Topic 4: Genetic Information, Variation and relationships between organisms

Question 1

What is DNA like in Prokaryotic cells and Eukaryotic cells?

Answer 1

Prokaryotic–> Short, circular and not associated with protein (Prokaryotes do not form chromosomes)

Eukaryotic–> Very long, linear and associated with proteins called ‘Histones’
(together, a DNA molecule and its associated proteins form a chromosome)

–>The mitochondria and Chloroplasts of eukaryotic cells also contain DNA, which is short, circular, not associated with protein and does not contain Introns
(because they were prokaryotes)

Question 2

What is a ‘Gene’?

Answer 2

A gene is a sequence of DNA bases that codes for the specific amino acid sequence of a polypeptide
(therefore determining the nature and development of organisms)

Question 3

What are ‘Homologous Chromosomes’?

Answer 3

–>A Homologous pair of chromosomes carries the same genes in the same Loci (location), but not necessarily the same alleles

–>It consists of a maternal and a paternal chromosome

Question 4

What is an ‘Allele’ and how do they arise?

Answer 4

An Allele is a different version of the same gene that codes for different types of the same characteristic

[e.g. the alleles B (brown eyes) and b (blue eyes) are different forms of the gene for eye colour]

–> Alleles arise due to mutations of a gene

Question 5

What is the ‘Genetic Code’?

Answer 5

The Genetic Code is a sequence of 3 nucleotide bases (a base triplet) which codes for a specific amino acid
(There are base triplets in DNA and mRNA)

–> The presence of 4 different nucleotides in DNA and RNA results in 64 (4x4x4) different DNA triplets or mRNA codons being produced to code for the 20 commonly occurring amino acids in living organisms

Question 6

What are the base triplets in mRNA called?

Answer 6

Codons

Question 7

What does it mean if the Triplet Code is ‘Degenerate’?

Answer 7

Different triplets (or codons) coding for the same amino acid

Question 8

What is a ‘Stop Code’?

Answer 8

Some triplet codes are called ‘Stop Codes’, they do not code for an amino acid but indicate the end of the code for a specific polypeptide

Question 9

What does it mean by the code being
‘Non-Overlapping’?

Answer 9

Each base is only read once as part of a specific triplet

Question 10

What does it mean by the code being ‘Universal’?

Answer 10

The same triplets code for the same amino acids in all organisms

Question 11

What is ‘ATG’?

Answer 11

ATG is the start code of all sequences coding for a polypeptide

Question 12

What is ‘Non-coding’ DNA and what are some examples?

Answer 12

In Eukaryotes, much of the DNA does not code for polypeptides (non-coding DNA), including:

-Introns–> sections of the gene that do not code for amino acids (the ones that do are known as ‘Exons’)

-Multiple Repeats–> these are some of the base sequences present between genes, often occurring of the same base sequences occurring again and again

Question 13

What is the structure of ‘RNA’?

Answer 13

RNA is a relatively short polynucleotide molecule, and it differs from DNA in that:

–> the pentose is Ribose (instead of deoxyribose)

–> the organic base Uracil replaces thymine
(so the 4 bases found in RNA are Adenine, Guanine, Cytosine, and Uracil)

–> mRNA and tRNA are single-stranded (in comparison to double-stranded DNA)

Question 14

What is the structure of mRNA
(messenger RNA)?

Answer 14

It has a linear structure and contains Codons (=mRNA base triplets)

Question 15

What is the structure of tRNA
(transfer RNA)?

Answer 15

–>Each molecule consists of a ‘clover leaf’ shape due to the presence of hydrogen bonds between complementary base pairs

–>At one end of the molecule, there is an ‘Anticodon’ (3 unattached tRNA nucleotide bases) and at the other end, a binding site for the attachment of a specific amino acid

(there are at least 20 different types found in the cytoplasm.)

Question 16

What is the ‘Genome’?

Answer 16

The genome is the complete set of genes (and the non-coding DNA) in a cell

Question 17

What is the ‘Proteome’?

Answer 17

The proteome is the full range of proteins that a cell is able to produce

Question 18

What are the 2 stages of Protein Synthesis?

