Base pairs
The genetic code is a 3 billion-character string written in base-4.
The nitrogenous bases are adenine, thymine, cytosine, and guanine. A base pair includes two nucleotides, and a nucleotide consists of a sugar, phosphate, and one of the nitrogenous bases. Each base pair includes two molecules, either a pair of AT (adenine-thymine) or CG (cytosine-guanine). The orientation of the pair matters, leading to two bits, or four possible combinations per base pair.
Three base pairs form a codon (or triplet). Each codon codes an amino acid. 64 possible codons (2 bits ^ 3). There are only 20 amino acids, so multiple codons can make an amino acid.
A protein is a string of ~50 amino acids, bounded by a start and ending codon. The start codon produces methionine. Ending codon doesn't produce anything.
"It seems to be a matter of five or six proteins, on average, from one gene." - McKusick
SNP - Single-nucleotide polymorphism
Refers to a change in a single basepair that appears often enough in a population (e.g. >1%). Estimated to be 4 to 5 million SNPs in a person's genome. 100 million SNPs have been found total. [source]
Genetic differences among individuals is 0.1%, which multiplied by 3 billion basepairs gets us directionally the 4-to-5 million estimate above.
Genes
A gene is a sequence of DNA, so maybe 900 base pairs (ex: 6 proteins * 50 amino acids / protein * 1 codon / protein * 3 basepairs / codon). It comes out to 448 base pairs per gene using the mDNA math below. A gene codes for a molecule that has a function.
An allele is a variant of a gene. The most common is variant type is dominant-recessive, where the recessive gene simply doesn't code for a functional protein, but still causes a phenotypical change.
The human genome is composed of 19,000-20,000 human protein-coding genes.
2.9 billion DNA base pairs that make up the human genome. Essentially ~6 billion molecules (nitrogenous bases) make up our DNA.
Functional composition
"Only 1.5 percent of the DNA in the genome actually codes for genes." [source]
The low junk calculation likely comes from the idea that only a small percentage of DNA is bounded by start and stop codons.
"... protein-coding sequences, which are what we mostly mean when we say 'genes,' occupy only a little over 1% of the human genome." [Powledge]
"8% is nearly all regulatory sequences" (Powledge)
Packing
146 base pairs pack into a nucleosome, so maybe one nucleosome per protein. Nucleosomes pack into chromatines. Chromatines pack into chromatine loops. Chromatine loops become 23 chromosomes.
Gene expression
There are 200 different types of cells in the human body, and about 20 different types of structures or organelles within those cells.
Chromosomes
Each cell has 46 chromosomes or 23 pairs of chromosomes.
A chromosome can be seen in two ways:
- An "X", which is two identical, sister chromatids joined by a centromere (dyad)
- Just a chromatid, or strip of DNA (useful karyotype)
The Y-chromosome is a misnomer because it's shaped more like a V when in dyad form, since its chromatid is shorter compared to the autosomes.
Homologous ("same" + "ratio, proportion" / "logos") means the genes line up at the same spot for each chromosome. Chromosome means "color" + "body", with the color referring to the color of the dye when they were discovered.
Diploid means "double" + "form", haploid means "single" + "form". Diploid cells have two homologous copies of each chromosome, one from father, and one from mother.
Zygote means "yoke" / "fertilized egg". -zygous means related to fertilization (i.e. homo-zygous, hetero-zygous).
Genetic recombination
Meiosis means "lessening", mitosis means "threading." Mitosis is just a cell duplicating itself. Meiosis is the production of gametes (sperm and eggs). These gametes have 23 chromosomes. Meiosis goes from diploid to haploid. Mitosis goes from diploid to diploid.
Chromosomal crossovers
23 chromosomes come from father, 23 from mother. The 23rd pair has two sex chromosomes.
"Only one or two crossovers will form between any given pair of chromosomes." [source]
Unequal crossovers result in the insertion or deletion of genetic material and are a general source of mutation in the genome. [source]
Mitochondria
"In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining control of the cell cycle and cell growth." [wikipedia]
16,569 base pairs of mDNA encode for 37 genes.
