topic 4A - DNA, RNA and protein synthesis Flashcards
DNA in prokaryotic cells
DNA molecules are short, circular and not associated with proteins
DNA in eukaryotic cells
DNA molecules are very long, linear and associated with proteins, called histones
-together a DNA molecule and its associated proteins form a chromosome
what DNA do the mitochondria and chloroplasts of eukaryotic cells contain?
DNA like the DNA of prokaryotes:
short, circular and not associated with protein
what is a gene?
a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA
DNA is a p____
polymer
which 3 parts make up a nucleotide?
-a phosphate group
-a 5 carbon sugar (a pentose), called deoxyribose
-an organic nitrogenous base
what is the locus?
the fixed position of a gene on a chromosome
what is a triplet/codon?
a sequence of three DNA bases that codes for a specific amino acid
how many amino acids are there?
20
how many different amino acids should be able to be made?
there are four bases so there are 64 different triplets possible (4^3), yet there are only 20 amino acids
-this is because multiple codon code for the same amino acids
what does the code being universal mean?
each triplet codes for the same amino acid in all organisms
(almost every organism uses the same code)
what does the code being degenerate mean?
most amino acids are coded for by more than one triplet
what does the code being non-overlapping mean?
each region is read discretely
in eukaryotes does all of the DNA code for polypeptides?
-in eukaryotes, much of the nuclear DNA does not code for polypeptides
-there are many non coding regions
what are introns?
non-coding regions within genes
what are exons?
the coding sequences
what separates exons within a gene?
one or more non-coding sequences (introns)
how are introns removed?
-during transcription, eukaryotic cells transcribe the whole gene (all introns and exons) to produce pre-mRNA molecules
-before the pre-mRNA exits the nucleus, splicing occurs, the introns are removed exons are joined together
what is the genome?
the complete set of genes present in a cell
why is every gene not expressed in every cell?
-the full genome is present within every cell of an organism, but not every gene is expressed in every cell
-which genes are expressed depends on the cell type
what is the proteome?
the full range of proteins that a cell is able to produce
is the proteome or genome larger and why?
the proteome:
-large amount of post-translational modification of proteins
-alternative splicing
is RNA single or double stranded?
single
what is each RNA polynucleotide strand made up of?
-alternating ribose sugars and phosphate groups linked together
-the nitrogenous bases of each nucleotide project out sideways from the single-stranded RNA molecule
which bonds form the sugar phosphate backbone?
covalent bonds known as phosphodiester bonds
key facts: mRNA
-single-stranded molecule
-made up of a sugar-phosphate backbone and exposed unpaired bases
-uracil bases are present
-carries this information from DNA to the ribosomes (for translation)
key facts: tRNA
-single-stranded molecule
-sugar-phosphate backbone
-folded shape
-hydrogen bonds between some of the complementary bases
-amino acids bind to a specific region of the molecule
-the specific anticodon found on the tRNA molecule is complementary to a specific codon on an mRNA molecule
which two stages does proteinsynthesis occur in?
-transcription
-translation
simple definition of transcription
DNA is transcribed and an mRNA molecule is produced
simple definition of translation
mRNA (messenger RNA) is translated and an amino acid sequence is produced
where does transcription occur?
in the nucleus of the cell
the stages of transcription
1) dna helicase catalyses the breakdown of hydrogen bonds between complimentary base pairs & the DNA molecule unwinds
2) nucleotides are exposed
3) free RNA nucleotides pair up (via hydrogen bonds) with their complementary (now exposed) bases on the template strand of the ‘unzipped’ DNA molecule
4) RNA polymerase moves along bonds the sugar-phosphate groups of these RNA nucleotides to form the sugar-phosphate backbone of the mRNA molecule
5) the DNA bases re-join as the RNA polymerase moves along
6) the mRNA leaves the nucleus via a pore in the nuclear envelope
what is the template strand?
the strand of the DNA molecule that was originally present in the nucleus
exons & introns (eukaryotes vs prokaryotes)
-prokaryotes only contain exons
-eukaryotes contain both exons and intron
alternative splicing
-the exons of genes can be spliced in many different ways to produce different mature mRNA molecules
-a single eukaryotic gene can code for more than one polypeptide chain
-this is part of the reason why the proteome is much bigger than the genome
where does translation occur?
cytoplasm/ribosome
steps of translation
1) a ribosome attaches to the start codon on the mRNA
2) a tRNA molecule with a complementary anticodon to the start codon binds to the mRNA
3) the ribosome moves along to the next codon, it can fit around two at a
time, a tRNA anticodon pairs, bringing an amino acid
4) the process is repeated and two amino acids form a peptide bond between them, forming a polypeptide
5) the ribosome continues to move along the mRNA and tRNAs arrive
with amino acids at each codon until a stop codon is reached
6) the polypeptide is complete
what is required for protein synthesis?
functional RNA molecules
examples of functional RNA molecules:
mRNA
tRNA
rRNA
genes & protein
-the genes in DNA molecules control protein structure & protein function
-they determine the exact sequence in which the amino acids join together
other than amino acids, what can triplets code for?
start and stop signals
how to calculate the number of amino acids:
bases / 3
how to calculate mRNA nucleotides
amino acids x 3
what is a genetic mutation?
a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
do mutations usually make an effect, why?
