CIE IGCSE Biology: 复习笔记：17.1.5 Which Proteins are Synthesised?
Control of Gene Expression: Extended
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.