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A-Level Biology > Unit 4 - Genetic Info, Variation & Relationships b/w Organisms > Flashcards

Unit 4 - Genetic Info, Variation & Relationships b/w Organisms Flashcards

1
Q

define gene

A

sequence of DNA bases that code for a polypeptide or for a functional RNA (inc. rRNA & tRNA)

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2
Q

define locus

A

fixed position on a chromosome that a gene occupies

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3
Q

define allele

A

different versions of a gene
different alleles have slightly different nucleotide sequences but occupy the same locus on the chromosome

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4
Q

how are chromosomes arranged?

A

homologous pairs
one inherited from the father & one inherited from the mother

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5
Q

eukaryotic DNA vs prokaryotic DNA

A

eukaryotic vs prokaryotic
linear (formed in chromosomes) vs circular
in nucleus vs in cytoplasm
no plasmids vs plasmids present
introns present vs no introns present
longer & more genes vs shorter & fewer genes
both are a double-stranded helix structure

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6
Q

how is DNA packaged in eukaryotes?

A

DNA double helix
DNA wraps around/associates with histones proteins
DNA-histone complex is coiled
coils fold to form loops
loops coil & pack together to form a chromosome (visible under light microscope)

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7
Q

what is the structure of a chromosome?

A

homologous chromosomes have same genes at the same specific loci but may have different alleles (paternal & maternal chromosomes)
duplicated chromosome - sister chromatids joined at the centromere

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8
Q

describe karyotype

A

picture of all the chromosomes from a single diploid cell
homologous pairs can be identified due to distinctive banding after staining
sex chromosomes displayed to one side
all other chromosomes are called autosomes

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9
Q

what sex chromosomes correspond to each gender?

A

xy - male
xx - female

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10
Q

define genome

A

complete set of genes in a cell

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11
Q

define proteome

A

full range of proteins that a cell can produce
or coded for by the cell’s DNA

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12
Q

what are the features of genetic code?

A

triplet code
degenerate
on-overlapping
universal

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13
Q

describe triplet code

A

a sequence of 3 bases (codon) codes for a specific amino acid

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14
Q

describe degenerate code

A

some amino acids are coded for by more than one codon
3 codons (stop codons) do not code for any amino acid & are used to mark the end of a polypeptide chain
the start of the sequence is always the same codon

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15
Q

describe non-overlapping code

A

each base in the sequence is only read once
code is always read in one direction along the DNA strand

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16
Q

describe universal code

A

DNA is the same in all organisms
same triplet code for same amino acids

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17
Q

why might mutations in nucleotide sequence of a gene not cause a change in structure of polypeptide?

A

triplets code for the same aa
mutation in introns/non-coding region

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18
Q

describe messenger RNA

A

function: transfers DNA code from nucleus to cytoplasm & determines the sequence of aas during protein synthesis
complementary to DNA code
small enough to leave nucleus via nuclear pores
associates with ribosomes in cytoplasm
easily broken down & only exists when needed to make a protein
sequence of nucleotides in mRNA = genetic code

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19
Q

describe the process of transcription

A

DNA helicase separates polynucleotides by breaking hydrogen bonds (b/w DNA bases)
the non-coding DNA strand acts as a template
free RNA nucleotides align by complementary base pairing
uracil base pairs with the adenine on DNA instead of thymine
RNA polymerase joins adjacent RNA nucleotides
phosphodiester bonds
in eukaryotes, pre-mRNA is spliced & introns removed to form mRNA

20
Q

describe the process of splicing

A

introns (non-coding sections) are removed
remaining exons (coding sections) join together
splicing does not happen in prokaryotes

21
Q

compare DNA replication & transcription

A

replication uses DNA polymerase to synthesise new DNA polynucleotides vs transcription uses RNA polymerase to synthesise new RNA polynucleotides

replication creates identical DNA molecules before cell division by mitosis vs transcription converts DNA into mRNA

22
Q

describe tRNA

A

small, single-stranded molecule
clover shape with one end of the chain longer
the longer section is the amino acid binding site, which binds to a specific aa
each tRNA carries a different aa to the ribosome
each amino acid has a specific anticodon, which pairs with the complementary codon on the mRNAby H bonds

23
Q

describe the process of translation

A

mRNA attaches to ribosomes (on RER)
anticodons bind to complementary mRNA codons by hydrogen bonds
tRNA brings a specific amino acid
amino acids join by peptide bonds
with the use of ATP
tRNA is released after aa is joined to polypeptide
the ribosome moves along the mRNA to form the polypeptide