-most mutations don’t alter the polypeptide / only alter it slightly so that its structure or function is not changed
-this is because the genetic code is degenerate
how can mutations alter polypeptides?
-DNA base sequence determines the sequence of amino acids that make up a protein
-mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for
2 types of mutation
-deletion
-substitution
what is deletion?
a mutation that occurs when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence
effect of a deletion:
-a deletion changes the amino acid that would have been coded for
-it has a knock-on effect by changing the triplets further on in the DNA sequence (frameshift)
-may change the ability of the polypeptide to function
what is substitution?
when a base in the DNA sequence is randomly swapped for a different base
effect of a substitution:
a substitution mutation will only change the amino acid for the triplet (a group of three bases)
three types of substitution:
-silent mutation
-missense mutation
-nonsense mutation
what is a silent mutation?
the mutation does not alter the amino acid sequence of the polypeptide
(this is because certain codons may code for the same amino acid)
what is a missense mutation?
the mutation alters a single amino acid in the polypeptide chain
what is a nonsense mutation?
the mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function
what are the different effects or gene mutations on polypeptides:
1) most mutations do not alter the polypeptide/only alter it slightly so that its appearance or function is not changed
2) a small number of mutations code for a significantly altered polypeptide with a different shape
↳ this may affect the ability of the protein to perform its function (eg: the shape of the active site on an enzyme changes)
what are natural mechanisms & what do they do?
-natural mechanisms that take place within cells to ensure the accuracy of DNA replication
-these mechanisms involve proofreading and repairing damaged DNA
when have natural mechanisms become ineffective?
when the mutation rate of a cell rises to above a normal level
what are mutagenic agents?
environmental factors that increase the mutation rate of cells
examples of mutagenic agents:
-high-energy radiation
-ionising radiation
-toxic chemicals
what is non-disjunction?
it occurs when chromosomes fail to separate during meiosis
(this occurs spontaneously)
what may the effects of non disjunction be?
-gametes will have an abnormal number of chromosomes compared to the normal haploid number
non disjunction & fertilisation:
if the abnormal gametes take part in fertilization, then a chromosome mutation occurs as the diploid cell will have the incorrect number of chromosomes
what does a chromosome mutation entail?
a change in the number of chromosomes
what does meiosis produce?
daughter cells that are genetically different from each other and to the parent cell (haploid gametes)
simple explanation of meiosis:
nuclear division that results in four cells (gametes) that have half the DNA (n) of the original organism (2n)
what are gametes used for?
to produce gametes which can then take
part in sexual reproduction
what happens during sexual reproduction?
-two haploid nuclei fuse & make a diploid nucleus
-a new unique organism with a different combination of alleles from each parent introduces genetic variation
simple steps of meiosis:
1) the DNA is replicated
2) homologous chromosomes are separated and the cell divides
(meiosis I, 2n)
3) each of the chromosomes divide into chromatids which enter four haploid cells
4) these haploid cells are non-identical to
each other and the parent cell
(meiosis II, n)
structure of chromosomes:
-chromosomes are held together at the centromere
-chromosomes are comprised of two chromatids
what do diploid cells have?
pairs of chromosomes:
one maternal and one paternal
what are the chromosomes in maternal & paternal pairs said to be?
-homologous
-these aren’t clones of each other as
one is from each parent
(blue = parent 1, red = parent 2)
what happens to this homologous pair?
-they pair up in meiosis
-the genes on these chromosomes are alleles of each other
-they code for the same protein, but these could be different versions e.g blue eyes (b) or brown eyes (B)
the genes of chromosomes as you go down the centromeres:
-gene for eye colour
-gene for enzyme A
-gene for cytochrome C
what is a homologous chromosome pair called?
a bivalent
what are alleles?
different forms of genes
alleles on a chromosome:
-different alleles of a gene have slightly different nucleotide sequences
-they still occupy the same position on the chromosome
why are the products of meiosis genetically different?
due to of independent assortment and crossing over
what happens during independent assortment?
each chromosome is inherited randomly and independent of other chromosomes (the inheritance of one chromosome does not affect the inheritance of another chromosome)
independent assortment:
(steps)
1) in meiosis I, homologous pairs of chromosomes line up opposite each other at the equator of the cell
2) it is random which side of the equator the paternal and maternal chromosomes from each homologous pair lie
3) these pairs are separated, so one of each homologous pair ends up in the daughter cell
the effect of independent assortment
there’s a large number of combinations of chromosomes that can be produced in the daughter cells
what is crossing over?
two homologous chromosomes come together and align, and then recombine and swap parts with each other
crossing over: steps
1) during meiosis I homologous chromosomes pair up and are in very close proximity to each other
2) the non-sister chromatids can cross over and get entangled
3) these crossing points are called chiasmata
4) the entanglement places stress on the DNA molecules
5) due to this, a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
what does crossing over result in?
