Sunday, 29 January 2017
BIOLOGY NOTES: CENTRAL DOGMA: AN OVERVIEW
BIOLOGY NOTES: CENTRAL DOGMA: AN OVERVIEW: central Dogma is the relationship between DNA, RNA and proteins. It was proposed by CRICK in 1958. Crick proposed that the in...
Thursday, 26 January 2017
BIOLOGY NOTES: CENTRAL DOGMA: AN OVERVIEW
BIOLOGY NOTES: CENTRAL DOGMA: AN OVERVIEW: central Dogma is the relationship between DNA, RNA and proteins. It was proposed by CRICK in 1958. Crick proposed that the in...
Wednesday, 25 January 2017
BIOLOGY NOTES: CENTRAL DOGMA: AN OVERVIEW
BIOLOGY NOTES: CENTRAL DOGMA: AN OVERVIEW: central Dogma is the relationship between DNA, RNA and proteins. It was proposed by CRICK in 1958. Crick proposed that the in...
CENTRAL DOGMA: AN OVERVIEW
central Dogma is the relationship between DNA, RNA and
proteins.
It was proposed by CRICK in 1958.
Crick proposed that the information present in DNA is transferred to DNA via replication and to RNA via transciprtion. Transciprtion leads to the production of tRNA, mRNA, rRNA. Now, mRNA undergoes translation to form proteins.
It was proposed by CRICK in 1958.
Crick proposed that the information present in DNA is transferred to DNA via replication and to RNA via transciprtion. Transciprtion leads to the production of tRNA, mRNA, rRNA. Now, mRNA undergoes translation to form proteins.
This relationship between RNA, DNA and protein is known as central dogma in
which information flows from DNA → RNA→ PROTEIN
and never in reverse.
Translation is the process by which a RNA sequence is converted into a set of amino acids (AKA a protein) using tRNA, ribosomes and translation factors.
Following is a basic idea of all these three processes.
DNA Replication is the process of making 2 identical copies of DNA from one original DNA copy.
Essentially the DNA “unzips” and each of the original strands acts as a template for the new strands.
DNA is synthesized in a 5’–>3’ direction during the S phase of the cell cycle.
Transcription is when template DNA strand is converted to complementary RNA.
The RNA is very similar to DNA, except that it has Uracil (U) in place of Thymine (T) and it is single stranded.
RNA is synthesized in a 5′ –> 3′ direction as the RNA Polymerase moves along the DNA in a 3′–> 5′ Direction.
Post-Transcriptional Modification (RNA Processing) is the process of converting immature RNA to mature RNA that is ready to be translated. 5'- capping, 3'- tailing, splicing are carried out.
NOTE:The central dogma of biology holds that genetic information normally flows from DNA to RNA to protein. As a consequence it has been generally assumed that genes code for proteins, and that proteins fulfill most structural and catalytic and also most of the regulatory functions, in all cells, from microbes to mammals.
However, the latter may not be the case in complex organisms.
This is because the concept of central dogma is just a simple interpretation of the exchange of genetic information going inside the cell; and, a simple interpretation often ignores many complications just to make things simple or easy to understand.
central dogma was proposed in 1950s and many other parts and processes related to DNA, RNA and proteins known then and the one which are continuously discovered today are not included.
central dogma was proposed in 1950s and many other parts and processes related to DNA, RNA and proteins known then and the one which are continuously discovered today are not included.
e.g. The presence of non-coding RNA (ncRNA), Alternative splicing, reverse transcriptase, introns, junk DNA, epigenetics, RNA viruses, trans-splicing, transposons, prions, epigenetics, and gene rearrangements are ignored to simplify things.
Exceptions also exist in the form of RETROVIRUSES. They carry out reverse transciption and RNA dependent RNA replication.
e.g. in TMV (tobacco mosaic virus),ΦR17, ΦMS2 etc. the viral RNA is able to replicate itself and carry out RNA to RNA change with the help of RNA replicase.
similarly, RSV (ross sarcoma virus) can change RNA to DNA using reverse transcriptase. This process is called reverse transcription.
