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Biology AQA > Genetic information, variation and relationships between organisms > Flashcards

Genetic information, variation and relationships between organisms Flashcards

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1
Q

How is eukaryotic DNA stored?

A

Eukaryotic cells contain linear DNA molecules that exist as chromosomes found in the nucleus. The DNA molecule is really long so it is wound around proteins called histones so the DNA can fit in the nucleus. The DNA and histones are then coiled up very tightly to make a compact chromosome. Histone proteins also provide support for the DNA.

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2
Q

What organelles contain their own DNA, separate from that in the nucleus?

A

Mitochondria and chloroplasts contain their own DNA but it is not associated with histone proteins and is circular and shorter than DNA molecules in the nucleus .

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3
Q

How is prokaryotic DNA stored?

A

In prokaryotes, DNA is also carried as chromosomes but the DNA molecules are shorter and circular. The DNA isn’t wound around histones, it condenses to fit the cell by supercoiling.

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4
Q

What is a gene?

A

A sequence of DNA bases that codes for either a certain polypeptide or functional RNA.

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5
Q

What makes one polypeptide different from another?

A

The different sequence and number of amino acids. The order of bases in a gene determines the order of amino acids in a polypeptide.

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6
Q

What is each amino acid coded for by?

A

A base triplet; a codon.

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7
Q

Genes that don’t code for a polypeptide code for functional RNA. What is fRNA?

A

Functional RNA is RNA molecules other than mRNA, which perform special tasks during protein synthesis.

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8
Q

Give two examples of functional RNA

A

tRNA- transfer RNA

rRNA- ribosomal RNA

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9
Q

What is a cell’s genome?

A

The complete set of genes in a cell.

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10
Q

What is a cell’s proteome?

A

The full range of proteins the cell is able to produce.

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11
Q

In eukaryotic DNA, genes that do code for polypeptides contain sections that don’t code for amino acids. What are these sections called?

A

Introns. There can be several introns within a gene.

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12
Q

What is the name for all the parts of a gene that do code for amino acids?

A

Exons

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13
Q

Eukaryotic DNA also contains regions of multiple repeats outside of genes, but these repeats don’t code for amino acids either. What are they called?

A

Non-coding repeats

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14
Q

What is an allele?

A

An allele is a different form of a gene, with a slightly different order of bases and therefore alleles code for slightly different versions of the same polypeptide.

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15
Q

What is the name for a pair of matching chromosomes?

A

A homologous pair.

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16
Q

What is the same in a homologous pair?

How are alleles positioned on homologous pairs?

A

Both chromosomes have the same size and genes, however they could have different alleles. Alleles coding for the same characteristic will have the same fixed position (locus) on each chromosome in a homologous pair.

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17
Q

What is the purpose of messenger RNA?

A

Messenger RNA, made during transcription, carries the genetic code from the DNA to the ribosomes, where it’s used to make a protein during translation. mRNA is a single polynucleotide strand. In mRNA, groups of 3 adjacent bases are usually called codons.

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18
Q

What is the purpose of transfer RNA?

What is the structure of tRNA?

A

Transfer RNA carries the amino acids that are used to make proteins to the ribosomes in translation. tRNA is a single polynucleotide strand that’s folded into a clover shape. Hydrogen bonds between specific complementary base pairs hold the molecule in this shape. Every tRNA molecule has a specific sequence of 3 bases at one end called an anticodon. They also have an amino acid binding site at the other end.

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19
Q

What is the first stage of protein synthesis?

A

Transcription

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20
Q

In short, what happens in transcription?

Where does transcription take place in eukaryotic and prokaryotic cells

A

An mRNA copy of a gene is made from DNA.
In eukaryotic cells, transcription takes place in the nucleus. Prokaryotic cells have no nucleus and so transcription occurs in the cytoplasm.

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21
Q

How does transcription start?

A

RNA polymerase enzyme attaches to the DNA double helix at the beginning of a gene. The hydrogen bonds between the two DNA strands in the gene break, separating the strands, and the DNA molecule uncoils at that point, exposing some of the bases.

