dna genes and protein synthesis Flashcards
define genome
as the complete set of genes in a cell
Compare and contrast DNA in eukaryotic cells with DNA in prokaryotic cells
Similarities:
● Nucleotide structure is identical - deoxyribose attached to phosphate and a base
● Adjacent nucleotides joined by phosphodiester bonds, complementary bases joined by hydrogen bonds
● DNA in mitochondria / chloroplasts have similar structure to DNA in prokaryotes
○ Short, circular, not associated with proteins
Differences:
● Eukaryotic DNA is longer
● Eukaryotic DNA is linear, prokaryotic DNA is circular
● Eukaryotic DNA is associated with histone proteins, prokaryotic DNA is not
● Eukaryotic DNA contain introns, prokaryotic DNA does not
● Eukaryotic DNA contain introns, prokaryotic DNA does not
What is a chromosome?
● Long, linear DNA + its associated histone proteins
● In the nucleus of eukaryotic cells
What is a gene?
A sequence of DNA (nucleotide) bases that codes for:
● The amino acid sequence of a polypeptide
● Or a functional RNA (eg. ribosomal RNA or tRNA)
What is a locus
Fixed position a gene occupies on a particular DNA molecule
Describe the nature of the genetic code
Triplet code - A sequence of 3 DNA bases, called a triplet, codes for a specific amino acid
Universal - The same base triplets code for the same amino acids in all organisms
Non-overlapping - Each base is part of only one triplet so each triplet is read as a discrete unit
Degenerate - An amino acid can be coded for by more than one base triplet
What are ‘non-coding base sequences’ and where are they found?
Non-coding base sequence- DNA that does not code for amino acid sequences / polypeptides:
1. Between genes - eg. non-coding multiple repeats
2. Within genes- introns
In eukaryotes, much of the nuclear DNA does not code for polypeptides.
What
What are introns and exons?
Exon - Base sequence of a gene coding for amino acid sequences (in a polypeptide)
Intron - Base sequence of a gene that doesn’t code for amino acids, in eukaryotic cells
define proteome
as the full range of proteins that a cell is able to produce.
describe how a gene is a code for the production of a polypeptide
- (Because) base/nucleotide sequence;
- (In) triplet(s);
- (Determines) order/sequence of amino acid sequence/primary
structure (in polypeptide);
define the term exon
/triplet sequence coding for polypeptide
an intron is a non coding sequence of dna, where is it positioned in the genome
positioned between genes
differences between dna in nucleus of plant cell and dna in prokaryotic cell
- (Associated with) histones/proteins v no histones/proteins;
- Linear v circular;
- No plasmids v plasmids;
4, Introns v no introns; - Long(er) v short(er);
not all mutations in nucleotide sequence of a gene cause change in structure of polypeptide. why?
Triplets code for same amino acid
Occurs in introns /non-coding sequence;
compare and contrast dna in pro vs eu cells
compare:
Nucleotide structure is identical;
Nucleotides joined by phosphodiester bond;
DNA in mitochondria / chloroplasts same / similar (structure) to DNA
in prokaryotes;
contrast:
Eukaryotic DNA is longer;
5. Eukaryotic DNA contain introns, prokaryotic DNA does not;
6. Eukaryotic DNA is linear, prokaryotic DNA is circular;
7. Eukaryotic DNA is associated with / bound to protein / histones,
prokaryotic DNA is not;
what is a homologous pair of chromosomes
(Two chromosomes that) carry the same genes;
explain how number of chromosones is halved during meiosis
- Homologous chromosomes (pair);
- One of each (pair) goes to each (daughter)
cell / to opposite poles;
crossing over increases genetic diveristy explain how
- Homologous pairs of chromosomes associate
/ form a bivalent; - Chiasma(ta) form;
- (Equal) lengths of (non-sister) chromatids /
alleles are exchanged; - Producing new combinations of alleles;
Explain the meaning of
Degenerate
Non overlapping
Degenerate : more than one base triplet can code for the same amino acid
Non overlapping: each base is part of only one triplet
mRNA is used during translation to form polypeptides.
Describe how mRNA is produced in the nucleus of a cell.
- Helicase;
- Breaks hydrogen bonds;
- Only one DNA strand acts as template;
- RNA nucleotides attracted to exposed bases;
- (Attraction) according to base pairing rule;
- RNA polymerase joins (RNA) nucleotides together;
- Pre-mRNA spliced to remove introns.