Exceptions also exist in the form of RETROVIRUSES. They carry out reverse transciption and RNA dependent RNA replication.
e.g. in TMV (tobacco mosaic virus),ΦR17, ΦMS2 etc. the viral RNA is able to replicate itself and carry out RNA to RNA change with the help of RNA replicase.
similarly, RSV (ross sarcoma virus) can change RNA to DNA using reverse transcriptase. This process is called reverse transcription.
Reverse transcription and RNA dependent RNA replication and other above mentioned parts and processes doesn't form a part of central dogma.
Only DNA replication, transcription, and translation forms a part of it.
A
number of startling observations and phenomena suggests that the traditional view
of genetic regulatory systems in animals and plants may be incorrect.
It will take years, perhaps decades, to construct a detailed theory that explains how DNA, RNA and the epigenetic machinery all fit into an interlocking, self- regulating system.
But there is no longer any doubt that a
new theory is needed to replace the central dogma that has been the foundation
of molecular genetics and biotechnology since the 1950s.
The central dogma, as
usually stated, is quite simple: DNA makes RNA, RNA makes protein, and proteins
do almost all of the work of biology.
Friday, 20 January 2017
TUMOUR SUPRESSOR GENES
TUMOUR SUPRESSOR GENES
It suppresses cell division and growth. It is a loss of function mutation and a recessive one too because both alleles of a TSG needs to be deactivated for tumorigenesis to take place. But, some dominant loss of function mutations are also known.
The two best characterized Tumor Suppressed Genes are:
1)RB 2) p53.
RB locus:
The gene RB is located in band q14 of human chromosome 13. It encodes RB protein .
The loss or inactivation of RB locus is mainly involved in retinoblastoma alongwith other type of cancer such as osteosarcomas and small cell lung cancer.
During the M-to-G1 transition,
pRb is dephosphorylated by PP1, to its growth-suppressive hypophosphorylated state Rb ; so that no growth takes place now.
E2F is actually a group of transcription factor and forms E2F-DP complex with dimerization partner (DP) protein . This dimer activates genes whose products are necessary to push a cell into S phase.
But, When Rb is bound to E2F of E2F-DP complex, the complex acts as a growth suppressor and prevents progression through the cell cycle.
The Rb-E2F/DP complex also attracts a histone deacetylase (HDAC) protein to the chromatin, reducing transcription of S phase promoting factors.
When it is time for a cell to enter S phase,
complexes of CDK and cyclins phosphorylate Rb to pRb, inhibiting its activity.The initial phosphorylation is performed by Cyclin D/CDK4/CDK6 and followed by additional phosphorylation by Cyclin E/CDK2. pRb remains phosphorylated throughout S, G2 and M phase.
Phosphorylation of Rb allows E2F-DP to dissociate from pRb and become active.When E2F is free it activates factors like cyclins (e.g. Cyclin E and A), which push the cell through the cell cycle by activating cyclin-dependent kinases, and a molecule called proliferating cell nuclear antigen, or PCNA, which speeds DNA replication and repair by helping to attach polymerase to DNA.
Similarly, p16, p21 and p27 ensure that CDK-4 and 6 remains inactive and thus contribute to tumour supression.
HOW CANCER TAKES PLACE:If both alleles of RB are inactivated; then it leads to absence of RB protein and absence of inhibition of E2F leading to continuous cell division.
Similarly, SV40 T Antigen and E1A protein of adenovirus binds with unphosphorylated RB and deter it from binding to E2F. As a result; E2F remains ever active and contribute to uncontrolled cell division.
p53 gene : p53 is the most important Tumor Suppressor Genes. It is involved in more than half of all cancers.
Initially, p53 was identified as an oncogene but later recognized as a dominant negative mutant.
Wild type p53 controls cell proliferation.
