An organism's "genome" is its collection of DNA . (Well, in RNA viruses, I guess it would be the RNA) The human genome happens to be about 3 thousand million base pairs of DNA= ~3 billion base pairs 3,000,000,000 ACGCGCGCGATATCACATTGGGCTGGCGCG... etc. etc. 30 base pairs TGCGCGCGCTATAGTGTAACCCGACCGCGC... etc. Like that, except a hundred million times longer The genome of the bacterium E. coli happens to be about 4.7 million base pairs Chloroplast genomes have one to two hundred thousand base pairs Mammal mitochondrial genome =16,500 base pairs (& was sequenced completely back in the 1980s) The smaller species of viruses have even smaller genomes. "The genome project" is an international research program that is in the course of finding out the exact base sequences (except for some parts!) of a hundred or so different species of animals, plants, fungi, bacteria, etc. including humans. Several local scientists are participating. So far, many bacteria, fruit flies, a nematode, & some human chromosomes are finished! For quite a few other organisms, a rough sequence has been obtained but it hasn't been completely assembled or annotated to identify all the genes. Summaries of the results, so far, are published in Nature, Science, and posted on the web. for example, the chloroplast genome of a certain alga Each normal human cell has 46 chromosomes two copies of each of 22 autosomal chromosomes In females, two copies of the X chromosome In males, one copy of the X chromosome and one copy of the Y chromosome. Total number of chromosomes per human cell= 46 One says, "The diploid chromosome number of humans is forty-six" Each sperm cell only has one copy of each autosome, and has EITHER an X or a Y. Sperm cells are said to be "haploid" And one says that "The haploid chromosome number of humans is twenty three." Each species of plant and animal has its own haploid chromosome number, which can be as low as one or as high as a hundred or more. For example, the fruit fly most used in research happens to have a haploid number of four. In some lower plants, each cell is haploid. In "Gray Tree Frogs", one species has diploid cells and a (very) closely related species is tetraploid. (we happen to be on the boundary between them!) "Triploid" means having 3 copies of each chromosome Guess what is meant by "pentaploid"? "Hexaploid"? Human chromosome Number One is the name of our biggest chromosome. The second biggest is called Chromosome Number 2 GUESS THE NAME OF THE THIRD BIGGEST!!?? The smallest human chromosome is number 22. (which is because somebody made a mistake!) (It should have been called chromosome #23) To have 3 copies of all chromosomes is to be triploid But to have 3 copies of any ONE chromosome, and two copies of all the rest is to be "trisomic". "Down Syndrome", which used to be called "Mongolian Idiocy", is caused by trisomy 22. Trisomy for any of the other (bigger) chromosomes results in spontaneous abortion of the embryo. The fraction of human pregnancies that abort spontaneously is about 20% in the US. (Which happens to be about the current rate of deliberate abortions; but the latter is dropping) Each chromosome is a single double-stranded length of DNA, many millions of base-pairs long. In the mitotic cell cycle diploid (after S-period effectively tetraploid) 2N -(S)-> 4N -(mitosis)-> 2N etc. In the meiotic cell divisions = meiosis 2N -(S)-> 4N -(meiosis I)-> 2N -(meiosis II)-> 1N. Meiosis looks like a sequence of two mitoses but with NO S PERIOD between them. Therefore meiosis produced 4 haploid cells. In sperm development, the cells undergo meiosis and then each of the 4 cells forms a sperm. In egg cell development ("oocyte") the second meiotic division       waits until after fertilization. Therefore, the human egg is diploid just before fertilization, then is effectively triploid for a few hours after fertilization, until meiosis II. In some species of animals (dogs, many others) BOTH meiotic division occurs after fertilization! (so they are briefly pentaploid!!) In meiosis I, the chromosome pairs (somehow!) find each other, line up next to each other, with the copies of each gene next to each other, "synapsis" and randomly cut and reconnect their DNA. "chiasmata" This is "genetic crossing-over". It is still an unsolved problem by what mechanism the chromosome pairs find each other and line up!        (or why they bother.) Prof Patricia Pukkila of the UNC Biology Dept is one of the leading world experts on this. She also runs the undergraduate research program. The evolutionary paradox of sexual reproduction Why go to all this trouble, ? alternating between haploid and diploid, ?? having recombination, ??? combining eggs and sperm, ????having two different sexes Why not just have some regular diploid cells go through the equivalent of embryonic development, and form clones of yourself. The two fold advantage of asexual reproduction! Each individual could produce as many children as if they were a female! Many animals and plants DO have asexual reproduction. Some by budding (aspens, coral, etc.)<BR> Others by development from non-fertilized eggs. Most plants and some animals don't have separate sexes! (& how is that related to non-motility?) The favorite explanation for mixing of genes is that it makes evolution more efficient; If you were to go into research about evolution, this would be one of the main questions to study Even Bacteria have their own kind of recombination discovered by Joshua Lederberg in his Ph.D. research Questions that you need to be able to answer: 1) How many different kinds of autosomes are there in human cells? 2) The numbering of chromosomes is related to the sizes of the chromosomes according to what rules? 3) Would chromosome number 20 be one of the biggest, or one of the smallest, or medium-sized? 4) What is the haploid chromosome number of humans? 5) Is this haploid number the same for all kinds of animals, or what? 6) What is the diploid chromosome number of humans? 7) How many base pairs in the human genome? 8) How many base pairs in the mitochondrial genome of humans and other mammals? 9) About how big is the genome of an average procaryote? **10) Are the relative sizes of human versus bacterial genomes about the same as you would have intuitively expected, or not? **11) Guess about how many base pairs there are in the genomes of yeast, flies, nematode worms, frogs, etc. (guess first; find out later) 12) Although triploidy or tetraploidy is fatal for humans, many (or most) kinds of animals and plants can survive as long as they have some exact multiple of their normal haploid number.

 

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