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22
Q

What happens in transcription after the two DNA strands separate and uncoil?

A

One of the strands is used as a template to make an mRNA copy. The RNA polymerase lines up free nucleotides alongside the exposed bases on the template strand. The free bases are attracted to the exposed bases. Specific, complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand, except thymine is replaced with uracil.

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23
Q

What happens once the RNA nucleotides have paired up with their specific bases on the DNA strand?

A

The RNA nucleotides are then joined together by RNA polymerase to form an mRNA molecule. The RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand. The hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by and the strands coil back into a double helix.

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24
Q

When does RNA polymerase stop making mRNA?

What happens to the newly made mRNA?

A

RNA polymerase stops making mRNA when it reaches a specific base triplet called a stop codon, which signals the enzyme to detach from the DNA.
In eukaryotes, mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place.

25
Q

What happens in eukaryotes with regards to introns and exons in transcription?

A

The introns and exons are both copied into mRNA during transcription. mRNA strands containing both introns and exons are called pre-mRNA. A process called splicing then occurs, where the introns are removed and the exons are joined together to form mRNA strands. This takes place in the nucleus.

26
Q

What happens in prokaryotes with regards to introns and exons in transcription?

A

In prokaryotes, mRNA is produced directly from the DNA, without the need for splicing, as there are no introns in prokaryotic DNA.

27
Q

What is the name of the second stage of protein synthesis?

A

Translation

28
Q

Where does translation occur?

A

In eukaryotes and prokaryotes, translation occurs at the ribosomes in the cytoplasm.

29
Q

What happens in translation? (basic process)

A

During translation, amino acids are joined together to make a polypeptide chain (protein) following the sequence of codons carried by the mRNA.

30
Q

What is the first step of translation?

A

The mRNA attaches itself to a ribosome and transfer RNA molecules carry amino acids to it. A tRNA molecule, carrying an amino acid, with an anticodon complementary to the first codon on the mRNA, attaches itself to the mRNA by specific, complementary base pairing.

31
Q

What happens in translation after the first tRNA molecule has attached to mRNA?

A

A second tRNA molecule attaches itself to the next codon on the mRNA in the same way. The two amino acids attached to the tRNA molecules are joined when a peptide bond forms between them. The first tRNA molecule then moves away , leaving its amino acid behind.

32
Q

What happens in translation after the first molecule of tRNA has moved away?

A

A third tRNA molecule binds to the next codon on mRNA. Its amino acid binds to the first two and the second tRNA molecule moves away. This process continues until there’s a stop signal on the mRNA molecule, producing a chain of linked amino acids (a polypeptide chain). The polypeptide chain moves away from the ribosome and the translation is complete.

33
Q

What is the genetic code?

A

The sequence of base triplets in mRNA which code for specific amino acids.

34
Q

Why can the genetic code be described as non-overlapping?

A

Each base triplet is read in sequence, separate from the base triplet before it and after it. Base triplets don’t share their bases and so the code is non-overlapping.

35
Q

Why can the genetic code be described as degenerate?

A

There are more possible combinations of triplets than there are amino acids (20 amino acids but 64 possible triplets). This means that some amino acids are coded for by more than one base triplet.

36
Q

What are start and stop codons?

A

Codons that are used to tell the cell when to start or stop production of the protein. UAG is a stop signal.

37
Q

Why can the genetic code be described as universal?

A

The same specific base triplets code for the same amino acids in all living things. For example, UAU codes for tyrosine in all organisms.

38
Q

What does the fact normal body cells have the diploid number of chromosomes mean?

A

Each cell contains two of each chromosome; one maternal chromosome and one paternal chromosome.

39
Q

How many chromosomes to gametes have?

A

Gametes have a haploid number of chromosomes, meaning they have one copy of each chromosome.

40
Q

What happens in fertilisation with regards to chromosomes?

A

A haploid sperm cell fuses with a haploid egg cell to form a diploid cell.

41
Q

Where does meiosis take place in humans?

A

The reproductive organs.

42
Q

What effect does meiosis have on chromosome numbers.

