U2T3 - DNA

Question 1

Describe the structure of DNA nucleotide.

Answer 1

Contains pentose sugar (deoxyribose), phosphate group + inorganic nitrogenous bases (adenine, guanine, cytosine, thymine). Found in nucleus + mitochondria.

Question 2

How do the nitrogenous bases pair up?

Answer 2

Adenine + thymine (2 H bonds), guanine + cytosine (3 H bonds) Relative amount of each is equal (A=T, G=C)

Question 3

How do the nucleotides in DNA join up?

Answer 3

Condensation reaction between sugar of one + phosphate of other creating phosphodiester bond + dinucleotide. If many join, double polynucleotide chain formed in double helix. RNA nucleotides are polymerised to form single polynucleotide chains but RNA has ribose as sugar + is found in cytoplasm.

Question 4

What are the 3 types of RNA?

Answer 4

Messenger, transfer + ribosomal.

Question 5

Give an example of a functional protein composed of more than 1 polynucleotide chain and explain why ‘1 gene, 1 protein’ is not accurate.

Answer 5

Haemoglobin. Some genes need multiple polypeptide chains so ‘1 gene, 1 polypeptide’ more accurate.

Question 6

How does DNA store information?

Answer 6

As linear sequence of bases in 1 strand of DNA. Acts as instructions to synthesise proteins. Spells out amino acids used to make protein + determines primary structure of polypeptide. If genetic code changes, phenotype changes.

Question 7

What are the features of the genetic code? (4)

Answer 7

Triplet code, degenerate code, non-overlapping code + universal code.

Question 8

How does the RNA know when to stop or start copying DNA?

Answer 8

There are 3 codons which don’t code for AAs but are instructions for ribosomes (stop sequences) UGA, UAG, UAA.

Question 9

Where does the production of proteins occur?

Answer 9

Cytoplasm.

Question 10

Describe the interruptions in DNA code?

Answer 10

Exons (coding sequences) are separated by long stretches of non-coding sequences (introns) In eurkaryotic chromosomes, genes contain short lengths of non-coding introns which are edited out during transcription.

Question 11

What does the DNA need to control in a fertilised egg?

Answer 11

Development of zygote into adult, metabolic activities of cell, characteristics of organism.

Question 12

What are the 4 stages of protein synthesis?

Answer 12

Transcription, mRNA carries into to ribosomes, Translation + functional protein production.

Question 13

Why is mRNA important?

Answer 13

Carries coded info from nucleus to ribosomes (site of polypeptide synthesis) so DNA protected from volatile conditions in cytoplasm and is always available for copying.

Question 14

Describe the process of transcription.

Answer 14

Complementary mRNA molecule made alongside spec. part of DNA molecule (gene) which codes for polypeptide. Catalysed by DNA helicase + RNA polymerase. Helicase binds to DNA near start of gene to be transcribed, breaking H bonds between base pairs + copying DNA unwinds. RNA polymerase moves along coding strand + free complementary RNA nucleotides attach to exposed DNA bases. Phosphodiester bonds form by condensation between mRNA nucleotides. When reach stop codon, RNA polymerase detaches + copying done. Releases mRNA + DNA rewinds. This is the ‘Primary Transcript’. Introns edited out of mRNA by enzymes by splicing. 20 base pairs of DNA exposed at once. This process results in series of exons, put together in diff orders to form diff proteins, allowing for flexibility.

Question 15

Describe stage 2 of protein synthesis.

Answer 15

mRNA leaves nucleus via nuclear pore + enters cytoplasm. Moves to ribosomes. In nucleus, bases of 2 DNA strands rejoin + gene protected as bases no longer exposed.

Question 16

Describe the process of translation.

Answer 16

Info in mRNA strand decoded into AA sequence. Ribosome attaches to mRNA + moves along mRNA strand, reading codon info. mRNA + tRNA strands brought close. tRNA anticodon attracted to first codon on mRNA by comp base pairing + held by ribosome. Second tRNA lines up beside second mRNA codon, peptide bond forms between 2 AA’s via condensation. Ribosome moves relative to mRNA. ATP provides energy for this. First P site tRNA released back into cytoplasm. tRNA on right moved from A site to P site, so another AA can attach. Process repeated, forming polypeptide. Synthesis begins at start codon AUG near 5’ end of mRNA. mRNA binds to P site of ribosome first, continues until stop codon. Polypeptide released. mRNA short lived but used num of times before broken down, avoids synthesising excess protein.

Question 17

Describe Stage 4 of protein synthesis.

Answer 17

Completed polypeptide taken in vesicles + processed by RER + Golgi apparatus (fuse with convex face, processed, pinched off from concave face + transported by vesicles for use within cell or secreted) to produce final functional protein. Parts of polypeptide chain may be removed, prosthetic groups added or multiple chains combined.

Question 18

What base sequence is associated with DNA methylation?

Answer 18

CpG sequences.

Question 19

What is the difference between prokaryotic and eukaryotic DNA?

Answer 19

Prokarytic doesn’t have introns. All DNA is coding DNA.

Question 20

Describe the role of ribosomes in translation?

Answer 20

Sub-units assembled in nucleolus + transported into cytoplasm. Link at start as lock onto mRNA strand. Ribosome has 2 sites, each cover codon (like a zip). A + P site. On A site, tRNA molecules link together in correct position on mRNA strand by linking comp anticodons + codons. Adjacent AAs linked by peptide bonds on P site.

Question 21

What do polypeptide chains usually start with?

Answer 21

Methionine

Question 22

Why does reverse transcription not normally happen in cells?

Answer 22

Requires reverse transcriptase which is only found in retroviruses like HIV.

Question 23

Give 1 example of DNA modification that acts to switch off a gene.

Answer 23

Methylation or histone modification.