizona.edu/CELL_BIO/tutorials/meiosis/page3.html

http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=related

http://www.youtube.com/watch?v=MqaJqLL49a0&feature=related

ΑΝΤΙΓΡΑΦΗ

Replication Bubble Forms

Helper proteinsHelper proteins

Directionality

Multiple Sites of Replication Origin of DNA

5’ to 3’ replication occurs in eukaryotes just like in prokaryotes

In this micrograph, a replication

bubble is visible along the

DNA of cultured cells.

Arrows indicate DNA replication direction at the two ends of each bubble.

Lagging strand

Leading strand

E. coli DNA replication

Helicase

RNA primer

DNA polymerase ILigase

DNA polymerase IIIActive sites

replicating

primase

primosomessDNA binding protein

Step 4. Elongation

Step 3: Primase

Okazaki Fragments

A. Ελικάση: απελίκωση και διαχωρισμός των αλυσίδων DNA.SSB: πρωτεϊνικό σύμπλοκο με μονόκλωνο DNA.

B. RNA πριμάση: δημιουργία υποκινητή RNA.

DNA πολυμεράση ΙΙΙ: έναρξη σύνθεσης της οδηγού αλυσίδας.

Γ. DNA πολυμεράση ΙΙΙ: έναρξη σύνθεσης της συνοδού αλυσίδας.

δημιουργία κομματιών Okazaki.

Δ. DNA πολυμεράση ΙΙΙ: επιμήκυνση και ολοκλήρωση σύνθεσης της οδηγού αλυσίδας.

Ε. Συνθετάση (Λιγάση): σύνδεση των τροποποιημένων κομματιών Okazaki.

ΣΤ. Ολοκλήρωση της σύνθεση της οδηγού αλυσίδας και της συνοδού.

ΑΝΤΙΓΡΑΦΗ DNA

Nucleosome Assembly

Unique to eukaryotic replication process

Animation:207.207.4.198/pub/flash/24/24.html

http://www.youtube.com/watch?v=teV62zrm2P0&feature=player_embedded#

http://www.youtube.com/watch?v=rpwjZX_z5rg&feature=related

http://www.youtube.com/watch?v=-mtLXpgjHL0&feature=related

ΚΕΝΤΡΙΚΟ ΔΟΓΜΑ

ΚΕΝΤΡΙΚΟ ΔΟΓΜΑ

The Central Dogma Of Molecular Biology

ReplicationDNA duplicates

TranscriptionRNA synthesis

TranslationProtein synthesis

4-Letter CodeA,T.G,C

4-Letter CodeA,U.G,C

20-Letter CodeAmino Acids

General Special Unknown

DNA → DNA RNA → DNA protein → DNA

DNA → RNA RNA → RNA protein → RNA

RNA → protein DNA → protein protein → protein

ΚΕΝΤΡΙΚΟ ΔΟΓΜΑ ????????

ribose

deoxyribose

Classification of RNAsRNAs

Functional Informational

transfer RNAsVital for protein

translation

Small nuclear RNAs snRNAsSmall nucleolar RNAs snoRNASmall cytoplasmic RNAs scRNASmall Interfering RNAs siRNAMicroRNA miRNA

messenger RNAsIntermediates

in decoding genes into proteins

ribosomal RNAsForms complexes with proteins

Vital for protein translation

Molecule of the year 2002

Sequences produced within the cell by transcription from individual miRNA genes, introns, or from polycistronic clusters of closely related miRNA genes. ‘pri-miRNAs’, are several thousand bases long.

miRNAs only have complementarity in a crucial ‘seed’ region 2-8 bases long in the 5’ region. This can make it possible for some miRNAs to pair with hundreds of high- and low-affinity mRNA targets (one-to-many), and conversely, multiple miRNAs may target a single mRNA (many-to-one). This mechanism seems to be a very ancient one in evolution, having been detected throughout plant and animal systems in various forms, and even in viruses.

microRNA (miRNA)

