User:Eileen Molzberger/Sandbox 1
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This process is especially important during DNA replication. During DNA replication, the two original DNA strands are separated and new, complementary strands are added to each of the original strands. The result is two new sets of double-stranded DNA (sometimes referred to as “duplexes”) exactly identical to the original double-stranded DNA set. So no unpaired DNA is present in either of the two new sets of DNA. | This process is especially important during DNA replication. During DNA replication, the two original DNA strands are separated and new, complementary strands are added to each of the original strands. The result is two new sets of double-stranded DNA (sometimes referred to as “duplexes”) exactly identical to the original double-stranded DNA set. So no unpaired DNA is present in either of the two new sets of DNA. | ||
A problem arises if one of the original strands is damaged; DNA damage is like a road block for the DNA replication machinery. If the original strand is damaged, a portion of that DNA strand won't be replicated. This results in a gap on the newly replicated strand. This gap is an unpaired segment of single-stranded DNA. Unpaired, single-stranded DNA is a red flag for the cell that something has gone wrong! | A problem arises if one of the original strands is damaged; DNA damage is like a road block for the DNA replication machinery. If the original strand is damaged, a portion of that DNA strand won't be replicated. This results in a gap on the newly replicated strand. This gap is an unpaired segment of single-stranded DNA. Unpaired, single-stranded DNA is a red flag for the cell that something has gone wrong! | ||
| - | The recombinational repair pathway (Figure 1) and the RecA protein will be used to fill in this gap. The basic steps of the pathway are as follows: [[Image:Gap_Strand_Exchange_small.jpg]] | + | The recombinational repair pathway (Figure 1) and the RecA protein will be used to fill in this gap. The basic steps of the pathway are as follows: [[Image:Gap_Strand_Exchange_small.jpg|right]] |
#The exposed single-stranded DNA is bound by RecA, forming a RecA-DNA filament. | #The exposed single-stranded DNA is bound by RecA, forming a RecA-DNA filament. | ||
#This single-stranded DNA-RecA filament finds a portion of homologous double-stranded DNA duplex. This DNA duplex is composed of two strands; one of these strands is exactly the same as the single-stranded DNA bound by RecA, and the other is exactly complementary. RecA is bound to the single stranded portion of DNA on the damaged duplex; this strand is missing its “pairing partner” and needs to be paired with a strand that is exactly complementary. The complementary portion that is added will fill in the gap on the damaged strand. | #This single-stranded DNA-RecA filament finds a portion of homologous double-stranded DNA duplex. This DNA duplex is composed of two strands; one of these strands is exactly the same as the single-stranded DNA bound by RecA, and the other is exactly complementary. RecA is bound to the single stranded portion of DNA on the damaged duplex; this strand is missing its “pairing partner” and needs to be paired with a strand that is exactly complementary. The complementary portion that is added will fill in the gap on the damaged strand. | ||
Revision as of 09:22, 30 April 2011
Contents |
RecA
Introduction
RecA is a protein involved in several processes in a cell. These processes include DNA repair, and signalling the cell that DNA damage has occurred (formally referred to as SOS response induction). A ubiquitous protein, RecA or a RecA homolog is present in almost all cells.
RecA Function in DNA Repair
DNA damage is a frequent occurrence. It can arise spontaneously, or can be caused other sources; these sources include reactive oxygen species formed during normal metabolic processes, Ultraviolet light and Ionizing Radiation. DNA damage that is not repaired can impair cell function or trigger programmed cell death (formally called "apoptosis"). DNA is composed to two complementary strands. When one of these strands is damaged, the cell can use the other undamaged strand to fill in the missing information on the damaged strand. Several different pathways exist to repair DNA, and RecA is involved in a pathway called recombinational repair. This pathway is one that uses the complementary strand to repair a damaged strand in a complex reaction called "strand exchange." This process is especially important during DNA replication. During DNA replication, the two original DNA strands are separated and new, complementary strands are added to each of the original strands. The result is two new sets of double-stranded DNA (sometimes referred to as “duplexes”) exactly identical to the original double-stranded DNA set. So no unpaired DNA is present in either of the two new sets of DNA. A problem arises if one of the original strands is damaged; DNA damage is like a road block for the DNA replication machinery. If the original strand is damaged, a portion of that DNA strand won't be replicated. This results in a gap on the newly replicated strand. This gap is an unpaired segment of single-stranded DNA. Unpaired, single-stranded DNA is a red flag for the cell that something has gone wrong!
The recombinational repair pathway (Figure 1) and the RecA protein will be used to fill in this gap. The basic steps of the pathway are as follows:
- The exposed single-stranded DNA is bound by RecA, forming a RecA-DNA filament.
- This single-stranded DNA-RecA filament finds a portion of homologous double-stranded DNA duplex. This DNA duplex is composed of two strands; one of these strands is exactly the same as the single-stranded DNA bound by RecA, and the other is exactly complementary. RecA is bound to the single stranded portion of DNA on the damaged duplex; this strand is missing its “pairing partner” and needs to be paired with a strand that is exactly complementary. The complementary portion that is added will fill in the gap on the damaged strand.
- RecA catalyzes the pairing of the single-stranded DNA to the one strand of the DNA duplex that is complementary. The other strand of the DNA duplex is displaced. This step is the actual "strand exchange."
RecA function in the SOS Response
Protein Structure
6 RecA monomers come together to form a filament that wraps around ssDNA.
