Organization of Genetic Material into Chromosomes

Abstract

The genetic information of most living organisms is stored in deoxyribonucleic acid (DNA). DNA usually exists as a double helix, with two strands of opposite polarity, held together by hydrogen bonds between the complementary bases. Adenine is paired with thymine with double hydrogen bonds, whereas guanine is paired with cytosine with triple hydrogen bonds. The specific base sequence and their complementarity make it a unique feature for each organism to store and transmit genetic information. Ribonucleic acid (RNA) usually exists as a single-stranded molecule containing uracil instead of thymine. The DNA molecules present in the bacterial chromosomes are circular and contain about 50 negatively supercoiled domains. Each eukaryotic chromosome contains one giant molecule of DNA packaged into 10 nm ellipsoidal beads called nucleosomes. The 10 nm fibers are further coiled into chromatin fibers about 30 nm in diameter, which are organized into domains by scaffolds comprising nonhistone chromosomal proteins. The chromosomes are maximally condensed during metaphase of meiosis and mitosis. The centromeres, which are spindle fiber attachment regions, and telomeres, which are the ends of chromosomes, have unique structures that facilitate their respective functions.

Glossary

Acentric

 Chromosome segment lacking a centromere.

Acrocentric

 Chromosome or chromatid that has its centromere near the end.

Anaphase

 A stage in cell division during which chromosomes are moved toward opposite ends of the spindle.

Autosome

 One of any pair of the chromosomes that do not carry the genes determining the sex.

Bacteriophage

 A virus that attacks bacteria.

Cell cycle

 The sequence of stages that a dividing eukaryotic cell goes through from the time it is created to the time it undergoes mitosis. It includes M, G1, S, and G2 phases.

Centromere

 The region joining the two sister chromatids during cell division.

Chromatid

 One of the daughter strands of a chromosome that has recently been copied during the cell division and which is still connected to the other daughter strand. Upon separation from each other, the chromatids become chromosomes.

Chromatin

 The entire complex of protein and DNA that makes up a eukaryotic chromosome.

Chromatin fibers

 A basic organizational unit of eukaryotic chromosomes that consists of DNA and associated proteins assembled into strands of average diameter 30 nm.

Chromomeres

 Small bodies that are identified by their characteristic size and linear arrangement along a chromosome.

Chromonema

 An optically single thread forming an axial structure within each chromosome.

Chromosome

 A single long molecule of DNA and any associated protein. Each chromosome carries a linear array of genes.

Dalton

 A unit of molecular mass approximately equal to that of a hydrogen atom or 1.66 × 10⁻²⁴ g. Alternatively, there is approximately 1 mole of daltons per gram.

Deoxyribonuclease

 Any enzyme that hydrolyzes DNA.

Dicentric chromosome

 One chromosome having two centromeres.

Diploid

 Containing two sets of homologous chromosomes and thus two copies of each gene.

Euchromatin

 Genetic material that is not stained so intensely by certain dyes during interphase and that comprises many different kinds of genes.

Eukaryote

 A member of the large group of organisms that have nuclei enclosed by a membrane within their cells.

Folded genome

 The condensed intracellular state of the DNA in the nucleoid of bacterium. The DNA is segregated into domains, and each domain is independently negatively supercoiled.

G1 phase

 The phase of a cell cycle that constitutes the first part of the interphase before DNA synthesis.

G2 phase

 The phase of the cell cycle between synthesis of DNA (S phase) and mitosis (M phase), the last part of interphase.

Genome

 All of the hereditary genetic information in an organism. A complete set of chromosomes (n) inherited as a unit from one parent.

Haploid

 Containing a single set of chromosomes. Mature eggs and sperm and bacteria are examples of haploid cells.

Helix

 Any structure with a spiral shape. The Watson and Crick Model is in the form of double helix.

Histones

 Group of proteins rich in the basic amino acids. They take part in the formation of nucleosomes.

Hydrogen bond

 A weak electrical attraction between two molecules, due to the partially positive hydrogen atoms of one molecule attracting particularly negative atoms of the other molecule. This is a bond between hydrogen and an electronegative element, and the above hydrogen is attached to another electronegative element.