1. Processed within the nucleus by a ‘microprocessor complex’ containing a double-stranded RNA-specific ribonuclease known as Drosha, and its binding partner Pasha, to give hairpin RNA precursors, the ‘pre-miRNAs’.2. Transported to the cytoplasm using Exportin-5. 3. Cleavage by the endonuclease Dicer results in a double-stranded miRNA and then incorporated into an RNA-induced silencing complex (RISC).4. A single mature miRNA strand is selected and matured and the other is degraded.5. The active miRNAs down-regulate gene expression by translational repression and/or messenger RNA (mRNA) cleavage, mediated by the RISC, in a manner similar to short interfering RNA (siRNA).

microRNA (miRNA)

http://www.youtube.com/watch?v=gZZyxVP02UU&feature=related

http://www.youtube.com/watch?v=AfXmDAqgFIg&feature=related

The brief life of an mRNA molecule begins with transcription and ultimately ends in degradation. During its life, an mRNA molecule may also be processed, edited, and transported prior to translation. Eukaryotic mRNA molecules often require extensive processing and transport, while prokaryotic molecules do not.

Messenger Ribonucleic Acid (mRNA) is a molecule of RNA encoding a chemical "blueprint" for a protein product.

5' capThe 5' cap is a modified guanine nucleotide added to the "front" (5' end) of the pre-mRNA using a 5',5-Triphosphate linkage. This modification is critical for recognition and proper attachment of mRNA to the ribosome, as well as protection from 5' exonucleases. It may also be important for other essential processes, such as splicing and transport.

Coding regionsCoding regions are composed of codons, which are decoded and translated into one (mostly eukaryotes) or several (mostly prokaryotes) proteins by the ribosome. Coding regions begin with the start codon and end with the one of three possible stop codons. In addition to protein-coding, portions of coding regions may also serve as regulatory sequences in the pre-mRNA as exonic splicing enhancers or exonic splicing silencers.

3' poly(A) tailThe 3' poly(A) tail is a long sequence of adenine nucleotides (often several hundred) added to the "tail" or 3' end of the pre-mRNA through the action of an enzyme, polyadenylate polymerase. In higher eukaryotes, the poly(A) tail is added onto transcripts that contain a specific sequence, the AAUAAA signal. The importance of the AAUAAA signal is demonstrated by a mutation in the human alpha 2-globin gene that changes the original sequence AATAAA into AATAAG, which can lead to hemoglobin deficiencies.

Transfer RNA (abbreviated tRNA)

a small RNA chain (73-93 nucleotides) that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. It has a 3' terminal site for amino acid attachment. This covalent linkage is catalyzed by an aminoacyl tRNA synthetase. It also contains a three base region called the anticodon that can base pair to the corresponding three base codon region on mRNA. Each type of tRNA molecule can be attached to only one type of amino acid, but because the genetic code contains multiple codons that specify the same amino acid, tRNA molecules bearing different anticodons may also carry the same amino acid.

The 5'-terminal phosphate group. The acceptor stem is a 7-bp stem made by the base pairing of the 5'-terminal nucleotide with the 3'-terminal nucleotide (which contains the CCA 3'-terminal group used to attach the amino acid). CCA sequence is important for the recognition of tRNA by enzymes critical in translation. In prokaryotes, the CCA sequence is transcribed. In eukaryotes, the CCA sequence is added during processing and therefore does not appear in the tRNA gene. The D arm is a 4 bp stem ending in a loop that often contains dihydrouridine. The anticodon arm is a 5-bp stem whose loop contains the anticodon. The T arm is a 5 bp stem containing the sequence TΨC where Ψ is a pseudouridine. Bases that have been modified, especially by methylation, occur in several positions outside the anticodon. The first anticodon base is sometimes modified to inosine (derived from adenine) or pseudouridine (derived from uracil).

http://telstar.ote.cmu.edu/Hughes/HughesArchive/tutorial/polypeptide/tutorial.swf

http://www.youtube.com/watch?v=4MRCH_J7Fhk&feature=player_embedded#

Ribosomal RNA (rRNA)

a type of RNA synthesized in the nucleolus by RNA polymerase I, is the central component of the ribosome, the protein manufacturing machinery of all living cells. The function of the rRNA is to provide a mechanism for decoding mRNA into amino acids and to interact with the tRNAs during translation by providing peptidyl transferase activity.

