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Biology A LEVEL > Genetics > Flashcards

Genetics Flashcards

1
Q

What is a gene?

A

A section of DNA that codes for the sequence of amino acids in a polypeptide and functional RNA

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2
Q

What happens when a protein is in demand?

A

DNA is unwound

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3
Q

What is the location of the gene on the strand of DNA/ Chromosome called?

A

Locus

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4
Q

What does ‘cracking the code’ refer to?

A

-Scientists realised a triplet code would provide enough combinations
-Scientists found that some amino acids were coded for by more than one triplet
-the genetic code is therefore degenerate

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5
Q

What is a start codon?

A

At the start of every gene there are 3 bases (start codon)

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6
Q

What is the start codon for DNA?

A

TAC

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7
Q

What is the start codon for mRNA?

A

AUG

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8
Q

What does the start codon code for?

A

The amino acid methionine which is later removed from the protein if it is not needed in the structure

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9
Q

What is a stop codon?

A

3 bases at the end of every gene that do not code for an amino acid

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10
Q

What is the function of a stop codon?

A

-Mark the end of a polypeptide chain
-Cause ribosomes to detach
-Stop translation

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11
Q

What are the 3 features of the genetic code?

A
  1. Degenerate
  2. Universal
  3. Non-lapping
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12
Q

How many amino acids can the genetic code code for?

A

20

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13
Q

How many bases are needed to code for the 20 amino acids?

A

3

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14
Q

What is the mathematical equation to prove that 3 bases can code for 20 amino acids?

A

4^n (4 to the power of n with n being the number of bases. 4^3 shows a sufficient number of amino acids being made)

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15
Q

What does degenerate relating to the genetic code mean?

A

Each amino acid is coded for by more than one triplet of bases

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16
Q

Why is the genetic code being degenerate advantageous?

A

If a point mutation occurs, the triplet of bases will be diff. It may still code for the same amino acid and therefore have no effect

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17
Q

What does universal regarding to the genetic code mean?

A

Each triplet codes for the same amino acid in every organism

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18
Q

Why is the genetic code being universal advantageous?

A

Makes genetic engineering possible

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19
Q

What does non over-lapping in regard to the genetic code mean?

A

-Each base in a gene is only part of one triplet of bases that codes for one amino acid.
-Each base is read only once

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20
Q

Why is the genetic code being non over-lapping advantageous?

A

-If a point mutation occurs, it will only affect one codon and therefore one amino acid

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21
Q

What are the names of the 2 coding and non-coding regions?

A

Introns and Exons

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22
Q

What are introns?

A

-Sections of DNA that do not code for amino acids and therefore polypeptide chains

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23
Q

What are exons?

A

-Sections of DNA that do code for amino acids.

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24
Q

Where are introns found?

A

-In eukaryotic DNA, but not in prokaryotic DNA.
-These get removed, spliced, out of mRNA molecules.

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25
Q

What does splicing do?

A

Removes non-coding regions of a gene and joins coding regions together to mRNA

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26
Q

What is the genome?

A

Complete set of genes within a cell.

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27
Q

What is the proteome?

A

Number of different proteins that a cell is able to produce

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28
Q

What is meiosis?

A

A type of cell division that produces daughter cells that are genetically different (gametes)

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29
Q

How many divisions occurs in meiosis?

A

2 nuclear divisions

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30
Q

What is produced from meiosis?

A

4 haploid daughter cells from a single diploid parent cell

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31
Q

Why do gametes need to be haploids?

A

To restore the chromosome number

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32
Q

What are the two mechanisms in meiosis which introduce variation?

A

-Independent segregation
-Crossing over (Chiasmata)

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33
Q

When do Independent segregation
and Crossing over (Chiasmata) occur?

A

Meiosis 1 (first round of division)

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34
Q

How does Independent segregation work?

A

-In meiosis I, homologous pairs of chromosomes line up opposite each other at the equator of the cell.
-It is random which side of the equator the paternal and maternal chromosomes from each homologous pair lie
-Pairs are separated, so one of each homologous pair ends up in the daughter cell

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35
Q

What does Independent segregation create?

A

Large number of possible combinations of chromosomes produced

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36
Q

How can you calculate the number of different combinations independent segregation can create?

A

2^n

n= number of homologous pairs

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37
Q

How does crossing over work?

A

-Homologous pairs line up opposite each other at the equator in meiosis I
-Parts of chromatids become twisted around each other
-Puts tension on chromatids causing pairs of chromatid to break
-Broken parts of chromatid recombine with another chromatid
-Results in new combinations of alleles.

