Genetic Information, Variation And Relationships Flashcards
What is a gene
A section of DNA that codes for a polypeptide
Where is a gene located
On a particular position called a locus
What is the gene code
The specific sequence of bases along the DNA molecule
What is every 3 bases coding for 1 amino acid called
The triplet code
Features of genetic code
- it’s degenerate
- the start of a DNA sequence that codes for a protein is always is always the same triplet (it codes for the AA methionine)
- there are ‘stop codes’
- the code is non-overlapping
- the code is universal
What does the code being universal mean
It’s the same in all organisms
What are stop codes
The 3 triplets that don’t code for any amino acids and mark the end of a polypeptide chain
What does a degenerate code mean
Most amino acids are coded for by more than 1 triplet
What does non-overlapping mean
Each base belongs to one triplet
What exons
The coding sequences
What are introns
The non-coding sequences
DNA + chromosomes in eukaryotes
- the DNA is linear and associate with proteins called histones to form chromosomes
What are homologous chromosomes
- a pair of chromosomes, 1 maternal and 1 paternal, that have the same gene loci and so determines the same features
What is a diploid
The total number of chromosomes - 2n
(46 in humans)
What is the haploid number
23 in sperm and egg cell only - n
What is an allele
A different version of a gene
DNA in prokaryotes vs eukaryotes
- prokaryotic DNA is circular, not linear
- prokaryotic DNA is not associated with histones
- prokaryotic DNA doesn’t contain introns
Structure of messenger RNA (mRNA)
- a single strand made of nucleotides
- involved in carrying the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm
Structure of transfer RNA (tRNA)
- small molecule
- folded into a cloverleaf shape
- has one end where amino acids attach
- at the opposite end with a sequence of 3 bases known as the anticodon
- for each amino acid there’s a different sequence of bases known as the anticodon
What part of the template strand is DNA polymerase complimentary to
The 3’ end of the template strand so it can only move along the template strand and add nucleotides in the 3’ to 5’ direction
What does DNA polymerase only being complimentary to 3’ mean
That the new strand is built 5’ to 3’ because the strands are antiparallel
What does antiparallel strands mean
That while one strand is continuously built the other is built in the other direction in sections as the DNA is unwound
-> the DNA polymerase on the opposite template strand has to detach and re-attach so it often moves more slowly
What group is attached the 5’
Phosphate group
What group is the 3’ attached to
The hydroxyl group
What is antiparallel
One strand of the DNA runs in the 5’ to 3’ direction while the other runs the opposite way in the 3’ to 5’ direction
What is mRNA used for
To transfer the genetic code form the nucleus to the cytoplasm to undergo protein synthesis
What I transcription
Process of making pre-mRNA from DNA
Transcription process
- DNA Helicase breaks the hydrogen bonds on a section of DNA containing the gene for the desired protein (this exposes nuelotides)
- RNA Polymerase moves along one of the two DNA strands (the template strand) -> it joins complementary nucleotides in the exposed template strand to form pre-mRNA strand
- The DNA bases rejoin as the RNA Polymerase moves along
- Pre-mRNA can be spliced before it leaves the nuclear pore
Where does splicing happen
In eukaryotes because prokaryotes don’t have introns
Why does splicing happen
Because DNA contains regions of DNA that code for proteins (called exons) and introns which are regions of DNA that don’t code for proteins
What is splicing
When introns are removed from pre-mRNA to only leave exons
-> enzymes are used to splice the introns
Genome
The entire set of genes or genetic material present in a cell
Proteome
The entire set of proteins that can be expressed from those genes
Where does mRNA go after transcription
It travels to the ribosome where it’s translated into a polypeptide chain using tRNA
tRNA Structure
- short single strand of RNA
- folded into a clover leaf and held together by hydrogen bonds between paired bases
- has an amino acid binding site (3 exposed bases)
- has an anticodon binding site (series of 3 bases which are complimentary to the codon on the mRNA which codes for the amino acid the tRNA molecule carries
What is translation
The process of making a polypeptide from mRNA
Translation Process