Protein p53 has different domains which help it perform different functions :
It's central domain binds the DNA by recognizing a 10 bp motif in it. SV40 binds to this central domain to deter it from binding DNA.
Now N terminal domian of p53 interacts with TBP to activate transcription of those genes ;whose promoter region contain 10 bp motif to which central domian of p53 has bound.
E1B protein of adenovirus binds with this N terminal domain to block p53 from functioning.
C terminal domain- it works during DNA damage.
If DNA damage is recognized during G1 phase;then p53 binds to the single stranded DNA. Now DNA damage is repaired first and then only cell may enter S phase.
But, if the cell has already committed to division; then p53 triggers apoptosis.
Functional p53 is a tetramer and C terminal domain works to oligomerize it.
It also has a signalling domain which binds to SH3 domain.
p53 as a transcription factor has the following 3 roles:
1) Apoptosis,
2) Cell cycle arrest in G1,
3) Prevention of genomic instability.
CELL CYCLE ARREST IN G1 - Protein p53 binds to single stranded region and to mismatches produced by deletion or addition of 1-3 bases. Thus binding activates the DNA binding and transcription activation domain of p53 and now it leaves the site of damage and activates the target gene.
Transcription of p21 is activated and it prevents the action of CDK4,6-cyclin D and CDK2-cyclin E action, which prevents the onset of S phase.
APOPTOSIS - Protein p53 following it's activation by damaged DNA activates genes leading to apoptosis.
PREVENTION OF GENOMIC INSTABILITY - Level of p53 is regulated by interaction between p53 and cellular oncoprotein Mdm2. An increase in p53 level induces transcription of Mdm2. Increase in Mdm2 level now inhibits p53 activity. This interaction maintains a low level of both proteins in normal cells.
Transcription activation by p53 requires the coactivators p300/CBP; . p300 binds the transcription activation domain of p53; thus helps Mdm2 to bind to p53. Binding of Mdm2 to p53 makes it an easy tsrget for degradation and thus helps to inhibit transcription activation by p53.
Protein p53 is modified in response to environmental signals thst influence cell growth. Signals like ionizing and UV irradiation cause acetylation of lysine, phosphorylation and dephosphorylation of serine at specific position of p53. These changes maty affect the stability, DNA binding ,oligomerization,, and binding to other proteins. Thus, p53 acts as a sensor that integrates information from many pathways that affect cell division.
NOTE- Both p53 and RB are activated by multiple pathways. Locus INK4A-ARF affects the function of both RB and p53. This locus produces p16 and p29. p16 prevents both assembly and activity of CDK-4,6-cyclin D ; as a result, RB remains active.
Similarly, p19 inactivates Mdm2 and this retains p53 active.
It suppresses cell division and growth. It is a loss of function mutation and a recessive one too because both alleles of a TSG needs to be deactivated for tumorigenesis to take place. But, some dominant loss of function mutations are also known.
The two best characterized Tumor Suppressed Genes are:
1)RB 2) p53.
RB locus:
The gene RB is located in band q14 of human chromosome 13. It encodes RB protein .
The loss or inactivation of RB locus is mainly involved in retinoblastoma alongwith other type of cancer such as osteosarcomas and small cell lung cancer.
During the M-to-G1 transition,
pRb is dephosphorylated by PP1, to its growth-suppressive hypophosphorylated state Rb ; so that no growth takes place now.
E2F is actually a group of transcription factor and forms E2F-DP complex with dimerization partner (DP) protein . This dimer activates genes whose products are necessary to push a cell into S phase.
But, When Rb is bound to E2F of E2F-DP complex, the complex acts as a growth suppressor and prevents progression through the cell cycle.
The Rb-E2F/DP complex also attracts a histone deacetylase (HDAC) protein to the chromatin, reducing transcription of S phase promoting factors.
When it is time for a cell to enter S phase,
complexes of CDK and cyclins phosphorylate Rb to pRb, inhibiting its activity.The initial phosphorylation is performed by Cyclin D/CDK4/CDK6 and followed by additional phosphorylation by Cyclin E/CDK2. pRb remains phosphorylated throughout S, G2 and M phase.