Small subunit ribosomal RNA, 5' domain taken from the Rfam database.

The ribosome is composed of two subunits, named for how rapidly they sediment when subject to centrifugation. tRNA is sandwiched between the small and large subunits and the ribosome catalyzes the formation of a peptide bond between the 2 amino acids that are contained in the tRNA.The ribosome also has 3 binding sites called A, P, and E.The A site in the ribosome binds to an aminoacyl-tRNA (a tRNA bound to an amino acid).

Type Size Large subunit Small subunit

prokaryotic 70S 50S (5S, 23S) 30S ((16S)

eukaryotic 80S 60S (5S, 5.8S, 28S) 40S (18S)

The NH2 group of the aminoacyl-tRNA which contains the new amino acid, attacks the carboxyl group of peptidyl-tRNA (contained within the P site) which contains the last amino acid of the growing chain called peptidyl transferase reaction. The tRNA that was holding on the last amino acid is moved to the E site, and what used to be the aminoacyl-tRNA is now the peptidyl-tRNA. A single mRNA can be translated simultaneously by multiple ribosomes.

http://telstar.ote.cmu.edu/Hughes/HughesArchive/tutorial/polypeptide/tutorial.swf

Proteins

large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by a gene and encoded in the genetic code. Although this genetic code specifies 20 "standard" amino acids, the residues in a protein are often chemically altered in post-translational modification: either before the protein can function in the cell, or as part of control mechanisms. Proteins can also work together to achieve a particular function, and they often associate to form stable complexes.

amino acid

a molecule that contains both amine and carboxyl functional groups. In biochemistry, this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent. In the alpha amino acids, the amino and carboxylate groups are attached to the same carbon, which is called the α–carbon. The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon. They can vary in size from just a hydrogen atom in glycine, through a methyl group in alanine, to a large heterocyclic group in tryptophan.

Name Abbr. Linear structure formula ======================================Alanine ala A CH3-CH(NH2)-COOH Arginine arg R HN=C(NH2)-NH-(CH2)3-CH(NH2)-COOH Asparagine asn N H2N-CO-CH2-CH(NH2)-COOH Aspartic acid asp D HOOC-CH2-CH(NH2)-COOH Cysteine cys C HS-CH2-CH(NH2)-COOH Glutamine gln Q H2N-CO-(CH2)2-CH(NH2)-COOH Glutamic acid glu E HOOC-(CH2)2-CH(NH2)-COOH Glycine gly G NH2-CH2-COOHHistidine his H NH-CH=N-CH=C-CH2-CH(NH2)-COOHIsoleucine ile I CH3-CH2-CH(CH3)-CH(NH2)-COOH Leucine leu L (CH3)2-CH-CH2-CH(NH2)-COOH Lysine lys K H2N-(CH2)4-CH(NH2)-COOHMethionine met M CH3-S-(CH2)2-CH(NH2)-COOH Phenylalanine phe F Ph-CH2-CH(NH2)-COOH Proline pro P NH-(CH2)3-CH-COOH Serine ser S HO-CH2-CH(NH2)-COOH Threonine thr T CH3-CH(OH)-CH(NH2)-COOH Tryptophan trp W Ph-NH-CH=C-CH2-CH(NH2)-COOHTyrosine tyr Y HO-p-Ph-CH2-CH(NH2)-COOH Valine val V (CH3)2-CH-CH(NH2)-COOH

The Central Dogma Of Molecular Biology

ΓΕΝΕΤΙΚΟΣ ΚΩΔΙΚΑΣ

A gene begins with a codon for the amino acid methionine and ends with one of 3 stop codons. The codons between the start and stop signals code for the various AAs of the gene product but do not include any of the 3 stop codons. When examining an unknown DNA sequence, one indication that it may be part of a gene is the presence of an open reading frame or ORF.