- A ribosome attaches to the start codon on the mRNA -> a tRNA molecule with a complimentary anticodon to the start codon bonds to the mRNA (it has a specific AA attached (always methionine)
- The ribosome moves along to the next codon, it can fit around 2 at a time - the process is repeated and an enzyme catalyses the condensation reaction between the AAs to join them with a peptide bond, forming a polypeptide
- The ribosome continues to move along the mRNA and tRNAs arrive with AAs at each codon until a stop codon is reached -> the polypeptide is released
DNA Replication
- The enzyme DNA Helicase causes the two strands to separate by breaking the hydrogen bonds between the bases
- As Helicase completes the splitting of the strand, free nucleotides have been activated that are attracted to their complimentary bases
- Once the nucleotides are lined up they are joined together by DNA Polymerase, forming phosphodiester bonds
- Finally, all the nucleotides are joined to form a complete polynucleotide (2 identical strands of DNA are formed)
What is DNA replication described as
Semi-conservative
Why is DNA replication described as semi-conservative
Because half the original DNA is save and built into the new DNA molecule
Definition of mitosis
Cell divides into 2 daughter cells that are genetically identical to each other and the parent cell
Definition of meiosis
Cell division into 4 unique daughter cells with half the chromosomes of the parent cell
What are the stages of the cell cycle
- Interphase
- Mitosis
- Cytokinesis
What happens during interphase
- Cells grows and makes copies of all its DNA and organelles
- Longest phase
- During interphase the chromatin doesn’t form chromosomes -> instead the chromatin looks unfolded
What are all the stages of mitosis
- Prophase
- Anaphase
- Metaphase
- Telophase
What happens during prophase
- During prophase, DNA/chromatin condenses into chromosomes which are made up of 2 sister chromatids -> these are joined together at the centromere
- The nuclear envelope breaks down, and the centrioles move towards the opposite ends of the cell
What happens during anaphase
- During anaphase, chromatids are pulled to opposite ends of the cell by the spindle fibres shortening
What happens during metaphase
- During metaphase, spindle fibres attach to each side of the centromere of each chromosome and adjust the chromosomes so that they are lined up on the equator of the cell
What happens during telophase
- During telophase, nuclear envelopes reform around each set of chromosomes, and the chromosomes decondense back into dispersed chromatin
What happens during cytokinesis
- During cytokinesis, the entire cells splits into 2 new cells
- After cytokinesis is complete, the 2 daughter cells are genetically identical to one another and to the parent cell
What is a gene mutation
A change to one or more nucleotide bases in DNA
What are 2 ways DNA bases can change
Deletion and substitution
What is substitution of bases
Where a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
What does substitution of bases cause
- the base change results in a different amino acid being coded for
- a substituted base can lead to the same amino acid being formed due to the code being degenerate
What is the deletion of bases
When a nucleotide is lost from the DNA sequence
What does the deletion of bases lead to
- 1 deleted base at the beginning of the sequence can alter every triplet in the sequence (frame shift)
- a deleted base near the end has a smaller impact
What are chromosome mutations
Changes in the structure or number of whole chromosomes
What 2 forms can chromosome mutations take up
Polyploidy and non-disjunction
Explain polyploidy
- changes in whole sets of chromosomes
- when organisms have 3 or more sets of chromosomes
- occurs mostly in plants
Explain non-disjunction
- changes in number of individual chromosomes
- where individual homologous pairs of chromosomes fail to separate during meiosis
- results in a gamete having either one more or fewer chromosome
Causes of mutations
- arise spontaneously during DNA replication -> no outside influence
- mutation rate is increased by factors known as mutagenic agents or mutagens
-> high energy radiation that can disrupt the DNA molecule
-> chemicals that alter the DNA structure or interfere with transcription
What is a diploid cell
- cells that contain 2 of each chromosomes
- 46 (2n) in normal body cells
What is a haploid cell
- cells with only 1 copy of each chromosomes
- 23 (n) in gametes
How is a diploid cell formed
