Biology Gene Polypeptide steps

Steps that show how one gene codes for more than one polypeptide

Step 1

Helicase unwinds and separates the double strand of DNA at the replication fork, by breaking weak hydrogen bonds, separating the 2 DNA strands. 

Step 2

RNA polymerase binds to the exposed template strand from the promoter and reads the gene in the 3’ to 5’ direction. 

Step 3

RNA nucleotides are synthesised to make a single strand of an mRNA copy of the template strand, synthesized in the 5’ to 3’ direction ONLY. The adenine bonds to uracil and cytosine binds to guanine. 

Step 4

The two strands of DNA re-join while the RNA polymerase continues. The polymerase proofreads for bases that aren’t complementary and removes them if found. 

Step 5

mRNA detaches from the template the template once it reaches the termination point. 

Step 6

DNA removes and replaces the primers and links the daughter strands into a continuous molecule which gives two identical DNA molecules that are made up of 1 parent and 1 daughter strand. The pre-mRNA synthesis is now complete. 

Step 7

A modified form of guanosine triphosphate is added to the 5’ end called the 5’ cap which prevents tells the ribosome where to begin translation.

Step 8

A tail of 30 to 200 adenine molecules are added to the 3’ end called a poly(A) trail which prevents the mRNA from being broken down. Having the 5’ and 3’ ends allow the cell to detect whether the full mRNA message exists. 

Step 9

RNA splicing begins to convert the pre-mRNA into mRNA. Molecules that remove a region which do not code for proteins known as introns must be removed before translation, and keep the region which does, known as exons. These molecules are called spliceosomes and are made of small nuclear ribonucleoproteins (or “snRNPs”). These snRNPs bind to the RNA transcript (pre-mRNA) along with other proteins and cause the intron to fold into a loop. 

Step 10

The intron forms a complete loop and detach from the exons which join to give mature mRNA (which will be transported to the cytoplasm), spliceosome components and excised intron (which will be recycled by the cell for energy or materials). 

Step 11

The mature mRNA is exported to the cytoplasm through the nuclear pores. 

Step 12

In the cytoplasm, the small ribosomal subunit binds to the mRNA at the 5’ cap, and the tRNA, which contains methionine as an amino acid. 

Step 13

The small ribosomal subunit and the tRNA move together towards the 3’ end to bind to the start codon of mRNA (generally AUG). 

Step 14

The large ribosomal subunit binds to the tRNA at the P site to complete the initiation complex.  

Step 15

A different tRNA molecule (at the A site of the large ribosomal subunit) pairs with the appropriate codon on the mRNA. 

Step 16

The two amino acids are linked and the amino acids at P site are broken off moves over so that all amino acids from P site are now at the A site 

Step 17

The tRNA in the P site and the A site move left, so that A moves to P and P moves to E where the E site tRNA is exited from the ribosome to be recycled. 

Step 18

The tRNA molecule detaches from the mRNA molecule and the mRNA molecule is moved down the ribosome.  

Step 19

Steps 15 – 18 are repeated until the entire mRNA molecule is one polypeptide molecule. A stop codon (generally either UAA, UAG, UGA) arrives at the P site and this causes a release factor to bind to the A site and make the entire complex fall apart. 

Step 20

The new protein is released, and it folds into its 3D shape and gets to work in the cell. This gene is now expressed!