Chapter 7A (2)

• Before learning more about genes, we should recall the basic structure and function of DNA. Deoxyribonucleic acid (DNA) sits inside the nucleus of your cells and is formed by the continuous pairing of base pairs into a longer, double-stranded nucleic acid chain (Figure 1). These base pairs are part of individual nucleic acid molecules known as nucleotide monomers, each of which consists of the same basic structure (Figure 1a): • a phosphate group • a deoxyribose sugar • a nitrogen-containing base, which can be one of four varieties: − adenine (A) − thymine (T) − guanine (G) − cytosine (C)
• Gene


  A particular section of DNA that codes for the creation of an individual polypeptide chain
• Gene


  • Acts like a set of instructions which a cell will then read to create the important protein molecules needed for a range of cellular functions, such as growth and regeneration
  • The order of bases in the double helix determines which protein gets made
• Sequence of bases


  • ATG
  • GGG
• Genome


  The collection of genes inside each of your cells, which houses all of the genetic information needed to build and maintain a complex organism
• Each person has two copies of each gene, one inherited from their mother and the other from their father
• There are roughly 25 000 different genes inside each of your cells
• Alleles


  Different forms of the same gene but with small differences in their base sequence
• Almost all genes are the same across every human being
• A small number of genes (<1 per cent) are slightly different between people, which contributes to the immense differences we see from person to person
• Eye colour


  • Green eyes
  • Brown eyes
• Differences in eye colour

  Caused by differences in the alleles you each possess for the genes involved with eye colour
• Allele variations


  • More than two possible alleles, e.g. green, brown, blue, grey eyes
• Gene locus


  Location of a gene on a chromosome
• Alleles


  Two alleles present at each gene locus, one inherited from each parent
• Phenotype


  The observable characteristics of an individual, e.g. eye colour
• Alleles interact with each other

  Determine which allele/s is dominant and expressed in the phenotype
• An organism’s genome sits inside the nucleus of each of their somatic cells in the form of DNA, which is then wrapped around proteins to form chromosomes. Chromosomes are arranged into homologous pairs based on the specific genes they possess. Because chromosomes are more condensed and easily visualised than DNA, biologists can analyse them using karyotypes
• What is a chromosome? As mentioned, there are roughly 25 000 different genes inside each of your cells, composed of around three billion individual base pairs in total. This genetic information, while immense, is not stored randomly inside the nucleus. Instead, each molecule of DNA is coiled tightly around histone proteins and packaged into thread-like structures called chromosomes. Human somatic cells mostly contain 46 chromosomes and therefore have a diploid number of 2n = 46.
• Telomeres


  A region of repetitive base sequences that is found at the end of every chromosome. It is used to protect the ends of chromosomes from fusing with other nearby chromosomes in the nucleus.
• DNA molecule


  Each chromosome is composed of a long DNA molecule that has been coiled tightly around histone proteins.
• Centromere


  A specialised sequence of DNA that holds together the two chromatids. It is very important for the process of meiosis, which you will learn about in lesson 7B.
• Sister chromatids


  The identical daughter strands of a replicated chromosome.
• Short arm


  Also known as the 'p arm' – this is the section of the chromosome that is shorter in length.
• Long arm


  Also known as the 'q arm' – this is the section of the chromosome that is longer in length.
• We call each of the sets of 23 chromosomes within a human nucleus (one set inherited from your dad and the other from your mum) a pair of homologous chromosomes (Figure 9). The criteria for homologous chromosomes are as follows: 1 they are the same in size and length 2 they have the same centromere position 3 they share the same genes at the same gene loci.
• Chromosomes


  Sets of chromosomes that are paired and identified together as they contain the same genes
• Chromosomes


  • Not identical in base sequence
  • Contain the same genes
• OCA2


  Gene most associated with eye colour, located on chromosome 15
• MC1R


  Gene most associated with hair colour, located on chromosome 16
• Each gene shares the same locus on each of the two homologues, allowing scientists to identify the exact location and base sequence for each gene of our genome
• Karyotypes


  Visualising chromosomes by arranging them into a karyotype or karyogram
• Karyotypes


  • Used to check for possible genetic abnormalities
  • Scientists check that the correct number of chromosomes are present
  • Scientists check that the size and length of each chromosome are correct
• As you can see, homologous pairs are arranged according to size. The pairs labelled 1–22 in Figure 11 are known as autosomes, each containing the genetic information for many thousands of genes. You will also notice that the final set of chromosomes (23) are labelled X or Y. These are known as sex chromosomes, and are responsible for determining the sex of an organism. In humans, two X chromosomes result in a female, whereas one X and one Y chromosome will result in a male.
• Aneuploidy


  A chromosomal abnormality in which an organism possesses an incorrect number of total chromosomes caused by the addition or loss of an individual chromosome
• In humans, aneuploidy would mean having more or less than the usual number of 46 chromosomes
• Types of aneuploidy


  • Monosomy (2n-1)
  • Trisomy (2n+1)
  • Tetrasomy (2n+2)
• Monosomy


  An organism has one missing chromosome
• Trisomy


  An organism has one extra chromosome
• Tetrasomy


  An organism has two extra chromosomes