Edexcel IGCSE Biology 复习笔记 3.2.1 DNA & the Genome

• The entire set of the genetic material of an organism is known as its genome
• In 2003, scientists completed a 13-year project in which they sequenced the genes that make up the whole human genome
  • This project was named the human genome project
  • It helped to develop medical understanding and treatment of genetic diseases as well as facilitating the study of human migration patterns

   

• Each gene within the genome is made up of a small section of DNA that codes for a particular sequence of amino acids
• These sequences of amino acids form different types of proteins
• Alleles are different versions of a gene
  • e.g. there are different alleles that code for brown or blue eye colour
  • Having 2 alleles of the same type is called homozygous
  • Having 2 alleles that are different to one another is called heterozygous

   

• There are many different types of proteins but some examples of these could be:
  • Structural proteins such as collagen found in skin cells
  • Enzymes
  • Hormones

   

• Genes control our characteristics as they code for proteins that play important roles in what our cells do

• DNA, or deoxyribonucleic acid, is the genetic material found in the nucleus of a cell
• DNA, is a polymer made up of two strands coiled around to make a double helix
• The strands are formed from a sugar phosphate backbone with bases joined together by complementary base pairing
  • Adenine pairs with thymine
  • Guanine pairs with cytosine

   

•  Cell division and protein synthesis both rely on these base-pairing rules because each half of the DNA double helix acts as a template to be copied in order to create a complete new double helix

The structure of DNA and complementary base pairs

• In the nucleus of a cell, the DNA double helix supercoils to form structures called chromosomes
• They are only visible during cell division
• Ordinary human body cells contain 23 pairs of chromosomes
  • This is a diploid number (often shown as 2n)
  • One chromosome from a pair is inherited from each parent
  • Each chromosome pair is called a homologous pair

   

• The 23rd pair of chromosomes are the sex chromosomes
  • In females, the sex chromosomes are the same, a homozygous genotype (XX)
  • In males, the sex chromosomes are different, a heterozygous genotype (XY)

   

• Gametes contain just 23 individual chromosomes
  • This is half the full number of a body cell and is called the haploid number (or n)

   

• Genes are found in specific locations on the chromosomes, these locations were identified in the human genome project

Genes are short lengths of DNA that code for a protein. They are found on chromosomes.