DNA Genetics Question 1 Most of the DNA of a human cell is contained in the nucleus. Distinguish - PowerPoint Presentation

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DNA

Genetics

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Question 1

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

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Question 1 - Answers

U

=Unique sequences; 

H

= Highly repetitive sequences

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;

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Question 2

Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands.

 

5 marks

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Question 2 - Answers

Award 1 for each of these structures clearly drawn and labelled.

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;

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Question 3

Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together.

 

8 marks

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Question 3 - Answers

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

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Question 4

Outline the structure of the nucleosomes in eukaryotic chromosomes.

 

4 marks

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Question 4 - Answers

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

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Question 5

State a role for each of four different named enzymes in DNA replication.

 

6 marks

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Question 5 - Answers

Award 1 mark for any two of the following up to 2 marks maximum.

helicase

DNA polymerase / DNA polymerase III

RNA primase

DNA polymerase I

(DNA) ligase

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Question 5 - Answers

Award 1 mark for one function for each of the named

enzymes.helicase

splits/ breaks hydrogen bonds/ uncoils DNA/ unwinds DNA

(DNA) polymerase III

adds nucleotides (in 5' to 3' direction) extending existing strand

(RNA) primase

synthesizes a short RNA primer (which is later removed) on DNA

(DNA) polymerase I

replaces RNA primer with DNA

(DNA) ligase

joins Okazaki fragments/ fragments on lagging strand/ makes sugar-phosphate bonds between fragments

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Question 6

Explain the process of DNA replication.

 

8 marks

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Question 6 - Answers

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 3direction;

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;

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Question 7

Explain how the process of DNA replication depends on the structure of DNA.

 

9 marks

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Question 7 - Answers

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 3` direction

the two strands have opposite polarity

discontinuous segments/ Okazaki fragments added to one strand

DNA ligase needed to connect the segments

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Question 8

Describe the genetic code.

 

6 marks

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Question 8 - Answers

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

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Question 9

Discuss the relationship between genes and polypeptides.

 

5 marks

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Question 9 - Answers

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

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Question 10

Explain briefly the advantages and disadvantages of the universality of the genetic code to humans.

 

4 marks

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Question 10 - Answers

genetic material can be transferred between species/ between humans

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

viruses can invade cells and take over their genetic apparatus

viruses cause disease

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Question 11

Compare the processes of DNA replication and transcription.

 

9 marks

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Question 11 - Answers

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

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

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Question 12

Distinguish between RNA and DNA.

 

3 marks

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Question 12 - Answers

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

);

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Question 13

Describe the roles of mRNA,

tRNA

and ribosomes in translation.

 

6 marks

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Question 13 - Answers

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

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Question 14

Outline the structure of

tRNA

.

 

5 marks

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Question 14 - Answers

Accept any of the points above if clearly explained using a suitably labelled diagram

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

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Question 15

Outline the structure of a ribosome.

 

4 marks

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Question 15 - Answers

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);

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Question 16

Explain the process of translation.

 

9 marks

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Question 16 - Answers

translation involves initiation, elongation/translocation and termination;

mRNA binds to the small sub-unit of the ribosome;

ribosome slides along mRNA to the start codon;

anticodon of

tRNA

pairs with codon on mRNA:

complementary base pairing (between codon and anticodon);

(anticodon of)

tRNA

with methionine pairs with start codon / AUG is the start codon;

second

tRNA

pairs with next codon;

peptide bond forms between amino acids;

ribosome moves along the mRNA by one codon;

movement in 5 to 3 direction;

tRNA

that has lost its amino acid detaches;

another

tRNA

pairs with the next codon/moves into A site;

tRNA

activating enzymes;

link amino acids to specific

tRNA

;

stop codon (eventually) reached;

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Question 17

Compare DNA transcription with translation.

 

4 marks

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Question 17 - Answers

both in 5` to 3` direction

both require ATP

DNA is transcribed and mRNA is translated

transcription produces RNA and translation produces polypeptides/ protein

RNA polymerase for transcription and ribosomes for translation/ ribosomes in translation only

transcription in the nucleus (of eukaryotes) and translation in the cytoplasm/ at ER

tRNA

needed for translation but not transcription