Genes and DNA UNIT 4 AOS 1

Question 1

what is the structure of DNA?

Answer 1

DNA is double stranded, running antiparallel meaning they run 5’-3’ and 3’-5’, the two strands are joined together by hydrogen bonds. The two ends are referred to as 5’ and 3’.

Question 2

what is DNA replication?

Answer 2

the synthesis of DNA on a DNA template to produce two identical copies

Question 3

outline the steps in DNA replication?

Answer 3

1. DNA helicase unwinds the DNA and breaks the hydrogen bonds between bases
2. RNA primase helps RNA nucleotides bind to the 5’ end of the 3’ to 5’ strand
3. DNA polymerase binds to the primer on the leading strands and adds nucleotides in the direction of 5’ to 3’. Another DNA polymerase binds to the primer on the other strand (lagging strand) and adds nucleotides 5’ to 3’.
4. The lagging strand only makes small fragments. At the end, the primers are removes and DNA polymerase fills in the gaps with extra nucleotides
5. DNA ligase winds the new strand up to form a double helix, joining the nucleotides together with phosphodiester bonds

Question 4

outline the steps in transcription?

Answer 4

1. DNA is read in the nucleus and an mRNA copy is made
2. RNA polymerase unzips DNA prior to gene transcription
3. RNA polymerase binds to transcription factors in the promotor region
4. RNA polymerase moves along the start region and copies the coding region to make mRNA
5. RNA polymerase assembles free nucleotides to the 3/ end so the mRNA grows from 5’ to 3’ as it moves along the DNA template
6. When RNA polymerase reaches the stop region, it unbinds itself and releases the mRNA

Question 5

what is alternative splicing?

Answer 5

the pre-mRNA has exons (coding parts) and introns (non-coding parts). Introns are removes via alternative splicing, making the final mRNA copy. A ‘poly A’ tail is also added. The mature mRNA now leaves the nucleus to be translated in the cytoplasm.

Question 6

outline the steps in translation?

Answer 6

1. Ribosomal sub-units attach to the mRNA in the cytoplasm or RER
2. A tRNA brings the corresponding anticodon to the start codon
3. The ribosome moves along the mRNA 5’ to 3’ and each time a new codon is exposed and new amino acid is made. When the corresponding tRNA brings in a new amino acid, the previous tRNA leaves
4. The amino acid joins via peptide bonds and forms a polypeptide chain until the stop codon is reached.