story of us - part 2 Flashcards
how do cells divide
via mitosis
what must cells do before they divide and what is that process called
make an exact copy of each DNA molecule in the nucleus
process is called replication
what does each cell formed in replication receive
the same amount and type of DNA
DNA replication:
1) DNA helicase (enzyme) unravels the double helix - separating hydrogen bonds holding polynucleotide strands of DNA together
2) each strand acts as a template for the formation of a new strand of DNA
3) free DNA nucleotides in the nucleus align with complementary bases on both strands, according to the base-pairing rule
4) DNA polymerase (enzyme) forms the sugar phosphate backbone (fills gaps between bases to form backbone)
5) two identical DNA molecules are formed - each contains a new strand from the parent DNA and a new complementary strand
in protein synthesis what is the strand of DNA molecules that codes for the manufacture of proteins in a cell called
the template strand
in protein synthesis what is the strand of DNA molecules that doesn’t code for the manufacture of proteins in a cell called
the non-template strand
functions of proteins:
- hormones
- enzymes - controls processes in cells
- haemoglobin
- collagen - structures in skin, keratin in har, myosin in muscles, structural proteins
what are proteins made of
chains of amino acids
what codes for one amino acid
a sequence of three bases in the template strand of the DNA
how is the DNA code a triplet code
three bases are needed to code for one amino acid
DNA code is universal - what does this mean?
the same in all organisms
how many different types of amino acids are there
20
how is DNA degenerate
- more than one code
- 64 different amino acids could be coded for, but only need 20
how is DNA read
as non-overlapping code
why can DNA not leave the nucleus
it may get damaged
where does protein synthesis take place
in ribosomes
what does protein synthesis taking place in ribosomes mean for proteins to be made
for proteins to be made, the genetic code must be copied, then transferred out of the nucleus into the cytoplasm
what transfers out of the nucleus instead of DNA
RNA
differences between DNA and RNA
- RNA is single-stranded, DNA is double-stranded
- in RNA the sugar is ribose, not deoxyribose
- RNA has uracil instead of thymine
similarities between DNA and RNA
- both have A/C/G nitrogenous bases
- both polynucleotides (made up of nucleotides)
- both have phosphate
what 3 types of RNA take part in protein synthesis
- messenger (mRNA)
- transfer (tRNA)
- ribosomal (rRNA)
mRNA:
forms a copy of the DNA code
-> formed in nucleus by copying DNA code
tRNA:
carries amino acids to the ribosomes to make the protein
rRNA:
where protein synthesis occurs
what 2 stages does protein synthesis take place in
transcription
translation
where does transcription take place
in the nucleus
where doe translation take place
in the ribosomes in the cytoplasm
in transcription what forms the genetic code
the base sequence of bases in DNA
what is transcription
the copying of the base sequence of DNA into the form of mRNA
transcription:
1) in the chromosome, part of the DNA double helix unwinds and ‘unzips’ so that the two strands separate, exposing the bases along the template strand. the enzyme DNA helicase is responsible for breaking apart the hydrogen bonds between the two strands of DNA. the DNA molecules unwinds, exposing a number of bases. template strand of DNA
2) RNA polymerase binds to the template strand and uses it as a template for synthesising a molecule of mRNA
3) building blocks of mRNA are free RNA nucleotides. complimentary RNA nucleotides are aligned against the DNA strand. the RNA nucleotides line up alongside the template strand according to the complementary base pairing rules. hydrogen bonds form between complementary RNA and DNA bases
4) enzyme RNA polymerase moves along the strand and joins the nucleotides together to form a pre-mRNA molecule. the RNA polymerase catalyses the formation of phosphodiester bonds. Bonds form between their ribose and phosphate groups, joining together to make the sugar-phosphate backbone of the molecule between neighbouring RNA molecules hence creating a new strand of mRNA