Contrast the structures of mRNA and DNA
- DNA double
stranded/double helix and mRNA single-stranded;
Contrast requires both parts of the statement - DNA (very) long and RNA short;
Accept ‛RNA shorter’ or ‛DNA bigger/longer’ - Thymine/T in DNA and uracil/U in RNA;
- Deoxyribose in DNA and ribose in RNA;
R Deoxyribonucleic/ ribonucleic acid
Ignore ref. to histones
Ignore ref. to helix and straight chain alone - DNA has base pairing and mRNA doesn’t/ DNA has hydrogen
bonding and mRNA doesn’t; - DNA has introns/non-coding sequences and mRNA doesn’t;
Single base deletion can lead to a non func protein
Why
Mutation) changes triplets / codons after that point / causes frame shift;
Accept changes splicing site
Ignore changes in sequence of nucleotides / bases
2. Changes amino acid sequence (after this) / codes for different amino
acids (after this);
Accept changes primary structure
Reject changes amino acid formed / one amino acid changed
3. Affects hydrogen / ionic / sulfur bond (not peptide bond);
4. Changes tertiary structure of protein (so non-functional);
Mutation in intron causes whag
Intron non- coding (DNA) / only exons coding;
Context is the intron
Do not mix and match from alternatives
Neutral references to introns removed during splicing
1. and 2. Ignore ref. to code degenerate and get same / different
amino acid in sequence
2. (So) not translated / no change in mRNA produced / no effect (on protein)
/ no effect on amino acid sequence;
Accept does not code for amino acids
OR
3. Prevents / changes splicing;
4. (So) faulty mRNA formed;
Accept exons not joined together / introns not removed
5. Get different amino acid sequence;
Explain how the structure of DNA is related to its function
Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability
/ protects bases / protects hydrogen bonds;
Must be a direct link / obvious to get the mark
Neutral: reference to histones
2. Long / large molecule so can store lots of information;
3. Helix / coiled so compact;
Accept: can store in a small amount of space for ‘compact’
4. Base sequence allows information to be stored / base sequence codes for
amino acids / protein;
Accept: base sequence allows transcription
5. Double stranded so replication can occur semi-conservatively / strands can act
as templates / complementary base pairing / A-T and G-C so accurate
replication / identical copies can be made;
6. (Weak) hydrogen bonds for replication / unzipping / strand separation / many
hydrogen bonds so stable / strong;
Accept: ‘H-bonds’ for ‘hydrogen bonds’
differences in primary structure in haemoglobin molecules can provide evidence of phylogenic (evolutionary relationships between species
explain how (5)
- Mutations change base / nucleotide (sequence);
Reject if mutation in amino acid - (Causing) change in amino acid sequence;
- Mutations build up over time;
- More mutations / more differences (in amino acid / base / nucleotide sequence / primary structure) between distantly related species;
OR
Few(er) mutations / differences (in amino acid / base / nucleotide sequence / primary structure) in closely related species; - Distantly related species have earlier common ancestor;
OR
Closely related species have recent common ancestor;
Accept “order” for “sequence”
compare and contrast DNA structure (5)
Comparisons
1. Nucleotide structure is identical;
Accept labelled diagram or description of nucleotide as phosphate, deoxyribose and base
2. Nucleotides joined by phosphodiester bond;
OR
Deoxyribose joined to phosphate (in sugar, phosphate backbone);
3. DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes;
Accept shorter than nuclear DNA/is circular not linear/is not associated with protein/histones unlike nuclear DNA;
Contrasts
4. Eukaryotic DNA is longer;
5. Eukaryotic DNA contain introns, prokaryotic DNA does not;
6. Eukaryotic DNA is linear, prokaryotic DNA is circular;
7. Eukaryotic DNA is associated with / bound to protein / histones, prokaryotic DNA is not;
describe the role of a ribosome in the production of polypeptide
- mRNA binds to ribosome;
- Idea of two codons / binding sites;
- (Allows) tRNA with anticodons to bind / associate;
- (Catalyses) formation of peptide bond between amino acids (held by tRNA molecules);
- Moves along (mRNA to the next codon)
Describe the two stages of protein synthesis
Transcription - Production of messenger RNA (mRNA) from DNA, in the nucleus
Translation - Production of polypeptides from the sequence of codons carried by mRNA, at ribosomes
Compare and contrast the structure of tRNA and mRNA
Comparison (similarities)
● Both single polynucleotide strand
Contrast (differences)
● tRNA is folded into a ‘clover leaf shape’
, whereas
mRNA is linear / straight
● tRNA has hydrogen bonds between paired bases,
mRNA doesn’t
● tRNA is a shorter, fixed length, whereas mRNA is a
longer, variable length (more nucleotides)
● tRNA has an anticodon, mRNA has codons
● tRNA has an amino acid binding site, mRNA doesn’t
Describe how mRNA is formed by transcription in eukaryotic cells
- Hydrogen bonds between DNA bases break
Only one DNA strand acts as a template - Free RNA nucleotides align next to their complementary bases on the template strand
- ● In RNA, uracil is used in place of thymine (pairing with adenine in DNA)
RNA polymerase joins adjacent RNA nucleotides - This forms phosphodiester bonds via condensation reactions
- Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA
Describe how production of messenger RNA (mRNA) in a eukaryotic cell is
different from the production of mRNA in a prokaryotic cell
● Pre-mRNA produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells
● Because genes in prokaryotic cells don’t contain introns so no splicing in prokaryotic cells
Describe how translation leads to the production of a polypeptide
- mRNA attaches to a ribosome and the ribosome moves to a start codon (AUG)
- tRNA brings a specific amino acid
tRNA anticodon binds to complementary mRNA codon - Ribosome moves along to next codon and another tRNA binds so 2 amino acids can be joined by a condensation reaction forming a peptide bond
- ● Using energy from hydrolysis of ATP
tRNA released after amino acid joined polypeptide - Ribosome moves along mRNA to form the
polypeptide, until a stop codon is reached
Role of atp in translation
Hydrolysis of atp to adp and pi releases energy
So amino acids join to tRNAs and peptide bonds from between amino acids
Role of tRNA in translation
Transports a specific amino acid in relation to its anticodon, tRNA anticodon complementary to base pairs to mRNA codon, forming hydrogen bonds
2 tRNAs bring amino acid together so peptide bond can form
Role of ribosomes in translation
- mRNA binds to ribosome
- allows tRNA anticodons to bind
- catalyses formation of peptide bond between amino acids (held by tRNA molecules)
- moves along to next codon
What is a gene mutation?