An ORF is any stretch of DNA that when transcribed into RNA has no stop codon.

A computer program can be used to check an unknown DNA sequence for ORFs.The program transcribes each DNA strand into its complementary RNA sequence and then translates the RNA sequence into an amino acid sequence. Each DNA strand can be read in three different reading frames. This means that the computer must perform six different translations for any given double-stranded DNA sequence.

ΜΕΤΑΓΡΑΦΗ

The Basic Transcription Unit Model

Let’s look closely at the process of transcription.

terminator

Promoter of RNA polymerase II

Affects rate of transcription

http://www.youtube.com/watch?v=WsofH466lqk

In principle, locating genes should be easy. DNA sequences that code for proteins begin with the three bases ATG that code for the amino acid methionine and they end with one or more stop codons; either TAA, TAG or TGA. Unfortunately, finding genes isn't always so easy

Four key components of transcription

• Promoter• Transcription start site• RNA coding region• The Terminator

Process of Bacterial Transcription

• Initiation• Elongation• Termination

Eukaryotic Transcription

• 3 distinct RNA polymerases in a eukaryotic cell nucleus define the three major classes of eukaryotic transcription unit:

• There may be as many as 14 subunits in an eukaryotic RNA polymerase; the total molecular weight is typically 500-700 kD.• Eukaryotic RNA polymerases cannot find or bind to a promoter by themselves. They require the binding of assembly factors and a

positional factor to locate the promoter and to orient the polymerase correctly. As we will see, the positional factor is the same in all cases.

• Class I Transcriptional Units: Class I genes or transcriptional units are transcribed by RNA polymerase I in the nucleolus. The best-studied examples are the rRNA transcription units. RNA polymerase I is a complex of 13 subunits.

• Class II Transcription Units: All genes that are transcribed and expressed via mRNA are transcribed by RNA polymerase II. RNA polymerase II (12 subunits) can transcribe RNA from nicked dsDNA templates or from ssDNA templates. However, by itself, it cannot initiate transcription at a promoter. In this respect, it resembles the core form of bacterial RNA polymerase.

• Class III Transcription Units: Class III genes are principally those for small RNA molecules in the cell. The best studied examples are the 5S rRNA gene -- which has been studied extensively in Xenopus laevis, and tRNA genes.The enzyme:RNA polymerase III is the largest of the three RNA polymerases with 17 subunits and a molecular weight of over 700 kD. It is moderately sensistive to a-amanitin. It is also the most active.

polymerase location type of RNA transcribed

I Nucleus/nucleolus Large rRNA

II nucleus pre-mRNA, some snRNAs, some snoRNAs

III nucleus small RNA such as tRNA and 5S Rrna, and snRNAs

The DNA strand that codes for the protein is called the sense strand because its sequence reads the same as that of the messenger RNA. The other strand is the antisense strand and serves as

the template for RNA polymerase during transcription.

Initiation

RNAP = RNA polymeraseIn transcription, one strand of DNA, the non-coding strand, is used as a template for RNA synthesis. As transcription proceeds in the 5' → 3' direction, and uses base pairing complimentarity with the DNA template to specify the correct copying, the DNA template strand is that oriented in the 3' → 5' direction. The strand that is not used as the template is called the coding strand, and has the DNA sequence that reflects that of the RNA produced.Transcription begins with the binding of RNA polymerase to the promoter. In prokaryotes, the RNA polymerase is a core enzyme consisting of five subunits: 2 α subunits, 1 β subunit, 1 β' subunit, and 1 ω subunit. At the start of initiation, the core enzyme is associated with a sigma factor (number 70) that aids in finding the appropriate -35 and -10 basepairs downstream of promoter sequences. Transcription initiation is far more complex in eukaryotes, the main difference being that eukaryotic polymerases do not recognize directly their core promoter sequences. Unlike DNA replication, transcription does not need a primer to start because RNA polymerase does not require a primer. The DNA unwinds and produces a small open complex and synthesis begins on only the template strand.