Answer 18

1) Transcription–> occurs in the nucleus, involves ‘rewriting’ (transcribing) part of the DNA code into a strand of messenger RNA

2) Translation–> occurs in the cytoplasm, involves ribosomes synthesising proteins using the information provided by messenger RNA

Question 19

How is ‘Transcription’ carried out?

Answer 19

[occurs in the nucleus, involves ‘rewriting’ (transcribing) part of the DNA code into a strand of messenger RNA]

1) The relevant section of the DNA molecule uncoils, and the 2 strands are separated as the Hydrogen bonds are broken

2) One of these 2 strands acts as the template

3) Individual RNA nucleotides line up alongside the DNA nucleotide bases on the template strand by complementary base pairing

4) Uracil (RNA) pairs with Adenine (DNA)

5) The individual RNA nucleotides are then joined together with phosphodiester bonds in reactions catalysed by RNA Polymerase

6) The 2 strands of the DNA in the nucleus will join back together one mRNA has been produced

Question 20

How is ‘Splicing’ of mRNA carried out?

Answer 20

[mRNA formed during transcription using DNA containing both exons and introns is known as ‘Pre mRNA’]

–>Splicing of pre-mRNA involves the removal of introns and the joining together of exons to form mRNA

Note: Splicing only occurs in eukaryotes, not prokaryotes (as they do NOT contain introns)

Question 21

How is ‘Translation’ carried out?

Answer 21

[occurs in the cytoplasm, involves ribosomes synthesising proteins using the information provided by messenger RNA]

1) mRNA molecules bind to a ribosome in the cytoplasm

2) A codon in mRNA is complementary to an anticodon in tRNA, so they bind

3) Amino acid on the first tRNA molecule binds to the amino acid in the second tRNA molecule by a peptide bond via a condensation reaction (requires hydrolysis of ATP and an enzyme that is part of the ribosome)

4) The first tRNA molecule then leaves and goes into the cytoplasm to collect another molecule of the same amino acid from the ‘amino acid pool’

5) The ribosome moves along the mRNA, and other tRNA molecules then join in the order determined by the codons on the mRNA strand

6) This process continues along the mRNA strand until all the codons have been ‘read’ up to the stop codon and the specific polypeptide has been produced

[The sequence of amino acids in this polypeptide has been determined by the codons on the mRNA strand]

[The polypeptide folds itself into its secondary and tertiary structure]

Question 22

What is ‘Genetic Diversity’ and what is it caused by?

Answer 22

Genetic diversity is the number of different alleles in a population

It is caused by:
–> Gene mutations

–> Meiosis (independent segregation of homologous chromosomes and crossing over)

–> Random fusion of gametes during fertilisation

–> Chromosome mutations

[This genetic diversity is acted upon by natural selection, resulting in species becoming better adapted to their environment]

Question 23

What is a ‘Gene Mutation’ and how do they occur?

Answer 23

Gene mutations are changes in the sequence of nucleotide bases in the DNA (mutations produce new alleles of genes)

They occur:
–> Spontaneously

–> Might arise during DNA replication

–> The mutation rate is increased by mutagenic agents (e.g., X-rays, benzene).

–> Mutations can result in a different amino acid sequence in the encoded polypeptide (due to the altered base sequence coding for a different sequence of amino acids)

[This may lead to a change in the shape or structure of the protein, which can alter or disrupt its function]

Question 24

What are the different types of ‘Gene Mutations’?

Answer 24

1) Some only change one triplet code [due to the Degenerate nature of the genetic code, not all mutations result in a change to the encoded amino acid]

2) Some mutations change the nature of all the base triplets downstream from the mutation (i.e. result in a frame shift)

Question 25

What are all the ways the sequence of nucleotide bases in a gene can be altered?

Answer 25

1) Substitution 2) Deletion 3) Addition 4) Duplication 5) Inversion 6) Translocation of bases

Question 26

What is 'Substitution'?

Answer 26

Substitution--> the replacement of one or more bases by one or more different bases [The substitution of a single base may result in]: --> a new triplet coding for a different amino acid in the polypeptide chain (which may result in a non-functional protein being formed) --> one different amino acid in the polypeptide changes, but a functional protein is still produced --> the same amino acid may be coded for due to degeneracy of the DNA code, e.g. CGA and CCG both code for the same amino acid, alanine so that the polypeptide remains unchanged

Question 27

What is 'Deletion'?

