DNA, RNA, Protein Synthesis, Meiosis

Question 1

Describe the structure of eukaryotic DNA

Answer 1

Eukaryotic DNA is linear, double-stranded helix with hydrogen bonds between the bases. DNA molecules are long and coil up around histones (proteins). This undergoes supercoiling to form chromosomes, which are stored in the nucleus.

Question 2

Describe the structure of prokaryotic DNA

Answer 2

Prokaryotic DNA is double stranded, but is shorter and circular. The DNA is not associated to histones and is stored in the cytoplasm of the cell as a part of the chromosomal loop or plasmids. This DNA is similar in structure to the DNA found within mitochondria and chloroplasts of eukaryotic cells.

Question 3

What is a gene?

Answer 3

A gene is a base sequence of DNA which codes for a specific polypeptide or functional RNA.

Question 4

What is a cell’s genome?

Answer 4

The complete set of genes in a cell (the entire of the cell’s genetic material)

Question 5

What is a cell’s proteome?

Answer 5

The full range of proteins a cell is able to produce

Question 6

Where are genes located within DNA?

Answer 6

A gene occupies a specific fixed position on a DNA molecule, called a locus

Question 7

Do all of the base sequences in DNA code for an amino acid/nucleotide?

Answer 7

In prokaryotic DNA, yes all bases code for amino acids (there are no introns). However, in eukaryotes, the DNA does contain introns (non-coding regions of DNA), and multiple repeats (another kind of non-coding DNA generally found between genes). Only exons are coding regions.

Question 8

What are alleles?

Answer 8

Alleles are alternative forms of a gene which have a slightly different order of bases, and as such, code for a slightly different polypeptide. They arise from mutations.

Question 9

What are homologous pairs?

Answer 9

A homologous pair is two chromosomes which are the same size and shape and code for the same genes. They are identical (unless they contain different alleles).

Question 10

What is protein synthesis?

Answer 10

The process by which new proteins are made by a cell, involving transcription and translation.

Question 11

Describe the structure of tRNA (transfer RNA)

Answer 11

tRNA has a cloverleaf shape. It is a single polynucleotide strand held together by hydrogen bonds to specific base pairs (on the other side of the structure, it is still single stranded despite this bonding). It has a anticodon consisting of three bases which binds to a specific mRNA codon. It also has an amino acid binding site at the other end, consisting of three bases (it binds to the amino acid coded for by the mRNA codon).

Question 12

Describe the process of transcription.

Answer 12

Transcription occurs the nucleus (eukaryotes) or in the cytoplasm (prokaryotes)
- DNA helicase binds to DNA and breaks the hydrogen bonds between bases, separating the two DNA strands
- RNA polymerase binds to the promoter region of DNA
- free RNA nucleotides bind to their complementary DNA bases (complementary base pairing)
- RNA polymerase forms phosphodiester bonds between adjacent RNA nucleotides. When RNA polymerase reaches a stop signal, mRNA detaches from DNA
- in prokaryotes, mRNA is formed and transcription is complete
- in eukaryotes, pre-mRNA is formed, which contains introns (non-coding DNA) and exons (coding DNA)
- splicing occurs, which removes the introns, forming mRNA
- the mRNA leaves the nucleus through a nuclear pore

Question 13

Describe the process of translation

Answer 13

Translation occurs either in the cytoplasm (free-floating ribosome) or on a ribosome associated to the RER.
- the mRNA strand attaches to a ribosome and a tRNA molecule with a complementary anticodon to the mRNA’s codon binds to the mRNA codon delivers a specific amino acid
- another tRNA molecule with a complementary anticodon to the next codon binds to the other attachment site on the ribosome, delivering the next amino acid in the polypeptide chain. A peptide bond forms between the adjacent amino acids and the first tRNA molecule detaches from the ribosome.
- the ribosome moves a distance of three bases (one codon) down the mRNA molecule and the process repeats until a stop codon is reached
- at this point, the ribosome detaches from the mRNA and the polypeptide is formed
- the polypeptide will then fold in a specific way to complete protein synthesis

Question 14

What is the genetic code?

Answer 14

The genetic code is a sequence of base triplets (codons) in mRNA which code for specific amino acids.

Question 15

What does non-overlapping mean?

Answer 15

Each base is only part of one triplet (and each triplet contains three bases)

Question 16

What does degenerate mean (genetic code)?

Answer 16

There is more than one base triplet for each amino acid (multiple different base combinations can code for the same amino acid).

Question 17

What does universal mean (genetic code)?

Answer 17

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

Question 18

What are gene mutations?

Answer 18

Gene mutations involve a change in the DNA base sequence of chromosomes. This may result in different amino acids being coded for.

Question 19

How do gene mutations occur?

Answer 19

Gene mutations occur spontaneously during DNA replication.

Question 20

What are the types of gene mutation?

Answer 20

• base substitution - one base is swapped for another (e.g. ATTCGA becomes ATTCAA). This is a point mutation as it only changes the one amino acid (potentially)
• base deletion - one base is deleted/removed from the sequence (e.g. GCTTCA becomes CTTCA). This is a frameshift mutation as it will affect all the amino acids downstream of the mutation (which may cause a very different protein to form).
• base addition - one base is added to the sequence (e.g. TTTCGA becomes TTTGCGA). This is also a frameshift mutation as it affects all amino acids downstream of the mutation, which could result in the formation of a very different protein.

