ch7: my genomes! ATAAGCTAGCATACGTACGATC-

Question 1

describe genetic code and define what is meant by the terms degenerate and universal (5)

Answer 1

genetic code determines how mRNA base sequences are expressed as amino acid sequences
based on triplets of bases on mRNA called codons
3 bases (a codon) code 1 amino acid
64 codons

degenerate: 2+ codons code for same amino acids
universal: all living organisms have this code

Question 2

describe the genetic code and its relationship to polypeptides and proteins (5)

Answer 2

genetic code is based on triplets of bases called codons
bases include adenine, guanine, cytosine and thymine in DNA and adenine, guanine, cytosine and uracil in RNA
3 bases (a codon) code 1 amino acid: some codons are start or stop codons

each gene codes for a polypeptide
DNA is transcribed into mRNA by complementary base pairing
mRNA is translated into a polypeptide by complementary base pairing
polypeptides may be joined to form proteins

Question 3

discuss the relationship between one gene and one polypeptide (7)

Answer 3

gene = DNA code for one specific sequence of a polypeptide
DNA is transcribed into mRNA by complementary base pairing
mRNA moves to a ribosome
mRNA is translated into a polypeptide by complementary base pairing

originally it was thought that one gene always codes for one polypeptide
some genes do not code for a polypeptide
some genes code for tRNA and rRNA
some sections of DNA code for regulators

antibody production does not follow this pattern
mutation will affect the primary structure of the polypeptide

Question 4

distinguish between RNA and DNA (3)

Answer 4

google docs

Question 5

explain the structure of a DNA molecule (8)

Answer 5

double helix: 2 strands coiled around
anti-parallel strands
made up of nucleotides
sugar-phosphate backbone: sugar + base + phosphate
complementary base pairing
Adenine=Thymine
Guanine≡Cytosine
google docs

Question 6

outline the structure and functions of nucleosomes (4)

Answer 6

found in eukaryotes
consists of DNA wrapped around histones in a group of 8
held together by another H1 histone in linker region
help to supercoil chromosomes + regulate transcription

Question 7

distinguish between unique and highly repetitive sequences in nuclear DNA (5)

Answer 7

google docs
satellite DNA is repetitive
repetitive sequences are used for profiling
prokaryotes do not (usually) contain repetitive sequences

Question 8

explain the process of DNA replication (8)

Answer 8

DNA replication is semi-conservative
1 new strand + 1 strand from parent molecule

helicase uncoils double helix → separates 2 strands by breaking H bonds between bases

DNA primase adds primer
short length of RNA
primer allows attachment of DNA polymerase III → starts to replicate DNA by adding free nucleotides in the cell in a 5’ – 3’ direction

complementary base pairing
Adenine=Thymine
Guanine≡Cytosine

leading strand: built up continuously towards the replication fork
lagging strand: built up in Okazaki fragments

DNA polymerase I removes RNA primers and replaces them with DNA
DNA ligase seals gaps between Okazaki fragments by making sugar-phosphate bond

2 phosphates are lost to gain the required energy to add nucleotides

Question 9

explain the role of Okazaki fragments in DNA replication (2)

Answer 9

DNA fragments formed on the lagging strand

replication must be in the 5’ –3’ direction → replication starts repeatedly, moves away from replication fork

Question 10

explain the roles of specific enzymes in prokaryotic DNA replication (7)

Answer 10

DNA gyrase: prep DNA for coiling and relieve stress
helicase: unwinds double helix by separating hydrogen bonds between two strands of DNA
DNA primase: adds an RNA primer
DNA polymerase III: synthesises complementary strand
replicate DNA at the primer by adding free nucleotides in the cell in 5’ – 3’ direction
DNA polymerase I: removes RNA primers and replaces them with DNA
DNA ligase: seals gaps between Okazaki fragments by making sugar-phosphate bond
DNA polymerase I/III: proofreads for mistakes

Question 11

explain the significance of complementary base pairing for replication, transcription and translation (8)

Answer 11

DNA: Adenine=Thymine + Guanine≡Cytosine
RNA: Adenine=Uracil + Guanine≡Cytosine

replication:
when DNA replicates the 2 strands separate
each single strand acts as template for base-pair matching
copying the opposite strand of the original DNA molecule
semi-conservative: original order of bases is conserved
identical nucleotide sequence of new complementary strands
new DNA identical to parent molecule
significance: information encoded in one DNA molecule is passed to others

transcription: mRNA sequence complementary to the DNA sequence

translation:
mRNA sequence → a specific polypeptide
tRNA carries specific amino acids
tRNA carries an anticodon (specific triplet of bases) that bind to codons by complementary base pairing
enables conservation of information transfer from DNA to RNA to polypeptide

Question 12

explain the process of transcription leading to the formation of mRNA (8)

Answer 12

RNA polymerase binds to a promoter on the DNA
unwind the DNA strands
bind nucleoside triphosphates to the antisense strand of DNA in a 5’ - 3’ direction

complementary pairing:
Adenine=Uracil
Guanine≡Cytosine
lose 2 phosphates to gain the required energy to add nucleoside triphosphates

until a terminator signal is reached
mRNA detaches from the template
DNA rewinds
RNA polymerase detaches from the DNA
many RNA polymerases can follow each other
introns have to be spliced in eukaryotes to form mature mRNA

