3.4.1 DNA, genes and chromosomes Flashcards

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1
Q

what is a gene?

A

a base sequence of DNA that codes for the amino acid sequence of a polypeptide and a functional RNA

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2
Q

what is a locus?

A

the fixed position of a gene on a particular DNA molecule

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3
Q

what is an allele?

A

one of a number of alternative forms of a gene

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4
Q

what is meant by a homologous pair of chromosomes?

A

they have exactly the same genes but might have different alleles

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5
Q

how is DNA stored in eukaryotes?

A
  • stored as chromosomes inside the nucleus
  • chromosomes are linear in shape
  • DNA is tightly wound around proteins called histones
  • this complex of DNA wrapped around a histone is called a nucleosome
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6
Q

how is DNA stored in prokaryotes?

A
  • DNA molecules are shorter and circular
  • DNA is not associated with a protein
  • instead, DNA is supercoiled to fit in the cell
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7
Q

how is DNA in mitochondria and chloroplasts stored?

A
  • similar to the DNA of prokaryotes
  • short, circular and not associated with proteins
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8
Q

what is a start codon?

A

the three bases (triplet) at the start of every gene which initiates translation

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9
Q

what is a stop codon?

A
  • the three bases (triplet) at the end of every gene that do not code for an amino acid
  • the stop codons mare the end of a polypeptide chain and cause ribosomes to detach
  • this therefore stops translation
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10
Q

what are the three features of the genetic code?

A
  • degenerate
  • universal
  • non-overlapping
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11
Q

what is meant by the genetic code being ‘degenerate’?

A

most amino acids are coded for by more than one triplet of bases

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12
Q

what is the advantage of the genetic code being degenerate?

A

if a point mutation (substitution) occurs, the triplet of bases will be different but it may still code for the same amino acid and therefore will have no effect

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13
Q

what is meant by the genetic code being ‘universal’?

A

the same triplet of bases codes for the same amino acid in all organisms

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14
Q

what is the advantage of the genetic code being universal?

A

it means that genetic engineering is possible (e.g., inserting the human gene for insulin into bacteria)

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15
Q

what is meant by the genetic code being ‘non-overlapping’?

A
  • each base in a gene is only part of one triplet of bases that codes for one amino acid
  • therefore each codon, or triplet of bases, is read as a discrete unit
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16
Q

what is the advantage of the genetic code being non-overlapping?

A

if a point mutation occurs, it will only affect one codon and therefore one amino acid

17
Q

what are introns?

A
  • sections of DNA that do not code for amino acids and therefore polypeptide chains
  • are found in eukaryotic DNA but not prokaryotic NDA
  • these get spliced (removed) out of mRNA molecules
18
Q

what are exons?

A

sections of DNA that do code for amino acids