5) as the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strand rejoins behind it. as a result, only about 12 base pairs on the DNA strand are exposed at any one strand
6) when the RNA polymerase reaches a particular sequence of bases on the DNA it recognises as a ‘stop’ triplet code, it detaches. and the production of pre-mRNA is complete
7) DNA helix then ‘zips up’ again
8) once the section of DNA corresponding to a protein has been transcribed, the mRNA molecule leaves the DNA and passes out of the nucleus to the cytoplasm. it leaves through holes in the nuclear membrane - the nuclear pores and is now attracted to a ribosome
what is the enzyme responsible for transcription
RNA polymerase
what does the template strand do
forms a framework upon which a molecule of mRNA is formed
what are the building blocks of mRNA
RNA nucleotides
what are the bases in DNA
- adenine
- thymine
- cytosine
- guanine
what are the bases in RNA
- adenine
- uracil
- cytosine
- guanine
how do RNA nucleotides line up alongside the template strand
according to the complementary base-pairing rules
what happens to form an mRNA molecule
the RNA nucleotides link up to form an mRNA molecule. bonds form between their ribose sugar and phosphate groups, joining together to make the sugar phosphate backbone of the molecule
how does the mRNA molecule leave the DNA and passes out the nucleus
it leave through holes in the nuclear membrane
why is the triplet code of DNA converted into a triplet code in the mRNA
because of complementary base pairing
what is translation
the conversion of the mRNA into a protein
wher does translation take place
the ribosomes
translation:
1) a ribosome becomes attached to the starting codon at one end of the mRNA molecule
2) first tRNA molecule (with the complementary anticodon sequence) moves to the ribosome and binds to the mRNA. it does so at the start start codon using complementary base pairing. the tRNA molecule carries a specific amino acid
3) a second tRNA molecule binds with the next complementary codon on the mRNA strand. the anticodon of the second tRNA binds to the next codon on the mRNA. it brings with it a second amino acid
4) a peptide bond forms between the amino acids
5) the first tRNA molecule is released
6) the ribosome moves along the mRNA, bringing another two tRNA molecules at any time, each pairing up with the corresponding codons on the mRNA
7) as two more tRNA molecules arrive at the mRNA and add their amino acids to the growing chain a polypeptide chain is formed (protein)
8) the process continues this way, with up to 15 amino acids being added each second - until the polypeptide chain is built
9) the synthesis of the polypeptide continues until a ribosome reaches a stop codon at the end of the chain, and it is released. the ribosome, mRNA and last tRNA molecule all separate and the polypeptide chain is released
what is a mutation
a change in the base sequence of DNA
when do mutations occur
randomly
what can sometimes happen when a DNA is replicating and what result can this have
- sometimes when DNA is replicating, mistakes are made and the wrong nucleotide is used
- this can alter the sequence of bases in a gene, resulting in the gene coding for the wrong amino acid and therefore wrong protein
what do mutations give rise to
give rise to changes in DNA sequence, which will be passed onto offspring and increase variation in species
duplications (genetic mutation):
- the nucleotide is inserted twice instead of once
- entire base sequence is altered - each triplet after the point where the mutation occurs is changed
- impact on organism: whole gene is different + will now code for an entirely different protein
deletion (genetic mutations):
- a nucleotide is missed out
- whole base sequence is missed out - each triplet after the mutation is altered and whole gene is different
- impact on organism: codes for a different protein
substitution (genetic mutation):
- different nucleotide is used
- triplet of bases in which the mutation occurs is changed and it may code for a different amino acid. if it does, the structure of the protein molecule will be different. this may be enough to produce a significant alteration in the functioning of protein or lack of function.
