3.4 genetic information, variation and relationships between organisms Flashcards
dna in prokaryotic cells
- short, circular and not associated with proteins
dna in eukaryotic cells
- very long, linear and associated with proteins called histones
- proteins + dna molecule = chromosome
- mitochondria and chloroplasts also have own dna, short and circular (like prokaryotic cells)
gene
- sequence of dna bases that codes for either a polypeptide or functional rna
primary structure of polypeptides
order or sequence of amino acids in the polypeptide
locus
- position of gene on a particular chromosome
triplet
sequence of 3 DNA bases
what do triplets do
code for a specific amino acid
genetic code is:
universal, non overlapping and degenerate
non coding DNA
- in eukaryotes, a lot of dna doesn’t code for polypeptides
- non coding dna = non coding multiple repeats of base sequences between genes
exons
- specific sequences between genes that code for amino acid sequences
introns
- one or more non-coding sequences that separate exons (within the gene)
concept of genome
- complete set of genes in a cell
concept of proteome
- full range of proteins that a cell is able to produce
structure of messenger RNA (mRNA)
- single stranded
- A,U,G,C
- mRNA made during transcription
- carries genetic code from the DNA to the ribosomes, where it is used to make a protein during translation
codons in mRNA
- group of 3 adjacent bases
structure of transfer RNA (tRNA)
- involved in translation
- carries amino acids that are used to make proteins to the ribosomes
- single stranded polynucleotide, folded into a clover shape
- hydrogen bonds between specific base pairs hold the molecule in shape
- every tRNA molecule has a specific sequence of three bases at one end called an anti codon
- also has amino binding site at other end
transcription
- mRNA copy of gene made from DNA
- eukaryotic cells: transcription takes place in nucleus
- prokaryotes don’t have a nucleus, so transcription takes place in cytoplasm
transcription in prokaryotes
- results directly in production of mRNA from DNA
transcription in eukaryotes
- results directly in production of pre-mRNA
- then spliced to form mRNA
stages of transcription
- DNA helix unwinds to expose bases to act as a template
- only one chain of dna acts as a template
- similar to dna replication, unwinding and unzipping is catalysed by dna helicase
- dna helicase breaks hydrogen bonds between bases
- free Mrna nucleotides in the nucleus align opposite exposed complementary dna bases
- enzyme rna polymerase bonds together the rna nucleotides to create a new rna polymer chain
- one entire gene is copied
- once copied, the pre-mrna is modified and then leaves nucleus through nuclear envelope pores
modification of pre-mRNA into mRNA
- introns spliced out by a protein called splicesome
- leaves behind only exons, the coding regions
process of translation
- once modified mRNA has left the nucleus, it attaches to a ribosome in the cytoplasm
- ribosome attaches at the start codon
- tRNA molecule with the complementary anticodon to the start codon aligns opposite the mRNA, held in place by the ribosome
- ribosome will move along the mRNA molecule to enable another complementary tRNA to attach to the next codon on the mRNA
- the two amino acids that have been delivered by the tRNA molecule are joined by a peptide bond. This is catalysed by an enzyme and requires ATP
- this continues until the ribosome reaches the stop codon at the end of the mRNA molecule. This stop codon doesn’t code for an amino acid and therefore the ribosome detaches and translation ends
- the polypeptide chain is now created and will enter the Golgi apparatus for folding and modification
gene mutations
- change in base sequence of chromosomes
- can arise spontaneously during DNA replication
types of gene mutations
- substitutions
- deletion
substitution
- one base is swapped with another
e.g ATGCCT -> ATTCCT
deletion
one base is removed
e.g ATGCCT becomes ATCCT
effects of mutations
- degenerate nature of code means that some amino acids are coded for by more than one codon (triplet code)
- means that not all substitution mutations will result in a change in the amino acid sequence of the protein
how does deletion lead to mutation
- changes number of bases present, which causes a shift in all base triplets after it (frame shift mutations)
what increases rate of gene mutation
mutagenic agents
name of a mutation
- non-disjunction
- mutations in number of chromosomes can arise spontaneously by chromosome non -disjunction during mitosis
genetic diversity
number of different alleles of genes in a species or population
large number of alleles for a characteristic
high genetic diversity
how is genetic diversity within a population increased
- mutations in DNA forming new alleles, some advantageous, some not
- different alleles introduced into a population when individuals from another population migrate into it and reproduce (gene flow)
purpose of genetic diversity
- allow natural selection to occur
- if population has low genetic diversity, it may not be able to adapt to change in environment and whole population could be wiped out by a single event
