Genome Structure and Chromosomes

Haplodiploidy and Chromosome Types

• Bees, wasps, and ants are haplodiploid, with haploid males and diploid females.
• Different types of chromosomes are inherited differently: autosomal genes, Y-linked genes, X-linked genes, and mitochondrial genes.

Mitochondrial Eve

• Mitochondrial Eve was not the first woman, nor was she the only woman alive at the time, nor was she the only woman who left descendants.
• She was just our last common ancestor if you only consider the female line.

Viroids

• Viroids are small RNA molecules that do not code for any proteins but use the host's RNA polymerase to replicate themselves.
• The smallest known pathogen is the coconut cadang-cadang viroid, which is a 246bp RNA molecule that kills 1 million trees a year.

Viruses and Giant Viruses

• Viral genomes tend to be compact.
• Genes in viruses often overlap and are multifunctional.
• Typical virus genome sizes are in the tens of thousands of base pairs, but there are extremes: single-stranded DNA viruses can be less than 2Kb in length, and 3Mb Pandoraviruses, discovered in 2013, infect amoebas.

Prokaryotes

• Prokaryotes have generally compact, circular genomes.
• Genes are arranged into operons.
• Plasmids are 'optional extras'.
• There are extremes: N. deltocephalinicola has the smallest prokaryotic genome at 112Kb, and S. hofmanni has the largest prokaryotic genome at 12Mb.

Eukaryotes

• Eukaryotes have typically large genomes organized into chromosomes.
• Organelles like mitochondria and chloroplasts can have genomes separate from the nuclear genome.
• Nobody has much of an idea what causes evolution/variation in chromosome number; it's an unsolved mystery.
• Within mammals, one of the monkeys has the most chromosomes at 97, and the muntjac deer has the fewest at 4.

Protists

• Protists are an extremely diverse group of unicellular eukaryotes.
• Some protists, like Paramecium, have two kinds of cell nuclei: one 'working memory' (macronucleus) containing many thousands of tiny, gene-sized 'chromosomes', and one 'master copy' micro nucleus containing the germline genome.

Human Genomes

• Humans are a terrible model organism due to long generation time, small litter sizes, and huge ethical problems with experimentation.
• The understanding of the human genome has been pieced together from various sources, including pedigrees and other genomes.

Gene Numbers

• In bacteria, identifying genes is relatively straightforward.
• In humans, it's not, because genes are hidden among a lot more junk.
• There are also large introns, splice sites, and alternative splicing patterns.
• ORFs can be found bioinformatically, but a very short ORF is unlikely to be transcribed.
• Homology with known genes from other species can also be used to help find genes.

Chromosome Structure

• Eukaryotic chromosome can be a sex chromosome or an autosome.
• Prokaryotic chromosome is typically circular, more gene-dense, and much smaller.
• One way the genome can be structured is that there can be sex chromosomes, which are subject to very different evolutionary pressures from autosomes.

Sex Determination

• XY systems like mammals have are not the only way to do it.
• Platypuses have ten sex chromosomes, and birds have a WZ system, as do moths.
• Some spiders have an X1X20 system with males heterogametic, and nematodes have an XO system where the female has more chromosomes than the male.
• Sex chromosomes aren't the only way to do sex determination; clownfish start male and turn female when they come to dominate a group, and alligators and crocodiles have temperature-dependent sex determination.

Explore the concept of the genome, its structure, and different types of chromosomes. Learn about the differences between eukaryotic and prokaryotic chromosomes, and how sex chromosomes are subject to unique evolutionary pressures.