Answer 27

Deletion--> the removal of one or more bases [this results in]: --> a Frame Shift (alteration in all the triplets from the point of mutation) --> the sequence of amino acids is altered from the point of addition, and the protein formed is non-functional

Question 28

What is 'Addition'?

Answer 28

Addition--> the adding of one or more bases (e.g. ATCCGT becomes ATCCTCGT) [this results in]: --> also results in a Frame Shift (the alteration in the base triplets from the point of addition) --> the sequence of amino acids is altered from the point of addition, and the protein formed is non-functional

Question 29

What is 'Duplication'?

Answer 29

Duplication--> where one or more bases are repeated, (e.g. CCGTAA becomes ccgCGtaa) [this results in]: --> frame shift (the alteration in the base triplets from the point of addition)

Question 30

What is 'Inversion'?

Answer 30

Inversion--> where a sequence of bases is reversed (e.g. TAGCAGCT becomes TCGACGAT)

Question 31

What is 'Translocation'?

Answer 31

Translocation--> where a sequence of bases is moved from one location in the DNA molecule to another part of the genome

Question 32

Why are 'Alleles' different versions of the same gene?

Answer 32

Alleles of a gene differ in their base sequence and code for different sequences of amino acids [--> this change in the polypeptide can make the protein produced non-functional] [--> this change in amino acid sequence may affect the position of hydrogen and ionic bonds (and if present, the position of disulfide bonds), which changes the tertiary structure] [--> if the protein coded for is an enzyme, the change in tertiary structure may alter the shape of the active site, and the enzyme is now non-functional as the substrate is not complementary and enzyme-substrate complexes do not form]

Question 33

What is 'Meiosis'?

Answer 33

Meiosis is a type of nuclear division that produces cells that are genetically different It has: --> 2 nuclear divisions --> 4 cells produced --> Genetically different cells produced --> Haploid cells produced (i.e. possess half the number of chromosomes) [Haploid cells are gametes (i.e. reproductive cells) ]

Question 34

Where does 'Meiosis' occur in the life cycle of an organism?

Answer 34

Meiosis occurs when the diploid number (2n) has been halved to the haploid number (n)

Question 35

What are the stages of 'Meiosis'?

Answer 35

2 successive nuclear divisions occur (i.e. meiosis I and II) to produce 4 haploid cells 1) [during late Interphase]: --> DNA replication --> Build-up of ATP --> Protein Synthesis and replication of cell organelles 2) Then meiosis I and II occur: -->[Meiosis I]: separates the members of each homologous pair -->[Meiosis II]: separates the chromatids of each chromosome

Question 36

How is Genetic Diversity produced in cells during meiosis?

Answer 36

1) Independent segregation of homologous chromosomes 2) Genetic recombination by crossing over within homologous chromosomes

Question 37

What happens in the 'Independent Segregation of Homologous Chromosomes'?

Answer 37

--> It gives different combinations of homologous chromosomes [=generating variation] 1) During the first meiotic division, homologous chromosomes pair together and then separate so that one member from each pair enters a gamete 2) Gametes produced from this cell will possess one member from each original homologous pair [due to the independent segregation of homologous chromosomes in meiosis] 3) The 2 members of each homologous pair line up opposite each other on the spindle during metaphase of the first meiotic division 4) The subsequent separation of the 2 members of a pair (during anaphase of the first meiotic division) is entirely independent from the separation of another pair [Therefore, the gametes can contain varied combinations of maternal and paternal chromosomes and alleles]

Question 38

What happens in 'Genetic recombination by crossing over within homologous chromosomes'?

Answer 38

-->[Shuffling of Alleles] 1) The 2 members of each homologous pair lie side by side, forming a 'Bivalent' [This only occurs in meiosis during prophase of the first meiotic division] 2) The chromatids of the homologous chromosomes then intertwine 3) Then, the chromatids break and equivalent portions of the chromatids are exchanged [i.e. Crossing Over] 4) The place where crossing over occurs is known as the 'Chiasma', resulting in the exchange of alleles of the same genes and can produce new combinations of alleles [i.e. Recombinants] -->The process is known as [Genetic Recombination], the chromosomes then separate. (The number of recombinants formed is relatively low as crossing over is relatively rare)