Elongation

Unlike DNA replication, mRNA transcription can involve multiple RNA polymerases on a single DNA template, so many mRNA molecules can be produced from a single copy of a gene. This step also involves a proofreading mechanism that can replace an incorrectly added RNA molecule.

Termination

Bacteria use two different strategies for transcription termination: in Rho-independent transcription termination, RNA transcription stops when the newly synthesized RNA molecule forms a hairpin loop, followed by a run of Us, which makes it detach from the DNA template. In the "Rho-dependent" type of termination, a protein factor called "Rho" destabilizes the interaction between the template and the mRNA, thus releasing the newly synthesized mRNA from the elongation complex. Transcription termination in eukaryotes is less well understood. It involves cleavage of the new transcript, followed by template-independent addition of As at its new 3' end, in a process called polyadenylation.

5' cap

The 5' cap is a modified guanine nucleotide added to the "front" (5' end) of the pre-mRNA using a 5',5-Triphosphate linkage. This modification is critical for recognition and proper attachment of mRNA to the ribosome, as well as protection from 5' exonucleases. It may also be important for other essential processes, such as splicing and transport.

3' poly(A) tail

The 3' poly(A) tail is a long sequence of adenine nucleotides (often several hundred) added to the "tail" or 3' end of the pre-mRNA through the action of an enzyme, polyadenylate polymerase. In higher eukaryotes, the poly(A) tail is added onto transcripts that contain a specific sequence, the AAUAAA signal. The importance of the AAUAAA signal is demonstrated by a mutation in the human alpha 2-globin gene that changes the original sequence AATAAA into AATAAG, which can lead to hemoglobin deficiencies.

RNA splicing

It is easier to locate genes in bacterial DNA than in eukaryotic DNA. In bacteria, the genes are arranged like beads on a string. Each gene consists of a single ORF. The situation in eukaryotic organisms is complicated by the split nature of the genes. Most eukaryotic genes take the form of alternating EXONS and INTRONS. Each exon is an ORF that codes for amino acids. The intron sequences do not code for amino acids and contain internal stop codons.

5’ phosphodiester of G 2’ OH of A

Splicing: 3 components are required

Note some important aspects of the 5’ and 3’ ends

http://www.sumanasinc.com/webcontent/anisamples/molecularbiology/mRNAsplicing.html

One of the surprises of the Human Genome Project was the relatively small number of genes found - about 25,000. One might ask, "How can something as complicated as a human have only 25 percent more genes than the tiny roundworm C. elegans?" Part of the answer seems to involve alternative splicing. Alternative splicing refers to the process by which a given gene is spliced into more than one type of mRNA molecule.

http://www.youtube.com/watch?v=OEWOZS_JTgk&feature=related

ΜΕΤΑΦΡΑΣΗ

In addition to the APE sites there is an mRNA binding groovethat holds onto the message being translated

Proper reading of theanticodon is the secondimportant quality controlstep ensuring accurateprotein synthesis

=EF-1

Elongation factors Introduce a two-step“Kinetic proofreading”

A second elongation factorEF-G or EF-2, drives the translocation of the ribosome along the mRNA

Together GTP hydrolysisby EF-1 and EF-2 help driveprotein synthesis forward

Termination of translationis triggered by stop codons

Release factor entersthe A site and triggershydrolysis the peptidyl-tRNAbond leading to release of the protein.

Release of the protein causesthe disassociation of the ribosome into its constituentsubunits.

http://www.youtube.com/watch?v=5bLEDd-PSTQ&feature=related

Inner life of cell: http://www.youtube.com/watch?v=CVUnzk40npw

Signaling: http://www.youtube.com/watch?v=tMMrTRnFdI4&feature=related