24
Q

extra notes on translation

A

start: 2 mRNA codons are exposed to the ribosome at once
mRNA is used as a template to which tRNA attaches
hydrogen bonds are formed b/w the start codon & complementary tRNA anticodon
cycle:
an enzyme in the ribosome catalyses the condensation of a peptide bond b/w 2 aas (needs ATP)
end:
stop codon at the end of mRNA - 3 stop codons in the genetic code
no tRNA has an anticodon which is complementary to stop codon
polypeptide released from ribosome & forms its specific tertiary structure to enable it to perform its function

25
Q

describe the role of ribosomes in translation

A

hold tRNA & mRNA together so tRNA anticodon can bind to complementary mRNA codon & correct aa can be added to polypeptide chain
ribosomes made up of rRNA & protein in nucleolus
have large & small subunits that attach to mRNA
ribosomes move along the mRNA

26
Q

comparison of DNA, mRNA & tRNA

A

see table

27
Q

describe the role of atp in the process of translation

A

releases energy
so aas join to tRNA & peptide bonds form b/w amino acids

28
Q

how are gene mutations caused?

A

DNA replicational errors in interphase
substitution, addition or deletion of bases from the normal sequence
can happen naturally
chance of mutation increased by mutagenic agents e.g. ionising radiation/UV/x-rays or chemicals/tar

29
Q

define mutagenic agent

A

a factor that increases the rate of gene mutations

30
Q

most mutations have a negative/neutral impact on the fitness of an organism (selected against) but some are beneficial & selected for by natural selection

A

.

31
Q

what are the types of base substitution mutations?

A

silent mutation
mis-sense mutation
non-sense mutation
all are single base substitutions

32
Q

describe silent mutation

A

new codon codes for same amino acid
so has no effect on protein’s primary structure bc the code is degenerate
so no effect on secondary & tertiary structure
so protein function is not affected

33
Q

describe mis-sense mutation

A

amino acid changes
primary structure different
this could change secondary/tertiary structure of the polypeptide
might be different H, ionic & disulfide bonds
so alters the specific 3d shape & function of the protein

34
Q

describe non-sense mutation

A

changed the amino acid to a stop codon
premature translation of the mRNA section so normal protein sequence is not completed
shorter primary structure so different 2/3 structure
might be different H, ionic & disulfide bonds
different specific 3d shape
loss of function e.g. no ESCs/Ab-Ag complexes formed

35
Q

describe frameshift mutation caused by deletion/addition base mutation

A

base has been deleted causing a shift to the left
addition of a base causes a shift to the right
causes frameshift, shifting the last base of each codon into the next one, to produce a different sequence after the mutation
different primary structure
effect on 2/3 structure of the polypeptide & affects the protein’s structure & function

nb addition/deletion of bases in multiples of 3 does not cause frameshift so is less detrimental to overall protein

36
Q

what is the function of meiosis?

A

produces four daughter gamete cells each with 1/2 the number of chromosomes as the parent cell & all genetically varied

37
Q

meiosis involves 2 nuclear divisions

A

one diploid cell in reproductive organ (ovary/testes) divides twice to produce 4 haploid daughter cells, which are all genetically different

38
Q

why is meiosis important?

A

1- it halves the number of chromosomes (from diploid to haploid), which allows sexual reproduction, the fusion of 2 gametes, without chromosome number increasing

2- it causes genetic variation by new combination of alleles
allows natural selection so species can adapt to environmental changes

39
Q

other than meiosis, what causes genetic variation?

A

random gene mutations –> change in DNA base sequence
creates new alleles

40
Q

summary of meiosis diagram

A

see notes

41
Q

how does meiosis result in genetic variation?

A

crossing over of homologous chromosomes
independent segregation of homologous chromosomes
which causes new allele combinations

random fertilisation of gametes

42
Q

describe the process of crossing over

A

chromatids of each pair twist around each other
tension –> often equal portions of chromatids break off = exchange of alleles
broken portions recombine with chromatids of its homologous partner = recombination
there are new genetic combinations of maternal & paternal alleles
each gamete contains a unique combination of alleles on a given chromosome = genetic variation

43
Q

describe independent segregation

A

homologous chromosomes pair up on the equator of the cell in metaphase 1
each pair can be positioned in 2 ways
the position of each pair is independent to the position of all other chromosome pairs

44
Q

what is the formula for # of possible gametes from independent segregation

A

2^# of homologous pairs

45
Q

define chromosome mutations

A

changes in structure or number of chromosomes

46
Q

why does non-disjunction happen?

A

1- homologous chromosomes fail to separate in meiosis 1

2- sister chromatids fail to separate in meiosis 2

47
Q

what is the effect of non-disjunction?

A

in both cases, gametes are formed with either 1 extra or 1 missing chromosome
e.g. Down’s syndrome, Turner’s syndrome

A-Level Biology (5 decks)

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