13. DNA and chromosomes GapFill

DNA and chromosomes

The genetic code for life is carried by a molecule called deoxyribonucleic acid, or DNA.

DNA is a very long and complicated molecule. The DNA is found in the   chloroplastscytoplasmnucleusvacuolesapmembrane of cells.

 An eggA chloroplastA chromosomeA nucleusA spermA gene is a long strand of DNA that contains a lot of genetic information.

 A spermA nucleusA nucleotideA chromosomeA geneAn egg is a small section of DNA that controls a single characteristic. Different ones control the development of different characteristics.

The genetic code is passed on to the next generation when organisms reproduce.

Organisms which reproduce sexually produce sex cells called  ciliagametesovarieseuglenasneuronesamoebas. Sex cells contain half the chromosomes found in body cells.

In human beings and other animals, the male sex cells are called sperm and the female sex cells are called eggs or ova.

In plants, the male sex cells are called   antherspollenseedsspermgranulesstamens and the female sex cells are called ovules.

Humans have   244623644232 chromosomes in our body cells, arranged in pairs. We inherit one of each pair from our mother and one from our father.

The 23^rd pair of chromosomes determines whether we are male or female. We call these X and Y chromosomes.

A woman has two X chromosomes, XX, while a man has one of each, XY.

All normal eggs produced by a human ovary have  no sex chromosomean X chromosome and a Y chromosomea Y chromosometwo X chromosomesan X chromosometwo Y chromosomes. In the case of sperm, however, half the sperm carry an X and half carry a Y.

A human baby's gender is determined when an egg is fertilised by a sperm. The baby will be a girl if the fertilising sperm has an X chromosome. The baby will be a boy if the fertilising sperm has a Y chromosome.

The DNA molecule has a shape like a coiling ladder, which we call a  triple spiraltriple helixhelixdouble helixspiraldouble spiral.

The rungs of the ladder are made up of pairs of chemicals called  phosphatesacidsbasesproteinssugarsgenes, connected to each other. It is the order of these pairs that carries the actual genetic code.

There are four different base molecules. Each is usually known by the first letter of its name:

adenine (A), cytosine (C), guanine (G) thymine (T)

The way the bases join up in pairs is fixed. A and T always join together, and C and G always join together.

Each group of three bases carries the genetic code for an amino acid. Amino acids combine together to make  cytoplasmcellsproteinsstarchsugarslipids. The order of bases controls the types of proteins that the cell makes.