DNA, histone-protein, and acidic-nuclear-protein assemblies {chromosome}| contain genes and replicate. Waldeyer invented name [1888].
shapes
Prokaryotes have one circular chromosome. Eukaryotes have linear chromosomes.
pairing
In organisms with sexual reproduction, chromosomes have pairs, one from father and one from mother. Both pair members have same nucleotide sequences, except for natural allele variations or mutation damage.
number
Red blood cells have no nuclei. Germ cells have 23 chromosomes. Other human cells have 46 chromosomes, in 23 pairs. Ape somatic cells have 24 chromosome pairs. Two ape chromosomes fused to make human chromosome 2.
synteny
Different species can have chromosomes with same genes {conserved synteny}.
linkage
Different species can have same chromosome gene order {conserved linkage}.
In female cells, one X-chromosome inactivates early in embryonic development and makes a nuclear body {Barr body}|. Xist gene makes active RNA that coats that X chromosome. Coated X also methylates. The other X-chromosome makes antisense RNA that binds Xist RNA.
Stained cell-nucleus parts {chromatin}| show nucleic acid-protein complexes. Inactivated X-chromosomes have chromatin {macrochromatin} that differs in structure and appearance.
Stained chromosomes show distinctive banding patterns {cytogenic map} under light microscopy.
In diploid organisms, somatic chromosomes {homologous chromosomes}| from father and mother contain same gene sequence, except for natural allele variations or mutation damage.
Offspring from mating two different species cannot produce offspring {sterility}|, because the chromosome sets cannot interact.
In all mammals, in most animals, in some fungi, and in some protozoa, chromosome ends {telomere}| have TTAGGG repeated 2000 times. In plants, TTTAGGG repeats. In ciliates, TTTTGGGG or TTGGGG repeats. Telomeres are nucleoprotein complexes at chromosome ends and can have caps.
Chromosome copying starts just inside chromosome ends, so copies are shorter than copied chromosome. Telomeres decrease in length with each replication. Over a lifetime, humans make 150 copies and shorten telomeres by half. Telomerase enzyme restores telomere length in sperm and egg cells, and in cancer cells, using RNA as template for reverse transcriptase. Telomerase prevents shortening in human immune system, hemopoietic system, germline cells, embryonic cells, stem cells, skin cells, intestinal-lining cells, hair-follicle cells, and cancer cells.
After telomeres reach threshold length, cells can have senescence. Perhaps, telomere shortening ends at age 40, related to cell-turnover reduction.
Rodent cells do not have telomere shortening, stop dividing after 10 to 15 doublings, have telomerase, and grow indefinitely in culture.
Chromosomes {autosome}| can be all chromosomes except X and Y.
In salivary glands, fruit flies have 1000 aligned chromosome copies {polytene chromosome}|.
X-chromosomes and Y-chromosomes {sex chromosome}| determine sexual characteristics.
male
Male cells have one X-chromosome and one Y-chromosome.
female
Female cells have two X-chromosomes.
genes
Sex-chromosome genes determine sex-linked traits and diseases, such as hemophilia and color blindness. Sex hormones influence some autosomal genes, such as baldness gene and horn gene.
crossing over
X-chromosome and Y-chromosome stopped crossing over in birds and mammals.
meiosis
Tips of Y recombine with tips of X, to allow meiosis.
evolution
Reptiles that led to mammals had two X-chromosomes. SRY gene arose 350,000,000 years ago. Between 320,000,000 to 240,000,000 years ago, ancient X-chromosome larger half, containing SRY, inverted or failed to recombine, making monotreme Y-chromosome. Between 170,000,000 and 130,000,000 years ago, a region on other centromere side inverted or failed to recombine, resulting in marsupial Y-chromosome. From -130,000,000 to -80,000,000 years ago, a large region on other centromere side inverted or failed to recombine, resulting in Eutheria Y-chromosome. From -50,000,000 to -30,000,000, a large region on other centromere side inverted or failed to recombine, resulting in human Y-chromosome. During this succession, SRY gene moved to other arm.
evolution: human
Y-chromosome variations track human migrations. M91 is only in south-Africa San people. Yap, M60, M2, MT68, M89M96, M35, M172, and M304 are in Africa. M170, M343, and LLY22 are in Europe. M9, M201, M17, M173, and M69 are in Near East. M20 and M45 are in India. M175, M174, and M122 are in southeast Asia. M130, M4, and M130 are in Pacific islands. M242 and M130 are in northeast Asia and Americas.
Sex chromosomes {X-chromosome}| can have 2000 to 3000 genes. Mammals inactivate X-chromosome in females, to prevent overproduction from X genes. X-chromosome started to develop 300 million years ago.
Sex chromosomes {Y-chromosome}| can have 80 genes. Some genes are only on Y. SRY is for making testes and derived from SOX3 of X-chromosome. TTY2, CDY, PRY, DAZ, and BPY2 are for making sperm. Other genes are TTY1, TSPY, and XKRY.
Some genes are on X and Y. DBY, EIF1AY, RPS4Y, SMCY, TB4Y, USP9Y, UTY, and ZFY are for housekeeping. RBMY and VCY work only in testes. Others are PCDHY and AMELY.
One Y-chromosome end is 95% of Y-chromosome and has no functioning genes. Y-chromosome has palindrome structure.
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Date Modified: 2022.0225