Biology 3.2 Chromosomes (Bioninja 1)

• Chromosomes carry genes in a linear sequence that is shared by members of a species.
• Different chromosomes will have distinctive sizes and other features such as arm lengths and banding patterns.
• Gene loci can be identified according to chromosome number, arm and banding region.
• DNA is a double stranded molecule that functions as the genetic material of a cell, composed of nucleotides made up of four distinct bases (A, T, G and C), and the order of these bases form gene sequences that code for specific proteins.
• A single double-stranded DNA molecule is called a chromosome.
• A single chromosome may contain thousands of unique genes on either strand.
• Chromosomal structure differs between prokaryotic and eukaryotic organisms, with chromosomes being circular in prokaryotes and linear in eukaryotes.
• Prokaryotes have one chromosome consisting of a circular DNA molecule and do not possess a membrane-bound nucleus, with the DNA contained in a region called the nucleoid.
• Prokaryotes only have a single chromosome (genophore) which is circular and naked (not bound to proteins), and the genome is more compact (usually no introns).
• Some eukaryotic organelles also possess circular DNA (chloroplast, mitochondria), indicating prokaryotic origins.
• The chromosome profile of an individual organism can be represented as a karyogram.
• The homologous pairs are arranged by length on karyograms.
• Karyograms display chromosomes from cells that have been arrested during mitosis (condensed DNA).
• Karyograms are generally used to identify sex or to identify chromosomal abnormalities (aneuploidy).
• Karyograms are used to deduce sex and diagnose Down syndrome in humans.
• John Cairns utilised a technique (autoradiography) to visualise chromosomes whilst uncondensed.
• Radioactive thymidine incorporated into DNA is a key component of the autoradiography method.
• Chromosomes treated with silver bromide are a crucial part of the autoradiography method.
• Radiation converts Ag + ions to insoluble grains, a process that is central to the autoradiography method.
• Autoradiography has other useful applications beyond identifying DNA length, such as identifying regions of active transcription.
• If radioactive uracil is used instead of thymine, regions of active transcription can be identified.
• For DNA, tritium (3 H) is used, while for RNA, tritium (3 H) is also used.
• More silver grains equals more transcription, as seen in low and high rates of transcription.
• Circular DNA (genophore) is a single chromosome in prokaryotes and is autonomous, capable of autonomous function, and can be exchanged by bacterial conjugation.
• Eukaryotic chromosomes are linear DNA molecules associated with histone proteins, with DNA stored in a membrane-bound nucleus, and eukaryotes possess multiple linear chromosomes, with DNA bound to histone proteins to form chromatin, and the genome is less compact (has many introns).
• In a eukaryote species, there are different chromosomes that carry different genes, with eukaryotic organisms having multiple (more than one) chromosomes, each carrying many different genes, located at specific gene loci.
• The number of chromosomes is a characteristic feature of members of a species.
• Different species have different chromosome numbers and gene distributions.
• Members of different species cannot sexually reproduce as the genetic content of their sex cells will not be complementary (they will have incompatible gametes).
• Horse = 64, Donkey = 62, Mule = 63.
• Hybrid species can sometimes occur, but are always infertile.
• Diploid nuclei have pairs of chromosomes while haploid nuclei have one chromosome of each pair.
• In sexually reproducing species, offspring will inherit genetic information (chromosomes) from both parents.
• Two copies of each chromosome = diploid (2n).
• All the body cells will have paired chromosomes, but the sex cells (gametes) must only have one of each pair.
• One copy of each chromosome = haploid (n).
• Organisms that only reproduce asexually are haploid.
• Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles.
• Homologous chromosomes are the paired maternal and paternal chromosomes found in diploid organisms.
• Homologous chromosomes will have identical structural features (size, band patterns, etc.) and the same genes at the same loci positions.