Question 13

state what is indicated by the presence of polysomes in a cell (1)

Answer 13

much protein of one type needed

mRNA is being repeatedly translated

Question 14

explain the control of gene expression in eukaryotes (8)

Answer 14

mRNA conveys genetic information from DNA to the ribosomes
gene expression requires the production of specific mRNA
some genes are only expressed at certain times + certain cells

transcription factors: ↑/↓ transcription by preventing/enhancing RNA polymerase binding
nucleosomes limit access of transcription factors to DNA
DNA methylation controls gene expression
introns may contain +ve or -ve gene regulators
chem env of cell can affect gene expression
e.g. auxin alters gene expression to promote cell growth

Question 15

distinguish between transcription and translation (4)

Answer 15

google docs

Question 16

distinguish between the sense and antisense strands of DNA during transcription (1)

Answer 16

only the antisense strand is transcribed to mRNA

sense strand has the same base sequence as mRNA with uracil instead of thymine

Question 17

explain why the process used during protein synthesis in cells is called translation (2)

Answer 17

codon → amino acid
mRNA → polypeptide
genetic code has to be translated

Question 18

explain the process of translation (8)

Answer 18

translation involves initiation, elongation, translocation and termination
ribosome slides along the mRNA in 5’ - 3’ direction to the start codon AUG → code for methionine
tRNA activating enzymes link amino acids to a specific tRNA
ribosome binds tRNA at the P site with the mRNA
anticodon of tRNA pairs with codon on mRNA using complementary base pairing

2nd tRNA binds at the A site
peptide bond forms between amino acids
polypeptide is transferred to the tRNA in the A site

translocation occurs as the ribosome moves down the mRNA
tRNA in A site holding the polypeptide moves to P site
tRNA in P site moves to E site
tRNA in E site without its amino acid detaches
another tRNA moves into A site

proceeds until stop codon is reached
polypeptide is released
mRNA detach from ribosome
subunits of ribosome separate

Question 19

outline the role of ribosomes in translation (4)

Answer 19

translation = production of polypeptides
the ribosome moves along the mRNA
mRNA binds to the ribosome
tRNA binds to the ribosome at the site where its anticodon corresponds to the codon on the mRNA
amino acids are bind by a peptide link in the ribosome

Question 20

outline how the structure of the ribosome is related to its function in translation (6)

Answer 20

translation = polypeptide synthesis
80S in eukaryotes + 70S in prokaryotes
can be free or bound to rER

ribosome is formed by rRNA and proteins → organised into a tertiary structure
small subunit + large subunit
large subunit: 3 binding sites (APE) for tRNA
2 tRNA can bind at the same time
small subunit: binding site for mRNA
rRNA catalyses formation of peptide bond

Question 21

explain how the amino acid was attached to the tRNA (3)

Answer 21

tRNA activating enzymes link amino acids to a specific tRNA
specific enzyme for specific tRNA: recognizes tRNA by its shape
has an end site for attaching an amino acid at the 3’ terminal (CCA)
ATP is needed

Question 22

outline the structure of tRNA (5)

Answer 22

composed of 1 chain of RNA nucleotides
has double stranded sections formed by base pairing
helical: 3 loops → distinctive 3D shape

has an end site attaching an amino acid at the 3’ terminal
has an anticodon of 3 bases which are not base paired

Question 23

explain the role of transfer RNA (tRNA) in the process of translation (3)

Answer 23

tRNA attaches to specific amino acid
tRNA with amino acid moves to the ribosome
anticodon of tRNA pairs with codon on mRNA using complementary base pairing

Question 24

explain the significance of polar and non-polar amino acids (3)

Answer 24

polar: hydrophilic R groups
at surface of proteins → water-soluble
hydrophilic channels @ membranes

non-polar: hydrophobic R groups
at centre of water-soluble proteins → stabilise their structure
proteins remain embedded in membrane

polar & nonpolar amino acids play a role in substrate interactions @ active site

Question 25

outline the four levels of protein structure (6)

Answer 25

primary structure: sequence of amino acids in a polypeptide joined by peptide bonds
secondary structures: beta-pleated sheets and a-helices held together by H-bonds
tertiary structure: folding of the polypeptide into a 3D structure stabilised by ionic bonds
quaternary structure
links 2+ polypeptides to form 1 protein
non-protein groups associated with the polypeptide

Question 26

distinguish between the secondary structure and tertiary structure of proteins (3)

Answer 26

google docs

Question 27

distinguish between globular and fibrous proteins (2)

Answer 27

google docs

Question 28

describe the relationship between genes, polypeptides and enzymes (4)

Answer 28

gene: a sequence of DNA bases
codes for a specific sequence of polypeptide

enzymes: composed of polypeptides
sequence of amino acids determines shape of active site
enzymes are involved in the replication of genes + synthesis of polypeptides