17.2) Chromosomes, genes and proteins
Chromosome is a thread of DNA, made up of a string of genes.
Genes is a length of DNA that codes for a protein.
Allele is a version of a gene.
A human body (somatic) cell nucleus contains 23 pairs of chromosomes.
These are difficult to distinguish when packed inside the nucleus, so scientists separate them and arranged them according to size and appearance. The outcome is called karyotype.
One of these pairs controls the inheritance of biological gender – whether offspring are male or female:
- males have two different sex chromosomes, X and Y
- females have two X chromosomes, XX
The ratio of female to male offspring is 1:1 – on average, half of the offspring will be girls and half will be boys
The genetic code:
- Each nucleotide carries one of four bases (A, T, C or G). a string of nucleotides therefore holds a sequence of bases. This sequence forms a code, which instructs the cell to make particular proteins.
- Proteins are made from amino acids linked together. The type and sequence of the amino acids joined together will determine the kind of protein formed.
- Its is the sequence of bases in the DNA molecule that decides which amino acids are used and in which order they are joined. Each group of three bases stands for one amino acid.
- A gene, then, is a sequence of triplets of the four bases, which specifies an entire protein.
- The chemical reactions that take place in a cell determine what sort of a cell it is and what its functions are. These chemical reactions are, in turn, controlled by enzymes.
- Enzymes are proteins. It follows, therefore, that the genetic code of DNA, in determining which proteins, particularly enzymes, are produced in a cell, also determines the cell’s structure and function. In this way, the genes also determine the structure and function of the whole organism.
- Other proteins coded for in DNA include antibodies and the receptors for neurotransmitters.
The manufacture of proteins in cells:
- DNA molecules remain in the nucleus, but the proteins they carry the codes for are needed elsewhere in the cell. A molecule called messenger RNA (mRNA) is used to transfer the information from the nucleus.
- mRNA is much smaller than a DNA molecule and is made up of only one strand. Also it contains slightly different bases (A,C,G and U). Base U is uracil.
- To pass on the protein code, the double helix of DNA unwinds to expose the chains of bases.
- One strand acts as template. A messenger RNA molecule is formed along part of this strand, made up of a chain of nucleotides with complementary bases to a section of the DNA strand.
- The mRNA molecule carrying the protein code then passes out of the nucleus, through a nuclear pore in the membrane. Once in the cytoplasm it attaches itself to a ribosome.
- Ribosomes make proteins. The mRNA molecule instructs the ribosomes to put together a chain of amino acids in a specific sequence, thus making a protein.
- Body cells do not all have the same requirements for proteins. For example, the function of some cells in the stomach is to make the protein pepsin. Bone marrow cells make the protein haemoglobin, but do not need digestive enzymes.
- Specialised cells all contain the same genes in their nuclei, but only the genes needed to code for the specific proteins are switched on (expressed). This enables the cell to make only the proteins it needs to fulfil its function.
Number of chromosomes:
Haploid nucleus: is a nucleus containing a single set of unpaired chromosomes present, for example, in sperm and egg cells.
Diploid nucleus: is a nucleus containing two sets of chromosomes present, for example, in body cells.
In a diploid cell, there is a pair of each type of chromosome and in a human diploid cell there are 23 pairs.