Protein synthesis and DNA

Protein Synthesis and DNA

DNA (deoxyribonucleic acid)

• The genetic material of eukaryotic cells is stored in the nucleus, for example in animal and plant cells

• DNA is found in thread-like structures called chromosomes

• Chromosomes are made from very long molecules of DNA

• A gene is a short section of DNA that controls a feature or trait

• Genes control traits by coding for specific proteins, such as enzymes, hormones, and structural proteins

• Almost every cell contains the same DNA; exceptions include red blood cells (no nucleus) and gametes

• The complete DNA of an organism is called its genome

Structure of DNA

• DNA is a long double helix made of two strands held together by weak hydrogen bonds

• Each strand has a sugar-phosphate backbone with bases projecting outwards

• There are four bases: adenine (A), thymine (T), cytosine (C), and guanine (G)

• Bases pair by complementary base pairing: A with T, and C with G

• The base sequence carries genetic information

Genes and Proteins

• Proteins are made from amino acids

• There are 20 different amino acids used to make proteins

• The type and order of amino acids determines a protein’s shape and function

• The order of bases in DNA determines the order of amino acids

• Three bases code for one amino acid; this triplet is called a codon

• Amino acids join to form polypeptide chains, which fold into functional proteins

Codons and Amino Acids

• Different base sequences produce different proteins

• Changing the base sequence changes the protein made

• One DNA molecule can contain many genes, each coding for a different protein

• Example: genes in skin cells code for melanin, which affects skin colour

RNA

• RNA is similar to DNA but is single stranded

• RNA contains uracil instead of thymine

• RNA can leave the nucleus, unlike DNA

• Two main types are involved in protein synthesis:

• mRNA:

  • It carries a copy of the gene from the nucleus to the ribosome

  • It is linear and looks like a single strand of DNA

  • The sequence of bases can vary molecule to molecule

• tRNA:

  • It carries specific amino acids to the ribosome

  • It is folded into a 3D shape

  • They all have the same sequence of bases apart from in one place, called the anticodon

  • Whihc amino acid a molecule carries depends on the sequence of its anticodon

Protein Synthesis

• A mRNA copy is made of the gene in the DNA and it is called transcription

• The mRNA leaves the nucleus and travels to a ribosome

• The tRNA also arrive at the ribosome carrying amino acids

• The tRNA arrive in the correct order as their anticodons have to match the codons of the mRNA

• The amino acids on the tRNA join to form a polypeptide (protein) chain and this is called translation

• Protein synthesis occurs in two stages: transcription and translation

• These processes allow instructions in DNA to be used to make proteins

Transcription

• Occurs in the nucleus

• DNA unwinds and the two strands separate at the gene

• One strand acts as a template

• mRNA is formed using complementary base pairing

• Base pairing rules during transcription:

  • DNA adenine pairs with RNA uracil

  • DNA thymine pairs with RNA adenine

  • DNA cytosine pairs with RNA guanine

  • DNA guanine pairs with RNA cytosine

• mRNA then leaves the nucleus and moves to a ribosome

• This is called transcription because you are copying the information between 2 similar molecules

Translation

• Occurs at the ribosomes in the cytoplasm

• The ribosomes binds to the first 2 codons of the mRNA

• Two tRNA molecules with the complementary anticodons arrive at the ribosome and bind to the first 2 codons of the mRNA

• The ribosome attaches to the mRNA

• tRNA molecules bring amino acids to the ribosome

• Anticodons on tRNA are complementary to codons on mRNA

• Amino acids are joined together to form a polypeptide chain

• The ribosome moves along the mRNA until the protein is complete

• Their amino acids are joined to the growing proteins

Mutations

• A mutation is a change in the DNA sequence or the amount of DNA

• The sequence of bases in the mRNA will also be different

• A different tRNA molecule could come to the ribosome and this could alter the order of the amino acids so that the wrong protein is made

• Mutations in genes can change codons and amino acid sequences

• This may change the shape and function of a protein

• Frequently mutations occur in an area of the DNA that is not a gene. This will not cause any proteins to be changed. It is known as neutral or silent

• If a mutation occurs in a gene it can change how it works and it will often have a small effect such as losing the ability to roll your tongue.

• However sometimes they could have a significant effect like diseases

• Many mutations are neutral, some are harmful, and a few are beneficial

• Mutations in body cells can lead to cancer

• Mutations in gametes can be inherited

Causes of Mutations

• Mutations occur naturally at low rates

• The rate can be increased by exposure to mutagens

Keywords Glossary