Topic 4 - DNA, RNA, and Protein Synthesis

Question 1

how is DNA stored in eukaryotic cells?

Answer 1

• eukaryotic cells contain linear DNA molecules that exist as chromosomes
• chromosomes found in nucleus
• DNA molecule is really long
• DNA wrapped around histones
• histone proteins help to support the DNA
• DNA is coiled tightly to make compact chromosomes
• mitochondria and chloroplasts in eukaryotic cells also have their own DNA

Question 2

describe mitochondrial DNA

Answer 2

• circular and shorter than DNA molecules in the nucleus
• not associated with histone proteins

Question 3

describe DNA in prokaryotic cells

Answer 3

• also carry DNA as chromosomes
• DNA is shorter and circular (diff. from eukaryotes)
• DNA isn’t wrapped around histones - condenses to fit in the cell by supercoiling

Question 4

what is a gene?

Answer 4

• sequence of DNA bases that codes for either a polypeptide or functional RNA
• the sequence of amino acids in a polypeptide forms the primary structure of a protein

Question 5

what is the difference between diff. polypeptides?

Answer 5

• have diff. no. and order of amino acids
• order of bases in a gene that determines the order of bases in a gene that determines the order of amino acids in a particular polypeptides
• each amino acid is coded for by a sequence of 3 bases in a gene called a triplet or codon

Question 6

how is a polypeptide made?

Answer 6

• DNA is first copied into messenger RNA (mRNA)
• this is the first stage of protein synthesis

Question 7

what do genes that don’t code for a polypeptide code, code for?

Answer 7

• code for functional RNA instead
• functional RNA are RNA molecules other than mRNA, which perform special tasks during protein synthesis (e.g. tRNA and rRNA), which forms parts of ribosomes

Question 8

what is a genome?

Answer 8

• complete set of genes in a cell

Question 9

what is a proteome?

Answer 9

• full range of proteins that a cell is able to produce

Question 10

what is an intron?

Answer 10

• sections of DNA that don’t code for amino acids
• can be removed via protein synthesis - don’t affect amino acid order
• prokaryotic DNA doesn’t have introns

Question 11

what are exons?

Answer 11

all the bits of DNA that code for amino acids

Question 12

what are non-coding multiple repeats?

Answer 12

• in eukaryotic cells
• DNA sequences that repeat over and over
  (e.g. CCTTCCTTCCTT)
• don’t code for amino acids either

Question 13

what are alleles?

Answer 13

• a gene existing in more than one form
• order of bases in each allele is slightly diff., so, code gor slightly diff. versions of the same polypeptide

Question 14

what are homologous pairs?

Answer 14

pairs of matching chromosomes

Question 15

what is DNA stored as in a eukaryotic cell membrane?

Answer 15

chromosomes

Question 16

describe characteristics of homologous pairs

Answer 16

• both chromosomes are the same size and have same genes (although they could have diff. alleles)
• alleles coding for the same characteristic will be found at the same locus (position) on each chromosome

Question 17

what is protein synthesis?

Answer 17

• production of proteins (polypeptides) from the info. contained within a cell’s DNA
• involves 2 stages: transcription (where DNA code copied into mRNA), and translation (where mRNA joins to ribosome and the code carried is used to synthesise a protein)

Question 18

describe mRNA

Answer 18

• messenger RNA
• made during transcription
• carries genetic code from DNA to ribosomes, where it’s used to make a protein during translation
• single polynucleotide strand
• groups of 3 adjacent bases = codons/triplets/base triplets

Question 19

describe tRNA

Answer 19

• transfer RNA
• involved in translation
• carries the amino acids that are used to make proteins to ribosomes
• single polynucleotide strand that’s folded into a clover shape
  (look at textbook pg. 206)
• hydrogen bonds between specific base pairs hold the molecule in its shape
• every tRNA molecule has a specific sequence of 3 bases at one end = anticodon
• also has amino acid binding site on the other end

Question 20

describe transcription

Answer 20

• This stage of protein synthesis occurs in the nucleus of the cell
  1. Part of a DNA molecule unwinds (the hydrogen bonds between the complementary base pairs break)
  2. This exposes the gene to be transcribed (the gene from which a particular polypeptide will be produced)
  3. A complimentary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
  4. Free activated RNA nucleotides pair up (via hydrogen bonds) with their complementary (now exposed) bases on one strand (the template strand) of the ‘unzipped’ DNA molecule
  5. The sugar-phosphate groups of these RNA nucleotides are then bonded together by the enzyme RNA polymerase to form the sugar-phosphate backbone of the mRNA molecule
  6. When the gene has been transcribed (when the mRNA molecule is complete), the hydrogen bonds between the mRNA and DNA strands break and the double-stranded DNA molecule re-forms
  7. The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

Question 21

describe eukaryotic transcription

Answer 21

• The genome within eukaryotic cells contains many non-coding sections
• Non-coding DNA can be found: between genes, as non-coding multiple repeats and within genes, as introns
  1. During transcription, eukaryotic cells transcribe the whole gene (all introns and exons) to produce pre-mRNA molecules
  (pre-mRNA contains the introns and exons of a certain gene)
  2. Before the pre-mRNA exits the nucleus, splicing occurs:
  The non-coding sections are removed
  The coding sections are joined together
  3. The resulting mRNA molecule carries only the coding sequences (exons) of the gene
  4. mRNA contains only exons and exits the nucleus before joining a ribosome for translation

Question 22

why does splicing not happen in prokaryotes?

Answer 22

because no introns are present

Question 23

describe translation

Answer 23

• This stage of protein synthesis occurs in the cytoplasm of the cell
  1. After leaving the nucleus, the mRNA molecule attaches to a ribosome
  2. In the cytoplasm, there are free molecules of tRNA (transfer RNA)
  These tRNA molecules have a triplet of unpaired bases at one end (known as the anticodon) and a region where a specific amino acid can attach at the other
  3. There are at least 20 different tRNA molecules, each with a specific anticodon and specific amino acid binding site
  4. The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome
  5. The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet (codon) on the mRNA molecule
  6. Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side
  7. A peptide bond is then formed between the two amino acids
  8. The formation of a peptide bond between amino acids requires energy, in the form of ATP
  9. The ATP needed for translation is provided by the mitochondria within the cell
  10. This process continues until a ‘stop’ codon on the mRNA molecule is reached – this acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
  This amino acid chain then forms the final polypeptide

Question 24

describe the features of the genetic code

Answer 24

• non-overlapping - each triplet is read separately from each other (base triplets don’t share their bases)
• degenerate - more possible combinations of triplets than amino acids (20 amino acids but 64 possible triplets), meaning some amino acids are coded for by >1 base triplet
• universal - same specific base triplets code for the same amino acid in all living things
  (e.g. UAU codes for tyrosine in all organisms)