Eukaryotes, archaea and bacteria
are the 3 domain system of organism classification. Archaea and Bacteria are sometimes group together and called prokaryotes, because they both lack nucleus and other membrane bound organelles (such as mitochondria).
- Eukaryotes: organisms whose cells have a nucleus, which sequesters DNA inside.
- Archaea and bacteria (prokaryotes) don’t have a nucleus, no chromosomes. DNA is loosely organized.
Evolutionarily, eukaryotes and prokaryotes split near the beginning of the evolution of life on earth, bacteria and archaea split later.
Mitochondria is considered to be absorbed by the ancestor of all eukaryotic cells, becoming part of every eukaryote.
The cell (division) cycle: the series of events that take place in a cell that cause it to divide to 2 daughter cells.
| State | Phase | Abbreviation | Description |
|---|---|---|---|
| Resting | Gap 0 | G_0 | Cells not dividing |
| Interphase | Gap 1 | G_1 | Cell growth. G1 checkpoint ensures cell is ready for DNA synthesis |
| ^ | Synthesis | S | DNA replication |
| ^ | Gap 2 | G_2 | Growth & prepare for mitosis. G2 checkpoint ensures cell is ready for mitosis phase and divide. |
| Cell division | Mitosis | M | Chromosome separation. Consists of prophase, prometaphase, metaphase, anaphase and telophase. Metaphase checkpoint ensures cell is ready to complete division. |
- Stem cells can differentiate into different types of cells, which involves another type of division where the 2 daughter cells are not identical to each other.
- Meiosis (reduction division): a special type of division in reproduction process that produces gametes, such as sperm or egg cells.
- Recombination/crossing over: prior to the meiosis division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. It is estimated to happen one crossover read per chromosome. It causes our chromosomes to be not identical to either of our parents’ chromosomes. The randomness introduced by recombination is one of the reasons that children of the same parents are not identical.
Haploid and diploid
- Haploid: a single set of chromosomes in an organism’s cells.
- Diploid: humans (and other sexually reproducing organisms) are diploid. Each cell have 2 sets of chromosomes, one from each parent. The exception being the egg and sperm cells, they are haploid.
- Haplotype, genotype, homozygous and heterozygous
- At each site of the genome, we have a paternal and a maternal “haplotype”. The two alleles form our “genotype”.
Homo/hetero-zygous
- A homozygous site means the two alleles (haplotypes) are the same type, a heterozygous site means they are different.
Karyotype
- A karyotype is a visual representation (usually by staining chromosomes and then pair them) of the complete set of chromosomes within a cell.
- Karyotyping is used to determine the number of chromosomes, to examine whether there are missing or extra chromosomes.