growth and repaircreation of gametes (sexcells)method of reproduction inunicellular organisms
Binary Fission – type of reproductionthat occurs in bacterial cells, single celled organism splits and becomestwo identical organisms
Chromosomes andDNA
Chromosomes are DNA wrappedaround proteins to form an X-shaped structure.
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The diagram will help you seethe relationship.
1. Chromosomes are found inthe nucleus2. Chromosomes are made of DNA3. Sections of chromosomes are called genes
DNA – deoxyribonucleic acid(it is the genetic code that contains all the information needed to buildand maintain an organism)
ChromosomeStructure
Chromosome Numbers
Each organism has a distinctnumber of chromosomes, in humans, every cell contains 46 chromosomes.Other organisms have different numbers, for instance, a dog has 78 chromosomesper cell.
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Somatic Cells – body cells,such as muscle, skin, blood …etc. These cells contain a complete setof chromosomes (46 in humans) and are called DIPLOID.
Sex Cells – also known as gametes.These cells contain half the number of chromosomes as body cells and arecalled HAPLOID
Chromosomes come in pairs,called Homologous Pairs (or homologs). Imagine homologs as a matchingset, but they are not exacly alike, like a pair of shoes.
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Diploid cells have 23 homologouspairs = total of 46
Haploid cells have 23 chromosomes(that are not paired) = total of 23
HomologousChromosomes
Sex Determination
Chromosomes determine the sexof an offspring. In humans, a pair of chromosomes called SEX CHROMOSOMESdetermine the sex.
If you have XX sex chromosomes- you are female
If you have XY sex chromosomes- you are male
During fertilization, spermcells will either contain an X or a Y chromosome (in addition to 22 otherchromosomes – total of 23). If a sperm containing an X chromosome fertilizesan egg, the offspring will be female. If a sperm cell containing a Y chromosomefertilizes an egg, the offspring will be male.
Creationof a Zygote
When two sex cells, or gametescome together, the resulting fertilized egg is called a ZYGOTE
Zygotes are diploid and havethe total 46 chromosomes (in humans)
Karyotype
A karyotype is a picture ofa person”s (or fetus) chromosomes. A karyotype is often done to determineif the offspring has the correct number of chromosomes. An incorrect numberof chromosomes indicates that the child will have a condition, like DownSyndrome
Compare the Karyotypesbelow
Notice that a person with DownSyndrome has an extra chromosome #21. Instead of a pair, this person has3 chromosomes – a condition called TRISOMY (tri = three)
Trisomy results when chromosomesfail to separate – NONDISJUNCTION – when sex cells are created. The resultingegg or sperm has 24 instead of the normal 23.
Other conditions result fromhaving the wrong number of chromosomes:
Klinefelters Syndrome – XXY(sex chromosomes)
Edward Syndrome – Trisomyof chromosome #13
**Try constructingyour own Karyotype athttp://www.biology.washington.edu/karyotyping/karyotypeSample1.html
