Gene, Genome and Chromosomes Segment 2

Summary

The lecture explains how chromosomes exchange genetic material during meiosis, then reviews chromosome and DNA structure and several features of genomes.

Crossing Over and Genetic Recombination

• Crossing over is the exchange of corresponding chromosome segments between homologous chromosomes, one inherited from each parent. It generally occurs during meiosis, rather than mitosis.
• The exchange happens at points called chiasmata (singular: chiasma). Crossing over is not equally likely along a chromosome; it occurs more frequently in some regions than others. The centromere is an important landmark.
• Because homologous chromosomes can exchange segments, the resulting gametes may carry new combinations of alleles. This increases genetic variation among offspring.
• For example, fruit-fly traits such as body color and wing length are represented by different alleles at particular chromosome locations. Crossing over can produce combinations of these traits that were not present together on either original parental chromosome.
• A gene’s position on a chromosome is its locus; different alleles can occupy the same locus.

Chromosome Examples

• Polytene chromosomes are exceptionally large chromosomes found in the salivary glands of fruit flies and some other insects. Their alternating dark and light bands are visible under a microscope and correspond to chromosome regions.
• Enlarged regions, or “puffs,” in these chromosomes are associated with active gene expression—the production of RNA from DNA.

DNA’s Chemical Building Blocks and Structure

• A nucleotide, the basic unit of DNA, consists of a phosphate group, a sugar, and a nitrogenous base. A sugar and a base together are called a nucleoside.
• DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Adenine and guanine are purines; cytosine and thymine are pyrimidines. RNA uses uracil (U) instead of thymine.
• DNA strands have directionality, conventionally written 5′ to 3′. Neighboring nucleotides are linked by phosphodiester bonds.
• DNA usually forms a right-handed double helix made of two complementary, antiparallel strands: one runs 5′ to 3′, while the other runs 3′ to 5′.
• The sugar-phosphate backbone is on the outside of the helix, while the bases pair on the inside. A pairs with T through two hydrogen bonds, and G pairs with C through three. These complementary pairings help stabilize the structure.
• The helix has a major groove and a minor groove, which are relevant to how proteins interact with DNA. The lecture gives approximately ten base pairs per helical turn.

What Genetic Material Does

The lecture identifies three broad functions of genetic material:

1. Store genetic information.
2. Replicate so information can be passed to daughter cells and offspring.
3. Enable gene expression: genetic instructions are used to make RNA and, in many cases, proteins that contribute to observable traits.

DNA Supercoiling and Topoisomerases

• DNA can be supercoiled, a more compact arrangement than relaxed DNA. This helps package DNA, and the lecture describes negative supercoiling as a common form.
• Topoisomerases alter DNA’s coiling by temporarily cutting and rejoining DNA strands:
  • Topoisomerase I cuts one strand.
  • Topoisomerase II cuts both strands.
• These enzymes help manage DNA structure, including during processes such as replication.

DNA Melting and Repeated Sequences

• Heating can separate the two DNA strands by disrupting the hydrogen bonds between paired bases. The temperature at which the strands separate is called the melting temperature; it can differ among DNA samples.
• Genomes contain DNA sequences that occur once as well as sequences that are repeated many times. Some repeated sequences are grouped as satellite DNA, including minisatellite and microsatellite repeats.
• Repeat patterns can vary among individuals. This variation can be used for DNA fingerprinting, such as investigating biological relationships or comparing forensic samples.

Speakers and Sources

• Primary speaker: An instructor or lecturer presenting a biology lesson; no name is clearly identifiable from the subtitles.
• Other voices: Brief, indistinct audience or student responses during the lecture.
• Other sources: No clearly identifiable external speaker or cited source.