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)



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.

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


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)


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 at