-a new combination of alleles on the two chromosomes
-there is usually at least one or more chiasmata present in each bivalent during meiosis
how many divisions does meiosis have?
two:
meiosis I and meiosis II
what happens before meiosis?
interphase:
S phase – DNA replication
(G1 + G2) – new proteins and organelles are made
what are the stages of meiosis?
prophase I, metaphase I, anaphase I, telophase I, cytokinesis I, prophase II, metaphase II, anaphase II, telophase II, and cytokinesis II
prophase I & metaphase I
(meiosis)
-the chromosomes condense and become visible
-crossing over occurs
-the chromosomes line up in the middle of the cell
anaphase I
(meiosis)
the sister chromatids separate and are pulled to opposite poles of the cell by the spindle fibers
telophase I
(meiosis)
-chromatids rrive at opposite poles
-spindle fibres start to break down
-nuclear envelopes form around the two groups of chromosomes
how can telophase I sometimes be different in plants?
some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
cytokines I
(meiosis)
-the cytoplasm divides
-cell organelles also get distributed between the two developing cells
-two haploid cells are produced
animal cytokines
(meiosis)
the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
plant cytokines
(meiosis)
-vesicles from the golgi apparatus gather along the equator of the spindle
-the vesicles merge with each other to form the new cell surface membrane
-the vesicles also secrete a layer of calcium pectate which becomes the middle lamella
(layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell)
what happens between meiosis I and II
-there is no interphase, the DNA is not replicated
-the second division of meiosis is almost identical to the stages of mitosis
prophase II
(meiosis)
-the nuclear envelope breaks down and chromosomes condense
-a spindle forms at a right angle to the old one
metaphase II
(meiosis)
chromosomes line up in along the equator of the cell
anaphase II
(meiosis)
centromeres divide and individual chromatids are pulled to opposite poles
telophase II
(meiosis)
nuclear membranes form around each group of chromosomes
cytokines (II)
-cytoplasm divides as new cell surface membranes are formed creating four haploid cells
-each cell has a unique combination of chromosomes from the parent cell
-this results in the production of four genetically different daughter cells
what is the purpose of meiosis?
-having genetically different offspring can be advantageous for natural selection
what is the number of possible chromosomal combinations resulting from meiosis?
2n / 2^23
n is the number of homologous chromosome pairs
fertilisation
-during fertilization any male gamete can fuse with any female gamete to form a zygote
-this random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
what is the number of possible chromosomal combinations resulting from fertilisation?
(2^23)^2
calculate how many different chromosomal combinations can result from meiosis in a plant species which has a diploid number of 16. assume no crossing over occurs.
diploid number (2n) = 16
haploid number (n) = 16/2 = 8
2^8 = 256
derive a formula to calculate the number of combinations of chromosomes after the random fertilisation of an ovule and pollen nuclei from this plant species
(2^n)²
n = 8
(2^8)²
mitosis v meiosis
(similarities)
-forms of cell division
mitosis v meiosis
(purpose)
-mitosis contributes to the growth of an organism / replaces dead/dying cells
-produces genetically different gametes for sexual reproduction
mitosis v meiosis
(outcomes)
daughter cells:
mit = 2 | mei = 4
ploidy of daughter cells:
mit = 2n | mei = n
daughter identical to parent?
mit = yes | mei = no
mitosis
(chromosome content)
mitosis:
-two daughter cells genetically identical to each other and the parent cell
-important so that growth and cell replacement can occur within a body continually
-every cell in an organism’s body (other than gametes) contain exactly the same genetic material - the full genome
meiosis
(chromosome content)
-ends with four daughter cells all of which contain half the genetic material of the parent cell and are all different from each other and the parent
-this is important for genetic variation within families and the population
-genetic variation can reduce the risk of inheriting genetic diseases
meiosis
(chromosome content)
-ends with four daughter cells all of which contain half the genetic material of the parent cell and are all different from each other and the parent
-this is important for genetic variation within families and the population
-genetic variation can reduce the risk of inheriting genetic diseases
what does the random fertilisation of gametes do?
increase genetic variation
