Homologous recombination dna repair animation
Parental strands of DNA separate and serve as templates for the synthesis of two new strands at the replication fork. One new DNA strand the leading strand is synthesized in homologous recombination dna repair animation continuous manner; the other strand the lagging strand is formed by the joining of small fragments of DNA that are synthesized backward with respect to the overall direction of replication.
DNA polymerases and various other proteins act in a coordinated manner to synthesize both leading and homologous recombination dna repair animation strands of DNA.
The fidelity of replication: DNA polymerases increase the accuracy of replication both by selecting the correct base for insertion and by proofreading newly synthesized DNA to eliminate mismatched bases.
Origins and the initiation of replication: DNA replication starts at specific origins of replication, which contain binding sites for proteins that initiate the process. In higher eukaryotes, origins may be defined by chromatin structure rather than DNA sequence. Telomeric repeat sequences at the ends of chromosomes are maintained by the action of a reverse transcriptase telomerase that carries its own template RNA.
Direct reversal of DNA damage: A few types of common DNA lesions, such as pyrimidine dimers and alkylated guanine residues, are repaired by direct reversal of the damage.
The resulting gap is filled by newly synthesized DNA, using the undamaged complementary strand as a template. In base-excision repair, specific types of single damaged bases are removed from the DNA molecule. In contrast, nucleotide excision repair systems recognize a wide variety of lesions that distort the structure of DNA and remove the damaged bases as part of an oligonucleotide. A third excision repair system specifically removes mismatched bases from newly synthesized DNA strands.
Specialized DNA polymerases are capable of replicating DNA across from a site of DNA damage, although the action of these polymerases may result in a high frequency of incorporation of incorrect bases. Repair homologous recombination dna repair animation double-strand breaks: Double-strand breaks are repaired by recombination to rejoin the damaged strands.
Recombinational repair can occur either by homologous recombination with an undamaged chromosome or by nonhomologous rejoining of the broken ends of a single DNA molecule. Switching between yeast mating types is mediated by site-specific recombination between active and inactive alleles of the mating type locus. In vertebrates, site-specific recombination plays a critical homologous recombination dna repair animation in generating immunoglobulin and T cell receptor genes during development of the immune system.
Additional diversity is provided to immunoglobulin genes by somatic hypermutation and class switch recombination. Gene amplification results from repeated replication of a chromosomal region. In some cases, gene amplification provides a mechanism for increasing gene expression during development.
Gene amplification also frequently occurs in cancer cells, where it can result homologous recombination dna repair animation the elevated expression of genes that contribute to uncontrolled cell proliferation. DNA Rearrangements Yeast mating types: Copyright Sinauer Associates.