Interphase

 The part of the cell cycle during which no cell division occurs. It includes G1 phase, the S phase, and G2 phase.

Karyotype

 A systematized arrangement of the chromosomes from a cell.

Kinetochore

 The attachment that forms during the cell division between the microtubules of a spindle fiber and a chromosome. It contains motor proteins that move the chromosomes along the microtubules.

M phase

 The phase of the cell cycle during which mitosis occurs. It includes karyokinesis and cytokinesis.

Metacentric

 A chromosome structure in which the centromere is roughly equidistant between the two ends.

Metaphase

 A stage in cell division during which chromosomes line up in the equator of the spindle.

Metaphase plate

 The imaginary plane along which the chromosomes line up during metaphase of cell division.

Mitosis

 Nuclear division in eukaryotes producing two daughter nuclei that are genetically identical to the parent.

Mitotic spindle

 An array of microtubules formed during cell division that moves chromosomes to opposite sides of the cell.

Nonhistone chromosomal proteins

 All of the proteins in chromosomes except the histones.

Nuclear envelope

 The double-layered membrane enclosing the nucleus of eukaryotic cell.

Nuclear pore

 An opening in the nuclear envelope that connects the inside of the nucleus with the cytoplasm and through which molecules can pass.

Nucleolus 

A structure in the eukaryotic nucleus where ribosomal RNA processing occurs and ribosomal subunits are assembled.

Nucleosome

 A repeating, bead-like structure of eukaryotic chromosome, consisting of about 200 nucleotides of DNA wrapped twice around eight molecules of histone proteins.

Nucleotide

 A monomer that can be polymerized to form the nucleic acids DNA or RNA. One nucleotide consists of a five-carbon sugar (ribose or deoxyribose), a phosphate group, and one of the nitrogenous bases.

Phosphodiester bond

 The bond formed in the polymerization reaction between the phosphate bond of one nucleotide and the hydroxyl group on the sugar component of another nucleotide.

Polynucleotide

 A linear sequence of joined nucleotides in DNA or RNA.

Polytene chromosomes

 Giant chromosomes produced by interphase replication without division and consisting of many identical chromatids arranged side by side in a cable-like pattern.

Prokaryotes

 A member of a large group of organisms that lack true nuclei in their cells including bacteria and blue-green algae.

Prophase

 The first stage of cell division during which chromosomes become visible, the mitotic spindle forms, and the nuclear membrane breaks down.

Protamines

 Small basic proteins that replace the histones in the chromosomes of some sperm cells.

Purine

 A double-ringed nitrogen-containing base present in nucleic acids; adenine and guanine are the two purines present in most DNA and RNA molecules.

Pyrimidines

 A single-ringed nitrogen-containing base present in nucleic acids; cytosine and thymine are commonly present in the DNA, whereas uracil replaces thymine in RNA.

Radioactive isotope

 An isotope that spontaneously decays to form a different isotope or element, by emitting radiation or a particle.

Recombination

 Any change in the combination of genes or alleles found on a chromosome or in a given individual. The production of gene combinations not found in the parents by the assortment of nonhomologous chromosomes and crossing over between homologous chromosomes during meiosis.

Ribosomal RNAs

 The RNA molecules that are structural components of ribosomes.

S phase

 The phase of the cell cycle during which DNA is synthesized and the chromosomes are duplicated.

Sex chromosomes

 One of the pair of chromosomes that carries the gene(s) that determine gender.

Sister chromatids

 The paired strands of a recently replicated chromosome that has not yet divided.

Telomerase

 An enzyme that adds telomere sequences to the ends of eukaryotic chromosomes.

Telomere

 The unique structure found at the end of eukaryotic chromosomes.

Telophase

 The last stage in cell division in which chromosomes are assembled at the poles of the mitotic spindle.

Transformation

 Genetic alteration of an organism brought about by the incorporation of foreign DNA into cells.

Virulence

 The ability to cause disease and death.