Biology- DNA and Inheritance

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DNA

Chromosomes are really long molecules of DNA. A DNA molecule has two strands coiled together in the shape of a double helix.

Each of the two DNA strands are made up of small groups called "nucleotides". Each nucleotide contains a small molecule called a "base".

The two strands are held together by four bases: adenine, cytosine, guanine and thymine. Bases are paired, A-T and C-G. This is called base-pairing. Base pairs are joined together by weak hydrogen bonds.

Watson, Crick, Franklin and Wilkins discovered DNA's structure.

Rosalind Franklin and Maurice Wilkins worked out DNA had a helical structure. They directed beams of x-rays onto crystalisised DNA, looked at patterns the x-rays formed when bounced off.

James Watson and Francis Crick used Franklin and Wilkins ideas, and knowledge that the amount of adenine+guanine matched amount of thymine+cystosine, made a model of the DNA molecule where all pieces fitted together.

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DNA

DNA copies itself every time a cell divides (each new cell still has the full amount of DNA).The DNA double helix unzips- forms two single strands. New nucleotides join base-pairing- makes an exact copy of the DNA on the other strand. Result- two double-stranded molecules of DNA (identical to the original DNA molecule).

EXPERIMENT TO EXTRACT DNA:

  • Chop up onion, put into beaker with solution of detergent and salt. Detergent will break down the cell membrane, salt will make the DNA stick together.
  • Put beaker into 60 C water bath for 15 minutes- this denatures enzymes, could digest DNA and helps soften onion cells.
  • Put beaker in ice to cool mixture down- stops DNA breaking down.Once mixture is ice cold, put into blender for a few seconds to break open cell walls and release the DNA.
  • Cool mixture down again, filter for froth, big bits cell out. Add ice-cold alcohol. DNA will start to come out of solution-it's not soluble in cold alcohol. Appears as stringy white substance.
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DNA Fingerprinting and Proteins

DNA fingerprinting is a way of cutting up a person's DNA into small sections and then seperating them. DNA fingerprinting is used in: Forensic Science and Paternity Testing.

Gene -a section of DNA. It contains instructions to make a specific protein. Cells make proteins by stringing amino acids together in a certain order. The order of the bases in a gene tells cells what order the put amino acids together.

DNA determines which genes are on or off- which proteins the cell produces.

A mutation is a change in an organisms's DNA base sequence. This could affect the sequence of amino acids in the protein, which could affect the shape of the protein and so its function.

HARMFUL- A mutation could cause a genetic disorder such as cystic fibrosis.

BENEFICAL- A mutation could produce a new characteristic that is benefical to an organism

NEUTRAL- Some mutations are neither harmful or benefical.

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Making Proteins

Proteins are made by ribosomes. DNA is in the nucleus and cant move as it is really big.

The cell needs to get the information from the DNA to the ribosome in the cells cytoplasm. This is done using a molecule called mRNA- very similar to DNA but it's shorter with one single strand.

mRNA is like a messenger between the DNA in the nucleus and the ribosomes.

  • Two DNA strands unzip. DNA is used as a template to make the mRNA. Base pairing ensures it's complementary (an exact match to the opposite strand). This step is called TRANSCRIPTION.
  • The mRNA molecule moves out of the nucleus and joins with a ribosome.
  • Amino acids that match the mRNA code are brought to the ribosome by the molecules called tRNA.
  • Job of the ribosome is to stick amino acids together in a chain to make a polypeptide. This follows the order of the triple of bases in the mRNA. This step is called TRANSLATION.
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