3.1.5 Nucleic Acids Are Important Information-Carrying Molecules Flashcards
Draw the structure of a nucleotide.
Name the pentose sugars in DNA and RNA.
DNA: deoxyribose
RNA: Ribose
State the role of DNA in living cells.
Base sequence of genes codes for functional RNA and amino acids sequence of poly peptides.
Genetic information determines inherited characteristics = influences structure and function of organisms.
State the role of RNA in living cells.
mRNA: Complementary sequence to 1 gene from DNA with introns (non-coding regions) spliced out. Codons can be translated into a polypeptide by ribosomes.
rRNA: component of ribosomes (along with proteins).
tRNA: supplies complementary amino acid to mRNA codons during translation.
How do polynucleotides form?
Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone).
Describe the structure of DNA.
Double helix of 2 polynucleotide strands.
Hydrogen bonds between complementary purine and pyrimidine bade pairs on opposite strands:
adenine + thymine
guanine + cytosine
Which bases are purine and which are pyrimidine?
Adenine and guanine = 2-ring purine bases
Thymine, cytosine and uracil = 1-ring pyrimidine bases
Name the complementary base pairs in DNA.
2 hydrogen bonds between adenine and thymine
3 hydrogen bonds between guanine and cytosine
Name the complementary base pairs in RNA.
2 hydrogen bonds between adenine and uracil
3 hydrogen bonds between guanine and cytosine
Relate the structure of DNA to its functions
- Sugar-phosphate backbone and many hydrogen bonds provide stability
- long molecule stores lots of information
helix is compact for storage in nucleus - base sequence of triplets codes for amino acids
- double-stranded for semi-conservative replication
- complementary base pairing for accurate replication
- weak hydrogen bonds break so strands separate for replication
Describe the structure of messenger RNA (mRNA).
- Long ribose polynucleotide (but shorter than DNA)
- Contains uracil instead of thymine.
- Single-stranded and linear (no complementary base pairing).
- Codon sequence is complementary to exons of 1 gene from 1 DNA strand
Relate the structure of messenger RNA (mRNA) to its functions.
Shorter than DNA: Breaks down quickly so no excess polypeptide forms.
Single stranded and linear: ribosome can move along strand and tRNA can bind to exposed bases.
Codon sequence is complementary to exons of 1 gene from 1 DNA strand: can be translated into a specific polypeptide by ribosomes.
Describe the structure of transfer RNA (tRNA).
- Single strand of about 80 nucleotide.
- Folded into clover shape (some paired bases).
- Anticodon on one end, amino acid binding site on the other:
a) anticodon binds to complementary mRNA codon
b) amino code corresponds and anticodon
Order DNA, mRNA and tRNA according to increasing length.
tRNA, mRNA, DNA
Why did scientists initially doubt that DNA carried the genetic code?
Chemically simple molecule with few components.
Why is DNA replication described as semiconservative?
- Strands from original DNA molecule act as a template.
- New DNA molecule contains 1 old strand and 1 new strand.
Outline the process of semiconservative DNA relication.
- DNA helicase breaks hydrogen bons between base pairs.
- Each strand acts as a template.
- Free nucleotides attach to exposed bases by complementary base pairing.
- DNA polymerase catalyses condensation reactions that join adjacent nucleotides on new strand.
- Hydrogen bonds reform.
Describe the Meselson-Stahl experiment.
- Bacteria were grown in a medium containing heavy isotope ¹⁵N for many generations.
- Some bacteria were moved to a medium containing light isotope ¹⁴N. Samples were extracted after 1 and 2 cycles of DNA replication.
3 Centrifugation formed a pellet. Heavier DNA (bases made from ¹⁵N) settled closer to the bottom of the tube.
Explain how the Meselson-Stahl experiment validated semiconservative replication.
After 1 division: all molecules have 1 strand ¹⁵N and 1 strand ¹⁴N.
After 2 divisions: 50% have 2 strands ¹⁴N and 50% have 1 strand ¹⁵N and 1 strand ¹⁴N.
Free nucleotides have ¹⁴N so new strand with have ¹⁴N.