● A change in the base sequence of DNA (on chromosomes)
● Can arise spontaneously during DNA replication (interphase)
What is a mutagenic agent?
A factor that increases rate of gene mutation, eg. ultraviolet (UV) light or alpha particles.
Explain how a mutation can lead to the production of
a non-functional protein or enzyme
- Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA
- So changes sequence of amino acids in the polypeptide
- So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
- So changes protein tertiary structure (shape) of protein
- Enzymes- active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form
Explain the possible effects of a substitution mutation
- Base / nucleotide in DNA replaced by a different base / nucleotide
- This changes one triplet so changes one mRNA codon
- So one amino acid in polypeptide changes
● Tertiary structure may change if position of hydrogen / ionic
/ disulphide bonds change
OR amino acid doesn’t change
● Due to degenerate nature of genetic code (triplet could
code for same amino acid) OR if mutation is in an intron
Explain the possible effects of a deletion mutation
- One nucleotide / base removed from DNA sequence
- Changes sequence of DNA triplets from point of mutation (frameshift)
- Changes sequence of mRNA codons after point of mutation
- Changes sequence of amino acids in primary structure of polypeptide
- Changes position of hydrogen / ionic / disulphide bonds in tertiary
structure of protein - Changes tertiary structure / shape of protein
Describe features of homologous chromosomes
Same length, same genes at same loci, but may have different alleles.
Describe the difference between diploid and haploid cells
● Diploid- has 2 complete sets of chromosomes, represented as 2n
● Haploid- has a single set of unpaired chromosomes, represented as n
Describe how a cell divides by meiosis
In interphase, DNA replicates → 2 copies of each chromosome (sister chromatids), joined by a centromere.
1. Meiosis I (first nuclear division) separates homologous chromosomes
● Chromosomes arrange into homologous pairs
● Crossing over between homologous chromosomes
● Independent segregation of homologous chromosomes
2. Meiosis II (second nuclear division) separates chromatids
● Outcome = 4 genetically
varied daughter cells
● Daughter cells are
normally haploid (if
diploid parent cell)
Draw
Explain why the number of chromosomes is halved during meiosis
Homologous chromosomes are separated during meiosis I (first division)
Explain how crossing over creates genetic variation
● Homologous pairs of chromosomes associate / form a bivalent
● Chiasmata form (point of contact between (non-sister) chromatids)
● Alleles / (equal) lengths of (non-sister) chromatids exchanged between chromosomes
● Creating new combinations of (maternal & paternal) alleles on chromosomes
Explain how independent segregation creates genetic variation
● Homologous pairs randomly align at equator → so random which chromosome from each pair
goes into each daughter cell
● Creating different combinations of maternal & paternal chromosomes / alleles in daughter cells
Other than mutation and meiosis, explain how genetic variation within a
species is increased
● Random fertilisation / fusion of gametes
● Creating new allele combinations / new maternal and paternal chromosome combinations
Explain the different outcomes of mitosis and meiosis
- Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells
● As 1 division in mitosis, whereas 2 divisions in meiosis
Mitosis maintains the chromosome number (eg. diploid → diploid or haploid → haploid)
whereas meiosis halves the chromosome number (eg. diploid → haploid)
● As homologous chromosomes separate in meiosis but not mitosis
Mitosis produces genetically identical daughter cells, whereas meiosis produces
genetically varied daughter cells
● As crossing over and independent segregation happen in meiosis but not mitosis
- Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells
Explain the importance of meiosis
● Two divisions creates haploid gametes (halves number of chromosomes)
● So diploid number is restored at fertilisation → chromosome number maintained between generations
● Independent segregation and crossing over creates genetic variation
How can you recognise where meiosis and mitosis occur in a life cycle?
● Mitosis occurs between stages where chromosome number is maintained (eg. diploid (2n)→ diploid (2n)
OR haploid (n)→ haploid (n))
● Meiosis occurs between stages where chromosome number halves (eg. diploid (2n)→ haploid (n))
Describe how mutations in the number of chromosomes arise
● Spontaneously by chromosome non-disjunction during meiosis
● Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis
● So some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)
Suggest how the number of possible combinations of chromosomes in
daughter cells following meiosis can be calculated
2n where n = number of pairs of homologous chromosomes (half the diploid number)
Suggest how the number of possible combinations of chromosomes
following random fertilisation of two gametes can be calculated
(2n)2 where n = number of pairs of homologous chromosomes (half the diploid number)