Phosphorylation of Rb allows E2F-DP to dissociate from pRb and become active.When E2F is free it activates factors like cyclins (e.g. Cyclin E and A), which push the cell through the cell cycle by activating cyclin-dependent kinases, and a molecule called proliferating cell nuclear antigen, or PCNA, which speeds DNA replication and repair by helping to attach polymerase to DNA.
Similarly, p16, p21 and p27 ensure that CDK-4 and 6 remains inactive and thus contribute to tumour supression.
HOW CANCER TAKES PLACE:If both alleles of RB are inactivated; then it leads to absence of RB protein and absence of inhibition of E2F leading to continuous cell division.
Similarly, SV40 T Antigen and E1A protein of adenovirus binds with unphosphorylated RB and deter it from binding to E2F. As a result; E2F remains ever active and contribute to uncontrolled cell division.
p53 gene : p53 is the most important Tumor Suppressor Genes. It is involved in more than half of all cancers.
Initially, p53 was identified as an oncogene but later recognized as a dominant negative mutant.
Wild type p53 controls cell proliferation.
Protein p53 has different domains which help it perform different functions :
It's central domain binds the DNA by recognizing a 10 bp motif in it. SV40 binds to this central domain to deter it from binding DNA.
Now N terminal domian of p53 interacts with TBP to activate transcription of those genes ;whose promoter region contain 10 bp motif to which central domian of p53 has bound.
E1B protein of adenovirus binds with this N terminal domain to block p53 from functioning.
C terminal domain- it works during DNA damage.
If DNA damage is recognized during G1 phase;then p53 binds to the single stranded DNA. Now DNA damage is repaired first and then only cell may enter S phase.
But, if the cell has already committed to division; then p53 triggers apoptosis.
Functional p53 is a tetramer and C terminal domain works to oligomerize it.
It also has a signalling domain which binds to SH3 domain.
p53 as a transcription factor has the following 3 roles:
1) Apoptosis,
2) Cell cycle arrest in G1,
3) Prevention of genomic instability.
CELL CYCLE ARREST IN G1 - Protein p53 binds to single stranded region and to mismatches produced by deletion or addition of 1-3 bases. Thus binding activates the DNA binding and transcription activation domain of p53 and now it leaves the site of damage and activates the target gene.
Transcription of p21 is activated and it prevents the action of CDK4,6-cyclin D and CDK2-cyclin E action, which prevents the onset of S phase.
APOPTOSIS - Protein p53 following it's activation by damaged DNA activates genes leading to apoptosis.
PREVENTION OF GENOMIC INSTABILITY - Level of p53 is regulated by interaction between p53 and cellular oncoprotein Mdm2. An increase in p53 level induces transcription of Mdm2. Increase in Mdm2 level now inhibits p53 activity. This interaction maintains a low level of both proteins in normal cells.
Transcription activation by p53 requires the coactivators p300/CBP; . p300 binds the transcription activation domain of p53; thus helps Mdm2 to bind to p53. Binding of Mdm2 to p53 makes it an easy tsrget for degradation and thus helps to inhibit transcription activation by p53.
Protein p53 is modified in response to environmental signals thst influence cell growth. Signals like ionizing and UV irradiation cause acetylation of lysine, phosphorylation and dephosphorylation of serine at specific position of p53. These changes maty affect the stability, DNA binding ,oligomerization,, and binding to other proteins. Thus, p53 acts as a sensor that integrates information from many pathways that affect cell division.
NOTE- Both p53 and RB are activated by multiple pathways. Locus INK4A-ARF affects the function of both RB and p53. This locus produces p16 and p29. p16 prevents both assembly and activity of CDK-4,6-cyclin D ; as a result, RB remains active.
Similarly, p19 inactivates Mdm2 and this retains p53 active.
