BIOG 1105-1106 | Cornell Introductory Biology, Individualized Instruction

(d) A translocation moves a segment from one chromosome to another, nonhomologous one. The most common type of translocation is reciprocal, in which nonhomologous chromosomes exchange fragments. Nonreciprocal translocations, in which a chromosome transfers a fragment without receiving a fragment in return, also occur.

Fig. 15.13. Alterations of chromosome structure. Vertical arrows indicate points of chromosome breakage. Dark purple highlights the chromosomal parts affected by the rearrangements. (From Campbell, Biology, 6th ed., p. 281)

Transposon-mediated duplication

Fig. 18.17. Insertion of a transposon and creation of direct repeats. (1) First, the transposase enzyme makes staggered cuts (red arrows) in the two DNA strands at a target site, leaving short segments of unpaired DNA as shown. Meanwhile, the transposon is cut out or copied at its initial site. (2) The transposon is then joined to the single-stranded ends at the target site. Presumably, the transposase holds all the components together during this process. (3) Finally, the gaps in the DNA strand are flled in by DNA polymerase and sealed by ligase. This results in direct repeats, identical segments of DNA on either side of the transposon. (Distances along the DNA are not to scale). (From Campbell, Biology, 6th ed., p. 346)

(1) Transcription of the retrotransposon DNA by RNA polymerase produces RNA.

(2) Translation of part of the RNA yields reverse transcriptase.

(3) Reverse transcriptase catalyzes the synthesis of a DNA strand on the retrotransposon RNA template.

(4) Reverse transcriptase also catalyzes the replacement of the RNA strand with DNA.

(5) Insertion of the double-stranded DNA version of the retrotransposon occurs at some other location.

Fig. 19.5. Retrotransposon movement. Notice that the retrotransposition mechanism is essentially identical to part of the retrovirus reproductive cycle (See Figure 18.7). By replicative transposition, as shown here, a retrotransposon can populate the genome of a multicellular eukaryote in huge numbers. (From Campbell, Biology, 6th ed., p. 360)

Unequal crossing over during meiosis

Exon duplication from unequal crossing over. Sometimes during meiosis two chromatids from homologous chromosomes (A) are misaligned during a crossocer event (B) as a result, one chromatid gains a second set of exons (and introns) from the misaligned region of the homologus chromatid (C). This duplication is inherited by one of the resulting gametes.

Duplication through chromosomal breakage and fusion during meiosis

Duplication of exons through breakage and fusion. Sometimes an end of one chromosome breaks off (A-B) and fuses to the end of another (C). In the example shown here, the fragement fuses to the corresponding end of the twin chromatid. If such an event occurs in the process of meiosis, which leads to gamete formation, one gamete will have two copies of all exons (and introns) on the fragment.