Question 21

What is the effect of genetic mutations?

Answer 21

The effect can vary dramatically depending on the type of mutation. A simple base substitution is unlikely to affect the protein much at all (it may not have any impact if the same amino acid is still coded for). However, base additions/deletions can have a huge impact on the final protein as many of the wrong amino acids could be coded for. If the mutation occurs in a non-coding region, it is much less likely to have an effect (although it may impact how well RNA polymerase can bind, so may result in more/less protein being made). Generally, gene mutations are detrimental to the protein, but they can be advantageous.

Question 22

What are mutagenic agents?

Answer 22

Mutagenic agents are factors which increase the rate of gene mutation. Some examples are UV radiation, ionising radiation, certain chemicals, and some viruses.

Question 23

What are chromosome mutations?

Answer 23

Chromosome mutations occur during meiosis and they are when the incorrect number of chromosomes are present in a cell. If the homologous chromosomes do not separate during meiosis 1, or the sister chromatids do not separate during meiosis 2, at least one cell may end up with a trisomy of a particular chromosome (3 copies of the chromosome rather than two), and other cells may get no copies or only one copy of the chromosome (these cells generally die). This is called chromosome non-disjunction and can lead to a variety of conditions, such as Down’s syndrome.

Question 24

What is the different between haploid and diploid cells?

Answer 24

Diploid cells have the diploid number (2n) of chromosomes (so has two copies of every chromosome (the full amount)). Haploid cells have the haploid (n) number of chromosomes (one copy of every chromosome (half the full amount)). Gametes are haploid, all other somatic (body) cells are diploid in humans. Other organisms may spend a large portion of their life cycle comprising mainly haploid cells.

Question 25

How do haploid cells become diploid?

Answer 25

During fertilisation, two gametes (haploid) fuse to form a zygote (diploid). It is important that gametes are haploid so that the correct number of chromosomes for that organism can be maintained over many generations (otherwise the number would double every generation).

Question 26

Describe the process of meiosis

Answer 26

• DNA unravels and replicates during interphase
• the DNA condenses to form two sister chromatids joined by a centromere (prophase)
• the chromosomes form homologous pairs (bivalents), which line up randomly on the equator of the cell (metaphase 1)
• the spindle fibres attach to the centromeres of the homologous chromosomes, which are separated and the cell divides in two (anaphase 1 and telophase 1). The cells are now haploid.
• the chromosomes line up on the equator of the cell again (not in pairs this time) and the spindle fibres attach to their centromeres (metaphase 2)
• the centromeres divide and the sister chromatids are separated (anaphase 2)
• the cells divide again (telophase 2 and cytokinesis)

Question 27

What are the differences between meiosis and mitosis?

Answer 27

• meiosis produces four genetically different haploid daughter cells, while mitosis produces two genetically different diploid daughter cells
• meiosis involves the formation of homologous pairs (bivalents), mitosis doesn’t
• meiosis has two cell divisions, mitosis only has one
• meiosis produces gametes, mitosis produces normal body cells
• meiosis involves crossing over, mitosis doesn’t

Question 28

What is crossing over and how can it lead to genetic variation?

Answer 28

Crossing over is when non-sister chromatids of the same homologous pair touch and exchange alleles at chiasmata during prophase 1 of meiosis. This results in new combinations of alleles, leading to greater genetic variation within the species.

Question 29

What is the independent segregation of chromosomes and how does it lead to greater genetic variation?

Answer 29

This means that when the homologous pairs/chromosomes line up on the equator of the cell, they do so randomly. This means that there are many different combinations of chromosomes which the daughter cells could end up with. This increases genetic variation as there are more different outcomes.

Question 30

How does fertilisation increase genetic diversity?

Answer 30

Fertilisation is random (which sperm fertilises which egg is random (in humans, other organisms have different gametes)). This increases genetic diversity as there are more combinations of genetic material which could form.

Question 31

What is genetic diversity?

Answer 31

The number of different alleles of genes in a species or population

Question 32

Why is genetic diversity important?

Answer 32

Having lots of genetic diversity in a population means that that population is more likely to be able to survive a change in the environment (e.g. the introduction of a new disease), as it is more likely that some individuals will be resistant. Having lots of genetic diversity also reduces the incidence of genetic diseases.

Question 33

What can increase genetic diversity?

Answer 33

• meiosis and random fertilisation increase genetic diversity by producing novel combinations of alleles
• random mutations also increase genetic diversity within a species, creating new alleles

Question 34

What is a genetic bottleneck?

Answer 34

This is when an event occurs which causes a big reduction in a population. This reduces the number of different alleles in the gene pool so reduces genetic diversity. The survivors reproduce and a large population is created from few individuals, so there is little genetic diversity.

Question 35

What is the founder effect?

Answer 35

This describes the reduction in genetic diversity which occurs when a small group splits from a population and starts a colony. There are few alleles in this colony, which, if it remains separated from the original population, will have low genetic diversity.