Thursday, 19 January 2017
BIOLOGY NOTES: ENZYMES INVOLVED IN DNA REPLICATION
BIOLOGY NOTES: ENZYMES INVOLVED IN DNA REPLICATION: ENZYMES INVOLVED IN DNA REPLICATION •Variety of enzymes help in DNA replication. •They can be listed as below: a) DNA Polymerase b) L...
Sunday, 15 January 2017
BIOLOGY NOTES: ENZYMES INVOLVED IN DNA REPLICATION
BIOLOGY NOTES: ENZYMES INVOLVED IN DNA REPLICATION: ENZYMES INVOLVED IN DNA REPLICATION •Variety of enzymes help in DNA replication. •They can be listed as below: a) DNA Polymerase b) L...
ENZYMES INVOLVED IN DNA REPLICATION
ENZYMES INVOLVED IN DNA REPLICATION
•Variety of enzymes help in DNA replication.
•They can be listed as below:
a) DNA Polymerase
b) Ligase
c) Primase
d) SSB
e) Helicase
f) Endonuclease
g) Pilot proteins
Let us first talk about DNA polymerase
1) DNA POLYMERASE : Also known as DNA replicase; it helps to synthesize a new DNA strand on a template DNA strand. It helps to add deoxyribonucleotides to the 3'-end of a pre-existing polynucleotide and so replication takes place in 5'-3' direction.
They also help in correct base pairing.i.e. only correct bases are paired against the bases on the template DNA strand.
DNA POLYMERASES ARE DIFFERENT IN PROKARYOTES AND EUKARYOTES.
In the current post, we will discuss only about prokaryotic DNA polymerase.
IN PROKARYOTES:
There are 3 types of DNA polymerases.
DNA polymerase I was first isolated by KORNBERG from E.COLI.
It possesses the following three activities; but it is chiefly a DNA REPAIR enzyme.
1) 5'-3' polymerase; (replication of nicks)
2) 5'-3' exonuclease; (repair DNA damaged by radiations)
3) 3'-5' exonuclease; (repair incorrectly placed bases of DNA during replication)
3'-5' EXONUCLEASE - It is the chief repair enzyme. It helps in proof reading. It removes incorrect bases from the 3'- end. Now, correct base is added against the template strand by 5'-3' polymerase.
5'-3' EXONUCLEASE - It removes DNA segments damaged by UV light, irradiation and other agents. It excises up to 10 bases of nucleotides.
It also excises RNA primers during replication after an endonuclease produces a nick. 5'-3' polymerase then adds the correct DNA bases. This is called nick translation.
NOTE :so, as we see 3'-5' exonuclease excises a single base as a result of proof reading; but 5'-3' exonuclease excises up to 10 bases in case of damage due to UV light and irradiation. and, in both cases 5'-3' POLYMERASE adds the missing bases. So, polymerase I helps in replication only in case of error, or damage or when RNA primers have been excised.
DNA polymerase I is encoded by the gene polA. The enzyme consists of two segments.
; one large and another small.
The larger segment is known as klenow fragment and exhibits the 5'-3' polymerase at the C-terminal and 3'-5' exonuclease activities at the N-terminal.
Smaller segment encodes for the 5'-3' exonuclease activity.
DNA polymerase II-It only has a 5'-3' polymerase activity and 3'-5' exonuclease activity.
The exact in vivo function is not known. Although, it most likely function in DNA repair in the absence of DNA polymerase I and III.
DNA polymerase III-It has 5'-3' polymerase activity and 3'-5' exonuclease activity. It is the MAIN DNA REPLICATION ENZYME.
It is made up of several subunits such as α, β, γ, δ, ε, θ, ψ, χ, τ.
now , we will deal with eukaryotic DNA polymerase along with other enzymes in next posts.
•Variety of enzymes help in DNA replication.
•They can be listed as below:
a) DNA Polymerase
b) Ligase
c) Primase
d) SSB
e) Helicase
f) Endonuclease
g) Pilot proteins
Let us first talk about DNA polymerase
1) DNA POLYMERASE : Also known as DNA replicase; it helps to synthesize a new DNA strand on a template DNA strand. It helps to add deoxyribonucleotides to the 3'-end of a pre-existing polynucleotide and so replication takes place in 5'-3' direction.
