CIE IGCSE Biology: 复习笔记：17.1.5 Which Proteins are Synthesised?
- Expression of a gene means whether that gene is transcribed and translated in a particular cell or not
- Most genes are not expressed in a particular cell
- They are 'switched off'
- Because that would be a waste of energy and other resources in the cell
- Only the genes whose proteins are vital to that cell's function are expressed ('switched on')
The triplet code of DNA (carried by mRNA) is read by the ribosome and amino acids are attached together in a specific sequence to form the protein
- In this way, DNA controls cell function by controlling the production of proteins
- The proteins may be enzymes, antibodies, or receptors for neurotransmitters
- Although all body cells in an organism contain the same genes, many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs
Haploid and Diploid Cells
- All humans have 23 different chromosomes in each cell
- In most body cells, not including the gametes (sex cells), we have 2 copies of each chromosome, leading to a total of 46 chromosomes
- Nuclei with two sets of chromosomes are known as diploid nuclei
- The gametes (egg and sperm cells) only have one copy of each chromosome, meaning they have a total of 23 chromosomes in each cell
- Nuclei with one set of unpaired chromosomes are known as haploid nuclei
An easy way to remember the difference between haploid and diploid is to remember:
Haploid = Half the normal number of chromosomes
It’s worth noting that the human diploid chromosome number is 46. In an exam, you may be given information about a different species, with a different number of chromosomes. Make sure you read exam questions carefully.