Nucleic Acids Flashcards
**Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. **
5 marks
- U: occur once in genome; H: occur many times;
- U: long base sequences; H: short sequences/5–300 bases;
- U: (may be) genes; H: not genes;
- U: (may be) translated/coding sequences; H: never translated;
- U: small differences between individuals; H: can vary greatly;
- U: exons (are unique sequences); H: introns (may be repetitive);
- U: smaller proportion of genome; H: higher proportion of genome;
- satellite DNA is repetitive;
- repetitive sequences are used for profiling;
- prokaryotes do not (usually) contain repetitive sequences;
Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands.
5 marks
- four nucleotides shown in diagram with one nucleotide clearly labelled;
- base, phosphate and deoxyribose (shown as pentagon) connected between the
- correct carbons and labelled at least once;
- backbone labelled as covalent bond between nucleotides correctly shown as 3 to 5 bond;
- two base pairs linked by hydrogen bonds drawn as dotted lines and labelled;
- two H bonds between A and T and three H bonds between C and G;
- adenine to thymine and cytosine to guanine; do not accept initials of bases
- antiparallel orientation shown;
Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together.
8 marks
- subunits are nucleotides
- one base, one deoxyribose and one phosphate in each nucleotide
- description/ diagram showing base linked to deoxyribose C1 and phosphate to C5
- four different bases - adenine, cytosine, guanine and thymine
- nucleotides linked up with sugar-phosphate bonds
covalent/ phosphodiester bonds - two strands (of nucleotides) linked together
base to base - A to T and G to C
- hydrogen bonds between bases
- antiparallel strands
- double helix drawn or described
Outline the structure of the nucleosomes in eukaryotic chromosomes.
4 marks
- contain histones
- eight histone molecules form a cluster in a nucleosome
- DNA strand is wound around the histones
- wound around twice in each nucleosome
- (another) histone molecule holds the nucleosome(s) together
State a role for each of four different named enzymes in DNA replication.
6 marks
helicase- unwinds and splits the strands
gyrase- relieves tortional strain
DNA polymerase III- attaches complimetrary nucleotides
DNA primase- attatches an RNA primer
DNA polymerase I- removes primer and adds complimetrary nucleotide
(DNA) ligase- joins okazaki fragments
Explain the process of DNA replication.
8 marks
- occurs during (S phase of ) interphase/in preparation for mitosis/cell division;
- DNA replication is semi-conservative;
- unwinding of double helix / separation of strands by helicase (at replication origin);
- hydrogen bonds between two strands are broken;
- each strand of parent DNA used as template for synthesis;
- synthesis continuous on leading strand but not continuous on lagging strand;
- leading to formation of Okazaki fragments (on lagging strand);
- synthesis occurs in 5’–> 3’ direction;
- RNA primer synthesized on parent DNA using RNA primase;
- DNA polymerase III adds the nucleotides (to the 3 end)
- added according to complementary base pairing;
adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.) - DNA polymerase I removes the RNA primers and replaces them with DNA;
- DNA ligase joins Okazaki fragments;
as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates
broken off releasing energy to form bond;
Explain how the process of DNA replication depends on the structure of DNA.
9 marks
- DNA molecule is double (stranded)
- hydrogen bonds linking the two strands are weak/ can be broken
- DNA can split into two strands
- split by helicase
- helicase moves progressively down the molecules
backbones are linked by covalent/ strong bonds
strands do not therefore break/ base sequence conserved - reference to semi-conservative replication
- base pairing/ sequences are complementary
A=T and C=G - the two original strands therefore carry the same information
- the two new strands have the same base sequence as the two original ones
- the strands have polarity
- base/ nucleotides added in 5
to 3direction - the two strands have opposite polarity
- discontinuous segments/ Okazaki fragments added to one strand
- DNA ligase needed to connect the segments
Describe the genetic code.
6 marks
- set of rules by which information encoded within mRNA sequences is converted into amino acid sequences (polypeptides) by living cells
- composed of mRNA base triplets
- called codons
- 64 different codons
- each codes for the addition of an amino acid to a growing polypeptide chain
- the genetic code is degenerate
meaning more than one codon can code for a partiuclar amino acid - the genetic code is universal
meaning it is the same in almost all organisms
(AUG is the) start codon - some (nonsense) codons code for the end of translation
**Discuss the relationship between genes and polypeptides. **
5 marks
- originally assumed one gene codes for one polypeptide
- (one) gene is transcribed into (one) mRNA
- mRNA is translated by a ribosome to synthesize a polypeptide
- many exceptions to one gene –> one polypeptide found
- many more proteins made than there are genes
- some genes do not code for polypeptides
- some genes code for tRNA/rRNA
- some genes regulate gene expression
- genetic information transcribed by eukaryotes is edited before it is translated
- polypeptides may be altered before they become fully functional proteins
Explain briefly the advantages and disadvantages of the universality of the genetic code to humans.
4 marks
ADVANTAGES:
- genetic material can be transferred between species/ between humans (genetic modification)
- one species could use a useful gene from another species
- transgenic crop plants/ livestock can be produced
bacteria/ yeasts can be genetically engineered to make a useful product
DISADVANTAGES:
- viruses can invade cells and take over their genetic apparatus
- viruses cause disease
- Compare the processes of DNA replication and transcription.
9 marks
SIMILARITIES:
- both involve unwinding the helix
- both involve spearating the two strands
- both involve breaking hydrogen bonds between bases
- both involve complementary base pairing
- both involve C pairing with G
- both work in a 5 --> 3 direction
- both involve linking/ polymerization of nucleotides
DIFFERENCES:
- replication with DNA nucleotides and transcritpion with RNA nucleotides
- details of ribose/ deoxyribose difference
- adenine pairing with uracil instead of thymine
- only one strand copied not both
- no ligase/ no Okazaki fragments with transcription
- DNA or RNA polymerase
- both require a start signal but this signal is different for each
- transcripiton has only one starting point
but replication has multiple starting points
- replication gives two DNA molecules whilst transcription gives mRNA
Distinguish between RNA and DNA.
3 marks
- DNA is **double-stranded **while RNA is single-stranded;
- DNA contains deoxyribose while RNA contains ribose;
- the base thymine found in DNA is replaced by uracil in RNA;
- one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);
Describe the roles of mRNA, tRNA and ribosomes in translation.
6 marks
- mRNA with genetic code/ codons
- tRNA with anticodon
- tRNA with amino acid attached
ribosome with two sub-units - mRNA held by ribosome
start codon - two tRNA molecules attached with mRNA on ribosome
- peptide bond between amino acids on tRNA
polypeptide forms - continues until a stop codon is reached
polypeptide is released
Outline the structure of tRNA.
5 marks
- tRNA is composed of one chain of (RNA) nucleotides
- tRNA has a position/end/site attaching an amino acid (reject tRNA contains an amino acid)
- at the 3’ terminal / consisting of CCA/ACC
- tRNA has an anticodon
- anticodon of three bases which are not base paired / single stranded / forming part of a loop
- tRNA has double stranded sections formed by base pairing
- double stranded sections can be helical
- tRNA has (three) loops (somethimes with an extra small loop)
- tRNA has a distinctive three dimensional / clover leaf shape
Outline the structure of a ribosome.
4 marks
- small subunit and large subunit;
- mRNA binding site on small subunit;
- three tRNA binding sites / A, P and E tRNA binding sites;
- protein and RNA composition (in both subunits);