A

Cells that divide by meiosis have a diploid number of chromosomes before they are split into two separate daughter cells with haploid numbers of chromosomes.

43
Q

What is the first step of meiosis?

A

Before meiosis starts (the cell division), the DNA unravels and replicates so that there are two copies of each chromosome, called chromatids. The DNA condenses to form double-armed chromosomes, each made from two sister chromatids. The sister chromatids are joined at the centre by a centromere.

44
Q

What happens in Meiosis I?

A

This is the first division of meiosis. The chromosomes arrange themselves into homologous pairs, which are then separated to halve the chromosome number.

45
Q

What happens in Meiosis II?

A

This is the second division of meiosis. The pairs of sister chromatids that make up each chromosome are separated when the centromere is divided. Four haploid cells (gametes) that are genetically different from each other are produced.

46
Q

Explain crossing over of chromatids during meiosis I

A

Homologous pairs of chromosomes come together and pair up. The chromatids twist around each other and bits of chromatids swap over. The chromatids still contain the same genes but now have a different combination of alleles.

47
Q

What is the effect of crossing over in meiosis I?

A

If crossing over occurs, all four daughter cells produced in meiosis II will contain chromatids with different combinations of alleles.

48
Q

Explain independent segregation

A

Each homologous pair of chromosomes in your cells is made up of one maternal and one paternal chromosome. When the homologous pairs are separated in meiosis I, it’s completely random which chromosome from each pair ends up in which daughter cell. This means the four daughter cells have different combinations of those maternal and paternal chromosomes. This ‘shuffling’ of the chromosomes leads to genetic variation in any potential offspring.

49
Q

Name three differences between the outcomes of mitosis and meiosis

A

Mitosis produces daughter cells genetically identical to their parent cell, while meiosis produces genetically different daughter cells.
Mitosis produces two daughter cells, while meiosis produces four daughter cells.
Mitosis produces cells with the same number of chromosomes as the parent cell, whereas meiosis produces cells with half the number of chromosomes as the parent cell.

50
Q

What causes chromosome mutations?

What kind of conditions do these mutations lead to?

A

Errors during meiosis.

Chromosome mutations lead to inherited conditions because the errors are present in the gametes.

51
Q

What is non-disjunction?

A

A type of chromosome mutation whereby the chromosomes fail to separate properly. In humans, non-disjunction of chromosome 21 ca lead to Down’s syndrome.

52
Q

Name three types of errors that cause genetic mutation.

A

Substitution- Where one base is substituted with another.
Deletion- Where one base is deleted, causing a frame shift.
Insertion- Where one base is added to the sequence, causing a frame shift.

53
Q

Why don’t all mutations affect the order of amino acids?

A

The degenerate nature of the genetic code means that most amino acids are coded for by multiple different codons. Therefore, substitution mutations may change one base but the base triplet it belongs to may still code for the same amino acid.

54
Q

What are mutagenic agents?

Give some examples

A

Mutagenic agents are factors that can increase the rate of mutations, which are usually spontaneous.
Examples of mutagenic agents are ultraviolet radiation, ionising radiation, certain chemicals and certain viruses.

55
Q

Define genetic diversity

A

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

56
Q

What increases genetic diversity in a population?

A
  • Mutations in the DNA forming new alleles
  • Different alleles being introduced into a population when individuals from another population migrate into them and reproduce. This is known as gene flow.
57
Q

What is a genetic bottleneck?

A

An event which causes a dramatic reduction in the size of a population and therefore the number of different alleles in the gene pool of the population. This reduces genetic diversity. The survivors reproduce and a larger population is created from a few individuals.

58
Q

What is the founder effect?

A

The founder effect is a type of genetic bottleneck whereby a few organisms from a population start a new colony and there are only a small number of different alleles in the initial gene pool. The frequency of each allele in the new colony may be very different to that in the original population, which may lead to a higher incidence of genetic disease. The founder effect can occur as a realist of migration leading to geographical separation or if a new colony is separated from the original population for other reasons, such as religion (Amish).

Biology AQA (8 decks)

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