They also help in correct base pairing.i.e. only correct bases are paired against the bases on the template DNA strand.
DNA POLYMERASES ARE DIFFERENT IN PROKARYOTES AND EUKARYOTES.
In the current post, we will discuss only about prokaryotic DNA polymerase.
IN PROKARYOTES:
There are 3 types of DNA polymerases.
- DNA polymerase I
- DNA polymerase II
- DNA polymerase III
DNA polymerase I was first isolated by KORNBERG from E.COLI.
It possesses the following three activities; but it is chiefly a DNA REPAIR enzyme.
1) 5'-3' polymerase; (replication of nicks)
2) 5'-3' exonuclease; (repair DNA damaged by radiations)
3) 3'-5' exonuclease; (repair incorrectly placed bases of DNA during replication)
3'-5' EXONUCLEASE - It is the chief repair enzyme. It helps in proof reading. It removes incorrect bases from the 3'- end. Now, correct base is added against the template strand by 5'-3' polymerase.
5'-3' EXONUCLEASE - It removes DNA segments damaged by UV light, irradiation and other agents. It excises up to 10 bases of nucleotides.
It also excises RNA primers during replication after an endonuclease produces a nick. 5'-3' polymerase then adds the correct DNA bases. This is called nick translation.
NOTE :so, as we see 3'-5' exonuclease excises a single base as a result of proof reading; but 5'-3' exonuclease excises up to 10 bases in case of damage due to UV light and irradiation. and, in both cases 5'-3' POLYMERASE adds the missing bases. So, polymerase I helps in replication only in case of error, or damage or when RNA primers have been excised.
DNA polymerase I is encoded by the gene polA. The enzyme consists of two segments.
; one large and another small.
The larger segment is known as klenow fragment and exhibits the 5'-3' polymerase at the C-terminal and 3'-5' exonuclease activities at the N-terminal.
Smaller segment encodes for the 5'-3' exonuclease activity.
DNA polymerase II-It only has a 5'-3' polymerase activity and 3'-5' exonuclease activity.
The exact in vivo function is not known. Although, it most likely function in DNA repair in the absence of DNA polymerase I and III.
DNA polymerase III-It has 5'-3' polymerase activity and 3'-5' exonuclease activity. It is the MAIN DNA REPLICATION ENZYME.
It is made up of several subunits such as α, β, γ, δ, ε, θ, ψ, χ, τ.
- α has the polymerase activity and ε has the proof reading activity. They generally function together.
- θ helps in assembly of enzymes and so bring α and ε together to form catalytic core.
- Though, the catalytic core has both polymerase and 3'-5' exonuclease activity but they dissociate after synthesizing only 11 bp long chain on only one strand.
- But, because there are two strands; so dimerization of this subunit is necessary in which τ helps.
- τ comes in contact with αθε to form αθετ complex which dimerizes to form α₂θ₂ε₂τ₂. this is called POL III*.
- Now γ complex is added to POL III* to form POL III'. γ- complex is known as clamp loader and loads β dimer onto DNA strands.
- There are 2 β-dimers. Each dimer forms a ring shaped structure that surrounds the DNA duplex providing a sliding clamp. It allows the holoenzyme to slide along DNA.
now , we will deal with eukaryotic DNA polymerase along with other enzymes in next posts.
Thursday, 5 January 2017
BIOLOGY NOTES: CANCER: immortal cells and their properties
BIOLOGY NOTES: CANCER: immortal cells and their properties: As discussed in the follwing article; HOW NORMAL CELL CHANGE INTO CANCER three steps lead to cancer; immortalization, transformation a...
CANCER: immortal cells and their properties
As discussed in the follwing article;
HOW NORMAL CELL CHANGE INTO CANCER
three steps lead to cancer; immortalization, transformation and metastasis.