At fertilisation, when a haploid sperm will fuse with a haploid egg
What is crossing over
- during meiosis 1, the individual chromosomes of each homologous pair come into close contact with each other
- they twist
- wherever chromatids cross over is called a chiasma
What is independent segregation
- happens twice in meiosis 1 and 2 (metaphase)
- the position of each homologous pair is independent of all the other chromosomes
- the maternal and paternal chromosomes can be on either side of the equator
How does meiosis give rise to genetic variation
- Crossing over -> gives a different combinations of alleles
- Independent segregation -> gives a different combination of chromosomes
-> genetic variation is further increased by random fertilisation
Key differences between meiosis and mitosis
- In meisiois, at probase 1, the chromosomes can cross over
- In meiosis, there’s no DNA replication before prophase II
- Meiosis produced genetically different daughter cells
- Meiosis produces haploid daughter cells
What is genetic diversity
The total number of different alleles in a population
When does speciation occur
When there’s no interbreeding with other populations
Natural selection process
- there’s variation in a population
- due to mutations some individuals have favourable alleles which help them to survive and reproduce
- they pass on these favourable alleles to offspring, so the frequency of these alleles in the population increases
Types of adaptations
- Anatomical
- Physiological
- Behavioural
What is discontinuous variations
- variation due to genetic factors
- usually controlled by 1 gene
What is continuous variation
- variation due to environmental influences
- usually controlled by many genes (polygenes)
What are the 2 types of selection
- Stabilising Selection
- Directional Selection
What is stabilising selection
- If environment conditions remain stable, it is the individuals with the phenotype closest to the mean that are favoured
- These individuals are more likely to pass their alleles onto the next generation
- Those with phenotypes at the extremes are less likely to pass on their alleles
- Stabilising selection therefore tends to eliminate the phenotypes at the extremes
What is directional selection
- If the environmental conditions change, the phenotypes that are best suited to the new conditions at most likely to survive
- Some individuals, which fall to either the left or right of the mean, will possess a phenotype more suited to the new conditions -> these are more likely to survive
- Over time, the mean will move in the direction of these individuals
- Directional selection therefore results in phenotypes at 1 extreme of the population being selected for and those at the other extreme being selected against
What are the 3 stages of interphase
- First growth phase (G1) -> where proteins are synthesised
- Synthesis phase (S) -> when DNA is replicated
- Second growth phase (G2) -> where organelle grow + divide -> energy stores are increased
General stages of the cell cycle
- Interphase
- Nuclear division -> when the nucleus divides, either in 2 (mitosis) or 4 (meiosis)
- Cell division -> process by which whole cell divides in 2 (mitosis) or 4 (meiosis)
What is cancer and what does it cause
The result of damage to the genes that regulate mitosis and the cell cycle
-> causes uncontrolled cell division
-> most mutant cells die by survivors are capable of dividing and forming tumours
What controls the rate of mitosis
2 particular genes
-> a mutation in one of them results in uncontrolled mitosis
Importance of mitosis
- Growth -> when 2 haploid cells fuse to form diploid cell, it has the genetic info needed to form the new organism
- Repair
- Reproduction
What is cell division in prokaryotic cells called
Binary fission
Cell division in prokaryotic cells
- Circular DNA replicates and both copies attach to the cell membrane
-> plasmids also replicate - Cell membrane begins to grow between the 2 DNA molecules dividing the cytoplasm into 2
- A new cell forms, dividing the original cells into 2 identical daughter cells
- Each cell has a single copy of circular DNA and variable number of plasmids
Replication of viruses
- Attachment - the virus attaches to the surface of the host cell
- Entry - the viral DNA/RNA enters the histamine cell
- Replication - viral DNA/RNA replicates and new viral proteins are made (forming new capsids)
- Assembly - new viral particles are assembled
- Release - host cell lyses (bursts) releasing the newly made viruses