38
Q

Compare Meiosis and Mitosis:

A

Meiosis:
1. Two nuclear divisions
2. Haploid cell (one set of chromosomes)
3. Introduces genetic variation

Mitosis:
1. One nuclear division
2. Diploid cells (two sets of chromosomes)
3. Creates genetically identical cells

39
Q

Exam technique: In an exam how would you identify meiosis occurring in a life cycle?

A

Involves a diploid cell parent cell (2n) dividing to become a haploid cell (n)

40
Q

Apart from independent segregation and crossing over, what else increases variation?

A

Random fertilisation

41
Q

How is variation increased by random fertilisation?

A

It is random which egg and sperm will fuse during fertilisation

42
Q

How can you calculate the possible combinations of chromosomes in a human when you consider random fertilisation?

A

(2^n)^2
(2^23)^2= 7.04 x 10^13

This is before crossing over is introduced to cause even more variation

43
Q

What is non-disjunction?

A

Failed separation of homologous pairs OR sister chromatids leading to incorrect number of chromosomes in gametes

44
Q

Exam technique: How does meiosis result in cells that have the haploid number of chromosomes and show variation?

A
  1. Homologous chromosomes pair up
  2. Crossing over
  3. produces new combination of alleles;
  4. Chromosomes separate;
  5. At random;
  6. Produces varying combos of chromosomes
  7. Chromosomes separated at meiosis 2
45
Q

What is mRNA?

A

A type of RNA synthesised during the process of transcription.

46
Q

What is the role of mRNA?

A

To carry genetic information from the DNA to the ribosomes

47
Q

What does a diagram of mRNA look like?

A
48
Q

What are the features of mRNA?

A

-Single-stranded, linear molecule.
-Contains a base sequence complementary to a DNA sequence.
-Contains codons, which are sets of three bases that code for an amino acid.
-Small enough to leave the nucleus.

49
Q

What is tRNA?

A

A type of RNA used in the process of translation.

50
Q

What is the role of tRNA?

A

-Works with ribosomes to translate mRNA
-Transfers free amino acids from the cytoplasm to the ribosome to begin protein synthesis
-Acts as taxi service for amino acids

51
Q

What are the features of tRNA?

A

-Single-stranded molecule folded into a clover-leaf shape.
-Hydrogen bonds between base pairs to hold in shape
-Contains specific sequence of three bases at one end (anticodon)
-Contains an amino acid binding site at the opposite end.

52
Q

What does a diagram of tRNA look like?

A
53
Q

Compare the structures of mRNA and tRNA:

A
  1. mRNA longer and has more nucleotides than tRNA
  2. mRNA is straight whilst tRNA is clover-leaf shaped
  3. mRNA contains no base pairs but tRNA does
  4. mRNA has no amino acid binding site but tRNA does
  5. tRNA has anticodon mRNA does not
54
Q

What is transcription?

A

The process of creating an mRNA copy of a gene from the DNA template

55
Q

Where does transcription occur in eukaryotic and prokaryotic cells?

A

Eukaryotes= Nucleus
Prokaryotes= Cytoplasm.

56
Q

What is the role of RNA polymerase in transcription?

A

Binds to DNA, separates the strands, and catalyses the formation of phosphodiester bonds between RNA nucleotides.

57
Q

What happens to DNA when transcription begins?

A
  • Hydrogen bonds between the DNA bases break
  • Two strands separate.
58
Q

Which strand acts as the template for mRNA synthesis? (transcription)

A

-Antisense strand acts as the template
-Sense strand has the same base sequence as the mRNA (except uracil replaces thymine).

59
Q

What is an antisense strand?

A

non-coding strand of DNA used to create the coding strand (mRNA) that will be translated into protein.

60
Q

How do free RNA nucleotides pair with the DNA template? (transcription)

A

Complementary base pairing

61
Q

How does transcription end?

A

RNA polymerase reaches a stop codon, detaches from DNA, and releases the mRNA strand.

62
Q

What happens to DNA after transcription?

A

Rewinds back into its double helix structure.

63
Q

What is the difference between mRNA processing in prokaryotes and eukaryotes?

A

Prokaryotes- mRNA directly produced.
Eukaryotes - pre-mRNA is spliced to remove introns before leaving the nucleus

64
Q

What are introns and exons?

A

Introns are non-coding regions removed during splicing, while exons are coding regions that remain in mature mRNA.

65
Q

What is RNA splicing?

A

Removing introns and joining exons to form mature mRNA in eukaryotic cells.

66
Q

Why is splicing necessary in eukaryotic cells?

A

It removes non-coding introns so that only coding exons remain in mature mRNA for translation.

67
Q

Where does translation occur in eukaryotic and prokaryotic cells?

A

Translation occurs in the cytoplasm, specifically on ribosomes, in both eukaryotic and prokaryotic cells.