This is a lead in the topic with me dealing with immortalization . Here it goes:-
IMMORTALIZATION -
Advantages:
Limitations
Methods for generating immortalized cell lines:
There are several methods for generating immortalized cell lines:
Examples of immortalized cell lines
There are several examples of immortalized cell lines, each with different properties.
Most immortalized cell lines are classified by the cell type they originated from or are most similar to biologically.
A549 cells – derived from the tumor of a cancer patient
HeLa cells – an extremely widely used human cell line isolated from a cervical cancer, probably derived from epithelial cells
HEK 293 – derived from aborted human fetal cells and a virus
Jurkat – a human T lymphocyte cell line isolated from a case of leukemia
3T3 – a mouse fibroblast cell line derived from a spontaneous mutation in cultured mouse embryo tissue
Vero cells – a monkey cell line
F11 Cells - a line of neurons from the dorsal root ganglia of rats.
NOTE: An immortalised cell line should not be confused with stem cells, which can also divide indefinitely, because stem cells form a normal part of the development of a multicellular organism. Whereas; immortalized cells are produced as a result of unpleasant mutation . Immortalization is an unfortunate situation and is an intial step towards TRANSFORMATION and then CANCER.
Well this was just a basic step into understanding the process of immortalization. A detailed discussion of the molecular level will be done in the coming articles. you can drop your queries or doubts in the comments; and if you want to discuss about something else. THANK YOU.
HOW NORMAL CELL CHANGE INTO CANCER
three steps lead to cancer; immortalization, transformation and metastasis.
This is a lead in the topic with me dealing with immortalization . Here it goes:-
IMMORTALIZATION -
- It is the process of becoming immortal. An immortal cell can indefinitely grow.
- Except for their capability to grow indefinitely, they are very much similar to normal cells and behave normally, when cultured in vitro.
- They don't form tumors when introduced in test animals like transformed cells do.
- Their ability to grow indefinitely makes them very useful in lab because this makes them easy to maintain . They are mostly used for understanding the normal processes and physiology of cells and drug testing.
Scanning electron micrograph of He-La cell
Advantages:
- The mutations required for immortality can occur naturally or can be intentionally induced.
- Immortalized cell lines can also be cloned giving rise to a clonal population which can, in turn, be propagated indefinitely. This allows an analysis to be repeated many times on genetically identical cells which is desirable for repeatable scientific experiments.
- Immortalized cell lines find use in biotechnology where they are a cost-effective way of growing cells similar to those found in a multicellular organism in vitro.
- The cells are used for a wide variety of purposes, from testing toxicity of compounds or drugs to production of eukaryotic proteins.
Limitations
- While immortalized cell lines often originate from a well-known tissue type they have undergone significant mutations to become immortal. This can alter the biology of the cell and must be taken into consideration.
Methods for generating immortalized cell lines:
There are several methods for generating immortalized cell lines:
- Isolation from a naturally occurring cancer. This is the original method for generating an immortalized cell line. Major examples include human He-La cells, obtained from a cervical cancer and mouse Raw 264.7 cells, obtained from a murine leukemia.
- Spontaneous or induced random mutagenesis and selection for cells which are able to undergo division.
- Introduction of a viral gene that partially deregulates the cell cycle (e.g., the adenovirus E1 gene was used to immortalize the HEK 293 cell line).
- Artificial expression of key proteins required for immortality, for example telomerase which prevents degradation of chromosome ends during DNA replication in eukaryotes
- Hybridoma technology, specifically used for the generation of immortalized antibody-producing B cell lines, where an antibody-producing B cell is fused with a myeloma (B cell cancer) cell.
Examples of immortalized cell lines
There are several examples of immortalized cell lines, each with different properties.
Most immortalized cell lines are classified by the cell type they originated from or are most similar to biologically.
