Revision for all of my tests on Friday

Homeostasis, genes and inheritance and evolution

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  • Created by: Elizabeth
  • Created on: 03-11-10 21:39

Genes and Inheritance

Chromosomes are found in the nucleus of most cells. They consist of long strands called DNA (deoxyribonucleic acid) A section of DNA has a genetic code that makes particular proteins is called a gene. These proteins could be enzymes or structural proteins such as the ones in our muscles and hair. The human body cells each contain 23 pairs of chromosomes. The parents pass on their genes to their children using their sex cells.A pair of chromosomes carry the same genes in the same place on the chromosome. There are different versions of a gene are called alleles. The alleles could be the same on each pair of chromosomes or they could be different, depending on the parents for example; one parent having blue eyes and one parent having brown eyes.

Sex cells only contain 23 chromosomes, unlike other cells which contain 46. This is because one chromosome is from the mother and one chromosome is from the father. The chromosomes that we get are random. This means that different children in the same family will get different combinations. This is why children and parents tend to look a little like their siblings and parents, although they are not identical. For female chromosomes, they look like ** and for male XY. The sex chromosome is always the last one (23)

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Determination of Gender

When sex cells form, the pairs of sex chromosomes are seperated (X   Y     X    X). All normal egg cells produced by a human ovary have an X chromosome. This also means that half of the sperm carry an X chromosome and half a Y. So a baby's gender is determined by the sperm that fertilises the egg. So if the X chromosome sperm fertilises the egg, the baby will be a girl. If the Y chromosome sperm fertilises the egg, the baby will be a boy. 

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Homoeostasis means keeping a constant internal environment. Homoeostasis is carried out throughout the whole body. If we did not have homoeostasis, our enzymes will not work properly meaning that we would die. The main things that have to controlled are, Carbon dioxide (If it is not removed, the body becomes too acidic and most of it is lost by breathing out and some by urination). 

Urea (a waste chemical which is produced when we digest amino acids in the liver. This is poisonous, mostly it is removed through sweating and urination.) 

Ions (if not kept at the right balance, our cells can become shrivelled, swollen or burst. Ions include sodium, potassium, hydrogen and phosphate. we lose some of these through our faeces and our sweat.) 

Sugar (having enough glucose for respiration plus a good store of glycogen is critical. If our blood glucose level falls too low, we die.)

Water (70% of our body mass is water. If we do not keep the right amount of water, we could die. The kidney contributes in this)



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Temperature (Our normal body temperature should be 37 degrees centigrade. If our bodies get too hot or too cold we would die and so would our enzymes. A way to remember these six things is to see this:

When (water)

Shall (Sugar)

I (ions)

Clean (Carbon dioxide)

The (Temperature)

Utensils (Urea)

Pituitary gland

Because of the pituitary glands, the brain has a huge effect on the controlling of the body involving hormones. 

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I worked really hard on that!!

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