- however it may also not code for a different amino acid as most amino acids have more than one code. the protein could therefore have a normal structure and function
- impact on organism: may or may not have an impact - genetic code is degenerate
inversion (genetic mutation):
- sequence of bases in a triplet is reversed
- triplet of bases in which the mutation occurs is changed and it may code for a different amino acid. if it does the structure of the protein will be different. this may be enough to produce a significant alteration in the functioning of protein or lack of function
- however it may also not code for a different amino acid as most amino acids have more than one code. the protein could therefore have normal structure and function
- impact on organism: depends if it codes for same amino acid - if code is degenerate or not
what happens if mutations are very harmful
the cell is likely to die and the mutation will be lost
what happens if the mutation does not affect the functioning of the cell in a major way
the cell may not die
if this cell divides again the a group of cells containing the mutant gene is formed. when the organism dies this mutation is lost
what happens if the mutation occurs in the gametes or in cells that divide to form the gametes
the mutation can be passed on to the next generation. this is how genetic diseases begin
examples of genetic diseases
- polydactyly
- achondroplasia
- cystic fibrosis
- sickle cell anaemia
- huntington disease
when are mutations advantageous
- insects resistant to insecticides
- antibacterial resistance
- camouflage - moths
what can increase the number of gene mutations
mutagens
examples of mutagens
- ionising radiation (UV light, x-rays, gamma rays)
- chemicals (including carcinogens, processed foods and preservatives, cosmetics and cleaning products)
- infectious agents (viruses, bacteria)
evolution definition
a change in the inherited characteristics of a population over time through the process of natural selection, which may result in the formation of new species
how does evolution happen
- the best suited organisms survive to reproduce
- this means that the characteristics that give an organism a better chance of survival are passed onto the next generation
- as a result fewer of the organisms which are less suited to the environment survive to reproduce
- the next generation will have more organisms that are better adapted and fewer organisms that are less adapted to
- this is repeated in every generation
why is it better to have a variety in organisms rather than lots of organisms that are the same
if there is an environmental change then the organism that is best adapted could change
what does the phrase survival of the fittest mean
those that are the best adapted to an environment survive and reproduce, so pass on alleles to the next generation
variation definition
all living things in a species are not the same
competition definition
there is not enough food or space for all of them
adaptation definition
some individuals have features which help them survive
reproduction definition
they are more likely to have offspring - more of the next generation have the useful feature
how are giraffes adapted
- a mutation in the DNA of some giraffes causes their necks to grow longer
- these giraffes are better adapted to their environment as they are able to reach the leaves on the highest trees and survive
- the giraffes that have the short necks cannot get the food and die
- the surviving giraffes reproduce and pass on the alleles for the longer neck to their offspring
- this process is repeated over and over and the longer necks are seen throughout the species
natural selection in peppered moths:
- mutation caused variation in coloured wings of moths
- change in environment -> results in a change of the beneficial phenotype
- dark coloured moths have advantage
- survive and reproduce to pass on alleles to next generation
- repeat over several generations -> change in phenotype that’s most common
what is antibiotic resistance
when a bacterium that used to be killed by antibiotics isn’t affected by it anymore. resistance starts when a random mutation gives bacterium resistance to a particular antibiotic
resistance bacteria have an advantage over the non-resistant bacteria of the same type. as bacteria reproduce so rapidly the resistant bacteria populations increases rapidly
what are antibiotics
chemicals to treat bacterial infections
what do antibiotics treat
bacteria ONLY
doesn’t work on viruses because they don’t have a cell wall
why is it dangerous to give antibiotics to treat minor ailments
if they are given when people don’t need them, there is an increased risk of bacteria mutating and becoming resistant
what is bacterial resistance a major problem today
they have a selective advantage as they are ‘fitter’ than the non-resistant bacteria. therefore, the bacteria have evolved as a result of antibiotic overuse / natural selection
example of antibiotic resistance
MRSA outbreak
-> can infect wounds and is difficult to treat without antibiotics
how does antibiotic resistance occur
1) a mutation occurs in the bacterium