A549 cells – derived from the tumor of a cancer patient
HeLa cells – an extremely widely used human cell line isolated from a cervical cancer, probably derived from epithelial cells
HEK 293 – derived from aborted human fetal cells and a virus
Jurkat – a human T lymphocyte cell line isolated from a case of leukemia
3T3 – a mouse fibroblast cell line derived from a spontaneous mutation in cultured mouse embryo tissue
Vero cells – a monkey cell line
F11 Cells - a line of neurons from the dorsal root ganglia of rats.
NOTE: An immortalised cell line should not be confused with stem cells, which can also divide indefinitely, because stem cells form a normal part of the development of a multicellular organism. Whereas; immortalized cells are produced as a result of unpleasant mutation . Immortalization is an unfortunate situation and is an intial step towards TRANSFORMATION and then CANCER.
Well this was just a basic step into understanding the process of immortalization. A detailed discussion of the molecular level will be done in the coming articles. you can drop your queries or doubts in the comments; and if you want to discuss about something else. THANK YOU.
Tuesday, 3 January 2017
BIOLOGY NOTES: CANCER : PART 1 (step by step notes on how normal...
BIOLOGY NOTES: CANCER : PART 1 (step by step notes on how normal...: CANCER : PART 1 All the cells in our body has a limited life-span; and their growth and division is controlled. But, occasionally, some...
CANCER : PART 1 (step by step notes on how normal cell change in to cancer cell)
CANCER : PART 1
All the cells in our body has a limited life-span; and their growth and division is controlled. But, occasionally, some cells might escape this control and give rise to clones of cells called tumor, which can undergo further changes to become cancer.
Any cell go through the following 3 steps to finally become a cancer.
1) Immortalization
2) Transformation
3) Metastasis.
These steps involve multiple genetic changes and these changes first make them immortal, then transformed and gradually malignant cancer
We will look at the above 3 properties in the following sections:-
IMMORTALIZATION :
- It is the process of becoming immortal. An immortal cell can grow indefinitely.
- They are very much similar to normal cells and behave normally, when cultured in vitro. They don't form tumors when introduced in test animals like transformed cells.
- They are very useful in labs because their property is similar to normal cells but they can grow indefinitely.
- This makes them easy to maintain and easy to use for understanding the normal processes and physiology of cells and drug testing.
TRANSFORMATION-In this process, the immortal cells lose a number of properties to become transformed.
Following are the properties of a transformed cell:
- They become anchorage independent.
- They need very little or negligible amount of growth factors for their growth.
- They show density independent growth. This leads to the foci formation in vitro and tumour formation in vivo.
- They lose cytoskeleton organization. This makes them look round unlike normal cells.
- Transformed cells are tumorigenic and have properties similar to that of cancer cells.
- when introduced in animal or humans; They give rise to tumors which may or may not become a cancer.
● METASTASIS- A tumor become a cancer only when it attains the property of metastasis. In which, part of the cell from a tumor detach and penetrate the blood vessels to get access to circulation and then move out of it to infect some other type of tissues to form a secondary tumor. This property , if acquired by a cell gives them the capability to infect different parts of our body and this is a very dangerous state and makes the treatment equally difficult; and THIS IS CANCER.
This is when we call a given tumor as cancer; and this is the property for which tumors are checked once doctor diagnoses a patient with a tumor. And if it is a benign tumor then it is removed just by a surgery; and depending upon the intensity to which it is malignant, doctors treat them with a combination of drugs, radiation,chemotherapy and immunotherapy.
now, we will have a look at which of the above properties are shown by the popularly known types of cancer; i.e. benign and malignant.
Benign tumour are immortal and transformed.they do not show the property of metastasis.
E.g. a skin wart.
They are mostly enclosed in fibroid connective tissue and are easily removed by surgery.
Malignant tumor (cancer) shows all the above 3 properties. e.g. sarcomas , lymphomas.
Well this was just a basic step into understanding the normal cell change into cancer. A detailed discussion will be done in the coming articles. you can drop your queries or doubts in the comments; and if you want to discuss about something else. THANK YOU.
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