68
Q

What is translation in protein synthesis?

A

Process of decoding mRNA to synthesise a polypeptide chain, which then folds into a functional protein.

69
Q

What role does tRNA play in translation?

A

Helps translate the mRNA sequence by bringing specific amino acids to the ribosome to form the polypeptide chain.

70
Q

Exam technique: How is mRNA produced in a plant cell?

A
  1. The DNA strands separate by breaking the hydrogen bond
  2. Only template strand is used to make MRNA
  3. Complementary base pairing occurs: A → U, T → A, C → C,G→ C
  4. Adjacent RNA nucleotides joined by RNA polymerase
  5. Pre-MRNA formed
  6. splicing/ introns removed to form mRNA
71
Q

Exam technique: Contrast structures of DNA and mRNA molecules.

A
  1. DNA has base pairing and mRNA doesn’t
  2. DNA has Introns/mRNA doesnt
  3. DNA very long, RNA short
72
Q

Exam technique: Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.

A
  1. MRNA associates with a ribosome
  2. Ribosomes moves to the start codon
  3. tRNA carries a specific amino acid
  4. Anticodon (on tRNA complementary) to codon (on mRNA)
  5. Ribosome moves along to next codon
  6. Process repeated and amino acid joins by peptide bonds/ condensation reaction to form polypeptide
  7. tRNA released and ribosome fits around two codons
73
Q

Exam technique: Describe the role of tRNA in the process of translation.

A
  1. tRNA anticodon complementary to codon on mRNA
  2. tRNA brings specific amino acid
  3. Amino acids are carried to ribosome
  4. Allows for correct sequence of amino acids along polypeptide
74
Q

How can you calculate the number of possible combinations of chromosomes?

A

2^n where n= the number pairs of homologous chromosomes

75
Q

Apart from independent assortment and chiasmata, how can genetic diversity be created?

A

Fertilisation of gametes

76
Q

What is a gene mutation?

A

Random change in the base sequence of DNA

77
Q

How do mutations occur?

A

Randomly but some factors can increase the rate of mutations (mutagens)

78
Q

What are examples of mutagens?

A

-Alpha, beta and gamma
- X-rays
-UV light
-Carcinogens (tar, petrol, phenols)

79
Q

Which point in the cell cycle do mutations most likely occur and why?

A

Interphase because this is where DNA replication is occurring

80
Q

What are the 3 types of mutations?

A
  1. Substitution
  2. Deletion
  3. Insertion
81
Q

What is substitution?

A

One base is replaced by another. Minor impact because the genetic code is degenerate and it may occur in intron.

82
Q

What is deletion?

A

One base is removed from the sequence. Big impact because all bases after inserted base shift to left called frameshift. all subsequent codons change.

83
Q

What is insertion?

A

One base is added into the sequence. Big impact because all bases after inserted base shift to right called frameshift. all subsequent codons change.

84
Q

Why are mutations dangerous?

A

-May change the amino acid sequence coded for by the gene
- Leads to a different polypeptide and potentially a different tertiary structure.

85
Q

What are the two types of chromosome mutations?

A

Polyploidy - Organisms have more than two sets of chromosomes, and is mostly seen in plants.

Non-disjunction - Occurs when homologous chromosomes fail to separate in meiosis, resulting in gametes with extra or missing chromosomes, and leads to individuals with extra or missing chromosomes in all cells.

86
Q

A mutation can lead to the production of a non-functional enzyme. Explain how.

A
  1. Change in base sequence of DNA
  2. Change in primary structure of enzyme, changes secondary structure
  3. Change in hydrogen, ionic and disulphide bonds
  4. Change in tertiary structure
  5. Change in active site
  6. No enzyme-substrate complexes can form
87
Q

Describe how a deletion mutation alters the structure of a gene.

A
  1. Removal of one or more bases
  2. Frameshift/ base sequence change
88
Q

Why might deletion have a greater effect than substitution?

A
  1. Deletion causes a frameshift
  2. Changes sequence of amino acids
  3. Whereas substitution alters one codon
89
Q

Suggest how a mutation can lead to the production of a protein that has one amino acid missing.

A
  1. Loss of three bases/ triplet/ codon
  2. Triplet for last amino acid is changed to a stop codon
90
Q

Suggest two ways a gene mutation can have no effect on an individual.

A

-Occurs in an intron
-Genetic code is degenerate
(amino acid sequence won’t change)

91
Q

How can a gene mutation have a positive effect on someone?

A

-Results in a change in the polypeptide that positively changes the properties of the protein
-May result in increased reproductive success OR increased survival (chances)

Biology A LEVEL (11 decks)

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