DNA and the Genome, KA 1-4

Question 1

Structure of DNA

Answer 1

Chemical bonds between nucleotides create a sugar-phosphate backbone.
Hydrogen bonds between complementary bases hold the two strands of DNA together.
Complementary bases are adenine - thymine, cytosine - guanine.
DNA has a double stranded and antiparallel structure, with deoxyribose and phosphate at 3’ and 5’ ends of each strand respectively, forming a double helix.

Question 2

DNA nucleotides

Answer 2

DNA is made up of subunits called nucleotides.

Each nucleotide contains a deoxyribose sugar, phosphate group and base.

Question 3

Organisation of DNA in prokaryotes

Answer 3

Prokaryotes have a single, circular chromosome and smaller circular plasmids.

Question 4

Organisation of DNA in eukaryotes

Answer 4

Eukaryotes all have linear chromosomes, in the nucleus, which are tightly coiled and packaged with associated proteins called histones.
They also contain circular chromosomes in their mitochondria and chloroplasts.
Yeast is a special example of a eukaryote as it also has plasmids.

Question 5

Replication of DNA

Answer 5

DNA is unwound and hydrogen bonds between bases are broken to form two template strands.
A primer binds to the 3’ end of each template strand.
DNA polymerase adds DNA nucleotides, using complementary base pairing, to the deoxyribose (3’) end of the new DNA strand.
DNA polymerase can only add DNA nucleotides in one direction resulting in the leading strand being replicated continuously and the lagging strand replicated in fragments.
These fragments are joined together by ligase.

Question 6

What is a primer?

Answer 6

A primer is a short strand of nucleotides which binds to the 3’ end of the template DNA strand allowing polymerase to add DNA nucleotides.

Question 7

Polymerase chain reaction (PCR)

Answer 7

DNA is heated to between 92 and 98°C to break hydrogen bonds between bases and separate the strands.
It is then cooled to between 50 and 65°C to allow primers to bind to target sequences.
It is then heated to between 70 and 80°C for heat-tolerant DNA polymerase to replicate the region of DNA.
DNA polymerase must be heat tolerant so it is not denatured by the high temperatures.
Repeated cycles of heating and cooling amplify the region of DNA.

Question 8

Primers in PCR

Answer 8

In PCR, primers are short strands of nucleotides which are complementary to specific target sequences at the two ends of the region of DNA to be amplified.

Question 9

Practical applications of PCR

Answer 9

PCR can amplify DNA to help solve crimes, settle paternity suits, and diagnose genetic disorders.

Question 10

Gene expression

Answer 10

Gene expression involves the transcription and translation of DNA sequences.
Only a fraction of the genes in a cell are expressed.

Question 11

RNA structure

Answer 11

Transcription and translation involves three types of RNA (mRNA, tRNA and rRNA).
RNA is single-stranded and is composed of nucleotides containing ribose sugar, phosphate and one of four bases: cytosine, guanine, adenine and uracil.

Question 12

Messenger RNA (mRNA)

Answer 12

mRNA carries a copy of the DNA code from the nucleus to the ribosome.
mRNA is transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm.
Each triplet of bases on the mRNA molecule is called a codon and codes for a specific amino acid.

Question 13

Transfer RNA (tRNA)

Answer 13

tRNA folds due to complementary base pairing.
A tRNA molecule has an anticodon (an exposed triplet of bases) at one end and an attachment site for a specific amino acid at the other end.
A tRNA anticodon is complementary to an mRNA codon (A-U, C-G).
Each tRNA molecule collects its specific amino acid in the cytoplasm and carries it to the ribosome.

Question 14

Ribosomal RNA (rRNA)

Answer 14

rRNA and proteins form the ribosome.

Question 15

Transcription

Answer 15

RNA polymerase moves along DNA unwinding the double helix and breaking the hydrogen bonds between the bases.
RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides by complementary base pairing.
RNA polymerase can only add nucleotides to the 3’ end of mRNA.

Question 16

RNA splicing

Answer 16

RNA splicing forms a mature mRNA transcript.
The introns of the primary transcript are non-coding regions and are removed.
The exons are coding regions and are joined together to form the mature transcript.
The order of the exons is unchanged during splicing.

Question 17

Alternative RNA splicing

Answer 17

Different proteins can be expressed from one gene, as a result of alternative RNA splicing.
Different mature mRNA transcripts are produced from the same primary transcript depending on which exons are retained.

Question 18

Translation

Answer 18

Translation begins at a start codon and ends at a stop codon.
Anticodons bond to codons by complementary base pairing, translating the genetic code into a sequence of amino acids.
Peptide bonds join the amino acids together to form a growing polypeptide chain.
Each tRNA then leaves the ribosome as the
polypeptide is formed and collects another specific amino acid in the cytoplasm.

Question 19

Final protein structure

Answer 19

Polypeptide chains fold to form the 3D shape of a protein, held together by hydrogen bonds and other interactions between individual amino acids.
Proteins have a large variety of shapes which determines their functions.

Question 20

What determines phenotype?

Answer 20

Phenotype is determined by the proteins produced as the result of gene expression.
It can also be influenced by environmental factors.

Question 21

What is cellular differentiation?

Answer 21

Cellular differentiation is the process by which a cell expresses certain genes to produce proteins characteristic for that type of cell. This allows a cell to carry out specialised functions.

Question 22

Unspecialised cells

Answer 22

Meristems are regions of unspecialised cells in plants that can divide (self-renew) and/or differentiate.
Stem cells are unspecialised cells in animals that can divide (self-renew) and/or differentiate. They are found in embryos and adult tissue.

Question 23

Embryonic stem cells

Answer 23

All the genes in embryonic stem cells can be switched on so these cells can differentiate into all the cell types that make up the organism and this means they are pluripotent.

Question 24

Tissue (adult) stem cells

Answer 24

Tissue stem cells are involved in the growth, repair and renewal of the cells found in that tissue.
Tissue stem cells are multipotent as they can differentiate into all of the types of cell found in a particular tissue type.
For example, blood stem cells located in bone marrow can give rise to all types of blood cell.

Question 25

Therapeutic uses of stem cells

Answer 25

Therapeutic uses involve the repair of damaged or diseased organs or tissues e.g. corneal repair, regeneration of damaged skin.
Stem cells from the embryo can self-renew, under the right conditions, in the lab.

Question 26

Research uses of stem cells

Answer 26

Research uses involve stem cells being used as model cells to study how diseases develop or being used for drug testing.
Stem cell research provides information on how cell processes such as cell growth, differentiation and gene regulation work.

Question 27

Ethics of embryonic stem cells

Answer 27

Use of embryonic stem cells can offer effective treatments for disease and injury; however, it involves destruction of embryos which some people think is murder.