genetic bottlenecks
- event that causes a BIG REDUCTION in a population
- reduces number of alleles in gene pool and so reduces genetic diversity
- survivors reproduce and larger population is created from few individuals
founder effect
- describes what happens when a few organisms from a population start a new colony and there’s only a small number of different alleles in the initial gene pool
- frequency of each allele in new colony might be different to frequency of these alleles in the original population
- may lead to higher incidence of genetic disease
principles of natural selection in evolution of a population
- randomly occurring mutations sometimes result in a new allele being formed
- can be harmful which means that the mutated allele quickly dies out
- however, some mutations can be beneficial to an organisms survival, its frequency in the population increases, leading to reproductive success - natural selection
- advantageous allele inherited by members of next generation
- as a result, over many generations, the new allele increases in frequency in the population
types of adaptations caused by natural selection
- behavioural
- physiological
- anatomical
behavioural adapation
- way an organism acts that increases chance of survival
e.g possums play dead, scorpions dance before mating (ensures they attract same species)
physiological adaptation
- processes inside an organism’s body that increases chance of survival
e.g brown bears hibernate, conserves energy
anatomical adaptation
- structural features of an organism’s body that increases chance of survival
e.g otters = streamlined shape, easy to glide through water
types of natural selection
- directional selection
- stabilising selection
directional selection
where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce.
- could be in response to an environmental change
example of directional selection
bacteria evolving antibiotic resistance:
- some individuals in a bacterial population have alleles that give them resistance to an antibiotic
- population is exposed to the antibiotic, killing the bacteria without resistance allele
- resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to offspring
stabilising selection
- where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
- occurs when environment isn’t changing and it reduces the range of possible characteristics
example of stabilising selection
human birth weight:
- very small babies are less likely to survive - high SA:V ratio. means its hard for them to maintain body temp
- very large babies are less likely to survive too. large babies = hard to fit through mother’s pelvis. leads to complications
classification definition
- arrangement of organisms into group of various sizes on the basis of shared features
taxonomy definition
form of classification that focuses on similarities between different species, for ease of naming and identification
phylogeny definition
classification of organisms by these evolutionary relationships, so that every group shares a common ancestor
products of meiosis
produces daughter cells that are genetically different from each other
what are gametes
male sperm cells
female egg cells
what is a zygote
- when a sperm cell and an egg cell have joined together in fertilsiation
what cells have the diploid number of chromosomes
normal body cells - 23 chromosome pairs
which cells have a haploid number of cells
sperm and egg cells - 23 chromosomes (gametes)
why is random fertilisation important
- produces zygotes with different combinations of chromosomes
- increases genetic diversity within a species
process of meiosis
- before; DNA unravels and replicates (2 copies of each chromosome)
- DNA condenses; made of 2 sister chromatids joined by a centromere
- meiosis I; chromosomes arrange themselves into homologous pairs
- pairs are separated; having number of chromosomes and creating two cells
- meiosis II; pairs of sister chromatids (that make up a chromosome) are separated
- 4 haploid cells that are genetically different are produced
explain process of crossing over in meiosis I
- homologous pairs of chromosomes pair up
- chromatids twist around each other and parts swap over
- still contain same genes but different alleles
2 main ways that lead to genetic variation
crossing over
independent segregation of chromosomes
how does independent segregation lead to genetic variation
when homologous pairs are separated during meiosis I, it is completely random which chromosome ends up with which daughter cell
differences between meiosis and mitosis
mitosis;
- produces cells with same number of chromosomes as parent cell
- daughter cells genetically identical to parent cell
- produces 2 daughter cells
meiosis;
- produces cells with haploid number of chromosomes
- daughter cells genetically different to parent cell and one another
- produces 4 daughter cells
what are chromosome mutations caused by
caused by errors in cell division when cells end up with different number of chromosomes than 23 whole ones
what does non-disjunction chromosome mutation cause
non disjunction = failure of chromosomes to separate properly
non disjunction of chromosome 21 can lead to downs syndrome