2) this mutation conveys resistance to an antibiotic for the bacterium
3) when antibiotics are added to a medium, the bacterium with the resistance will survive
4) those bacteria without resistance will die
5) surviving bacteria reproduce rapidly by mitosis and the gene for antibiotic resistance is passed on to the offspring bacteria
6) if reproduction is allowed to continue for long enough, this specific species of bacteria may become resistant to this specific antibiotic
why are doctors reluctant to prescribe antibiotics
increased resistance -> less likely to work against bacteria in the future
how can we prevent antibiotic resistance from spreading
- don’t give others antibiotics
- use full prescription
- don’t use in farming
why is it easier to use bacteria or insects to study natural selection
reproduce rapidly
what is a stem cell
an undifferentiated cell of an organism that can divide to produce many more cells of the same type
eg. more undifferentiated stem cells
what process do stem cells undergo to develop into other types of cells
differentiation
what are the different type of stem cells in humans
- embryonic stem cells
- adult stem cells
embryonic stem cells:
- taken from embryo
-> egg 3-5 days after fertilisation
-> IVF fertilised eggs -> ethical issues - pluripotent (ability to differentiate into any type of cell except umbilical placenta)
why are embryonic stem cells important
they help o form all the different tissues and organs needed during development to form a whole new individual
adult stem cells:
- multipotent (can differentiate into a limited range of cell types)
-> eg. bone marrow stem cells can only differentiate into blood cells - less ethical issues
what are adult stem cells used for
- can only differentiate to produce a few different cell types
- they are predominantly used to replace cells lost through damage or to produce new cells for growth
where are stem cells found in plants
the root and shoot tips, in the meristem tissue
what are meristem cells
unspecialised cells that can differentiate into the cells needed by the plant in regions where growth is occurring
what can plant stem cells do throughout their life
plant stem cells retain the ability to differentiate into any type of plant cell throughout the life of the plant
it is possible to use plant stem cells to clone plants with desired characteristics
this could be useful if plants had resistance to a particular disease
adult stem cells in medicine:
- adult stem cells can be cultured in the lab and made to differentiate into specialised cells (predominantly blood cells) but into fewer cell types than is possible with embryonic stem cells
-> eg. somatic cells (skin cells) can become induced to become pluripotent -> can differentiate into any cell type - stem cells are already used to treat some diseases (eg. leukaemia) but there is a huge potential for stem cells to be used to cure many more diseases in the future (eg. diabetes and paralysis)
embryonic stem cells in medicine:
- modern scientific techniques mean that it is possible to grow human embryos in the lab and extract embryonic stem cells from them
- these embryonic stem cells can then be stimulated to differentiate into most types of specialised cell
- as a result, they are potentially very effective in treatment of certain diseases or to repair damaged organism by growing new tissue from stem cells
how could stem cells be used to treat diabetes (type I)
stem cells could be differentiated into insulin - producing pancreatic cells which are transplanted into the patients body
how could stem cells be used to treat paralysis
stem cells could be differentiated into nerve cells (neurones) which are transplanted into the damaged region of the nervous system
benefits of using stem cells
- great potential to treat a wide variety of diseases
- organs developed from a patients own stem cells reduces the risk of organ rejection and the need to wait for an organ donation
- adult stem cells are already used successfully in a variety of treatments acting as proof of benefits
risks/issues of using stem cells
- stem cells cultured in the lab could become infected with a virus which could be transmitted to the patient
- there is a risk of cultured stem cells accumulating mutations that can lead to them developing into cancer cells
- low numbers of stem cell donors
social issues of using stem cells
- it is possible for embryonic stem cells to be collected before birth (from amniotic fluid) or after birth (umbilical chord blood) and stored by a clinic, but this can be expensive and isn’t an option for everyone
- a lack of peer reviewed clinical evidence of the success of stem cell treatments
- educating the public sufficiently about what stem cells can and cannot be used for
ethical issues of using stem cells
- stem cells may be sourced from unused embryos produced in IVF
-> it is right to use them? who gives permission? - is it right to create embryos through therapeutic cloning and then destroy them? who owns the embryo?
- should an embryo be treated as a person with human rights? or as a commodity?
- lack of consent
- potential for clones
