Chromosomes are linear pieces of DNA that are highly folded and compacted (DNA Packaging). If all of the DNA strands from each of your 46 chromosomes (23 pairs) were joined end to end, the DNA would stretch about 2 meters. Therefore, DNA molecules must be highly organized and compacted to fit into the nucleus of a single cell.
As you learned in the Carbon and Life tutorial, DNA is a double-stranded molecule, containing two antiparallel strands of nucleotides arranged in the form of a double helix. The two strands are held together by hydrogen bonds and contain nitrogenous bases (nucleotides) paired in a complementary way that appear as "rungs" on a spiral ladder. The complementary nucleotides pair specifically, such that adenine and guanine nucleotides, the purines (A and G) pair with corresponding thymine and cytosine nucleotides, the pyrimidines (T and C); A pairs with T, and G pairs with C. The figure at right depicts this in two ways. The image on the left clearly shows the base-pairing rules, whereas the image on the right depicts the helical arrangement of the paired polynucleotide strands.
Homologous chromosomes have similar nucleotide sequences but they are not identical. Indeed, it is these subtle differences in nucleotide sequences that form the molecular identity of alternative alleles for a given gene. During Prophase I, homologous chromosomes pair together due to their similarity in structure, length, and gene sequence. This sets up a physical association, synapsis, that enables the exchange of genetic sequences (i.e., pieces of DNA) between homologous chromosomes through crossing over.
The pairing of homologous chromosomes at Prophase I is different than the
pairing that occurs between complementary strands of DNA., The DNA of
nonsister chromatids becomes precisely aligned when the
synaptonemal complex is formed. When the homologs pair, both a maternal
copy and a paternal copy of genetic information line up against each other.
While the chromosomes are in synapsis, the two homologs may swap genetic
material in a process called crossing over.
This process is a source of genetic recombination
and
produces recombinant chromosomes
. That is, a piece of a
maternal chromatid exchanges with a piece of the paternal chromatid on the
homologous chromosome. There can be multiple crossovers between the non-sister
chromatids of homologous chromosomes. Furthermore, crossover configurations
can occur in any combination and can lead to different outcomes. Recall,
crossing over was introduced in the tutorial on Meiosis.
Only two nonsister chromatids are involved in any single crossover event, and crossing over occurs at different points along the chromosome during each meiosis. Every meiotic process does not involve crossing over in the same set of genes each time, but there is at least one crossover per homologous pair. Still, when crossing over is observed in a meiotic division, up to half of the products of that division result in a recombination of DNA between the homologs, producing novel chromosomes. This recombination means the homologs are now different than the parental chromosomes, therefore, each may carry different genetic information.
Many factors affect crossing over, and the position on the chromosome where crossing over will occur is unpredictable. Crossing over is a random event. While the location of the break points on the DNA sequence of the chromosomes are fairly random, the recombination frequency is relatively constant between homologous chromosomes. (For a given chromosome, N number of cross overs will occur, but where they will occur is random.)
The probability of crossing over between genes on a chromosome is dependent on the distances between the genes. This shouldn't surprise you because the greater the distance between two genes, the greater the chance a break will occur.
Genes that are located on the same chromosome and that tend to be inherited together are called linked genes because the DNA sequence containing the genes is passed along as a unit during meiosis unless they are separated by crossing over. The closer together that genes are located on a particular chromosome, the higher the probability that they will be inherited as a unit, since crossing over between two linked genes is less frequent the closer together the two genes are (genes with complete linkage are close enough together on a chromosome that they never recombine and are always inherited as a unit).
Because of this, linked genes do not follow the expected inheritance patterns predicted by Mendel's Theory of Independent Assortment when observed across several generations of crosses. For two heterozygous genes that are unlinked and undergoing independent assortment, you expect to see parental and recombinant gametes in a ratio of 1:1:1:1.
When two genes are linked on a chromosome,crossing over between the two genes will be less common than having no crossing over, so fewer recombinant chromosomes will be produced. Under this circumstance, a ratio that deviates from the usual 1:1:1:1 will be observed, indicating that the genes are linked. The number of parental genotypes in the gametes will be higher and the number of recombinant genotypes will be lower.
Chromosome Behavior and Gene Linkage Part 1 VoiceThread Transcript