genetic info, variation & relationships Flashcards
what is a gene? (3) (genetic info)
- a section of DNA at a locus (particular position) on a DNA molecule
- codes for the amino acid sequence of a polypeptide & a functional RNA (ribosomal & transfer)
- made up of a specific sequence of bases along the DNA molecule
why do scientists theorise that there are 3 bases that code for each amino acid? (5) (genetic info)
- only 20 amino acids regularly occur in proteins
- only 4 different bases (A, T, C, G) are present in DNA
- if each base coded for an amino acid, only 4 different amino acids could be coded for
- using a pair of bases, 16 (4 X4) different codes are possible (still inadequate)
- 3 bases produce 64 different codes (enough for 20 amino acids)
why is each different code called a triplet? (genetic info)
- it holds three bases for each amino acid
outline 8 features of the genetic code (genetic info)
- a few amino acids are coded for by a single triplet
- remaining amino acids are coded for between 2 & 6 triplets each
- code is known as a ‘degenerate code’ as most amino acids are coded for by more than 1 triplet
- a triplet is always read in one particular direction along the DNA strand
- the start of a DNA sequence that codes for a polypeptide is always the same (methionine). If this first molecule doesn’t form part of the final polypeptide chain it is later removed
- three triplets don’t code for any amino acids. These are ‘stop codes’ & mark the end of a polypeptide chain
- code is non-overlapping (each base sequence is read only once)
- code is universal (with a few minor exceptions each triplet codes for the same amino acid in all organisms). This is indirect evidence for evolution
outline the differences in DNA structure in prokaryotes & eukaryotes (6) (genetic info)
prokaryotes:
- circular
- not wrapped around histones
- made of nucleotides
- short
- plasmids
- no introns
eukaryotes:
- line at
- wrapped around histones
- made of nucleotides
- long
- no plasmids
- introns
which two eukaryotic sub-cellular structures have a similar form of DNA to prokaryotes? (genetic info)
- chloroplasts
- mitochondria
why do chromosomes appear as two attached chromatids at the start of division? (genetic info)
- because DNA has already been replicated to give two identical DNA molecules
- DNA is held by histones
what is a homologous pair of chromosomes? (4) (genetic info)
- a pair of matching chromosomes that have the same genes
- don’t necessarily have the same alleles
- referred to as the diploid number (46 in humans)
- one chromosome is provided by each parent
what is an allele? (3) (genetic info)
- one of a number of alternative forms of gene
- each individual inherits one allele from each parent
- can be the same or different
what happened when two alleles are different? (genetic info)
- each allele has a different sequence, therefore a different amino acid sequence, so produces a different polypeptide
what do base changes to a gene cause? (4) (genetic info)
- results in a new allele being produced, so a new sequence of amino acids being coded for
- this results in a different polypeptide being produced & therefore a different protein
- this protein may not function well/at all
- in an enzyme, it may change shape so its substrate no longer binds to it because the active site is no longer complimentary (causes issues for organism)
what is a codon? (genetic info)
- the sequence of 3 bases on mRNA that codes for a single amino acid
define ‘genome’ (genetic info)
- the complete set of genes in a cell
- includes DNA in the mitochondria & chloroplasts
define ‘proteome’ (genetic info)
- the full range of proteins produced by the genome
- sometimes refereed to as the complete proteome (in which case ‘proteome’ refers to the proteins produced by a given type of cell under a certain set of conditions
state what RNA is made up off & the two types that are important in protein synthesis (genetic info)
- made up of a ribose pentose sugar, an organic base (A, U, C, G) & a phosphate group
- messenger RNA (mRNA) & transfer RNA (tRNA) are important in protein synthesis
what is mRNA? Describe its role in protein synthesis (6) (genetic info)
- a long strand of mononucelotides that is arranged in a single helix
- base sequence in determined by the sequence of bases on a length of DNA during transcription
- there is many types
- once formed, mRNA leaves the nucleus via the nuclear pores (in the nuclear envelope) & enters the cytoplasm, where it associated with the ribosomes
- it then acts as a template for protein synthesis
- it’s structure it suited to its functions as it has info in the form of codons. This sequence of codons determines the amino acid sequence of a specific polypeptide that will be made
what is tRNA & what is its role in protein synthesis? (7) (genetic info)
- a small molecule that is made up of around 80 nucleotides
- single stranded chain folded into a clover-leaf shape (with one end of the chain extending beyond the other - this is the part of tRNA that an amino acid can easily bond to)
- there are many types, each of which binds to a specific amino acid
- at the opposite end of a tRNA molecule is the anticodon (a sequence of 3 other organic bases)
- the genetic code id degenerate so there must be as many tRNA molecules as there are coding triplets
- however each tRNA molecule is specific to an amino acid acid & has an anticodon that is specific to that amino acid
- tRNA is structurally suited to its role of lining up amino aids on the mRNA template during protein synthesis
what is an anticodon? (genetic info)
- a sequence of 3 organic bases at the end of a tRNA molecule opposite the end chain
What happens to tRNA molecules during protein synthesis? (genetic info)
- an anticodon pairs with 3 complimentary bases that make up the codon on the mRNA molecule
how are mRNA & tRNA suited to their roles in protein synthesis? (genetic info)
- mRNA is structurally suited to its role as it has info in the from of codons. This sequence of codons determines the amino acid sequence of a specific polypeptide that will be made
- tRNA’s end chain is for attaching amino acids & its anticodon is for complimentary base pairing with the codon of an mRNA molecule, so is suited to line up amino acids on the mRNA template
outline the differences in structure, function & composition of DNA, mRNA & tRNA * (8) (genetic info)
DNA:
- double polynucleotide chain
- largest molecule of the 3
- double helix
- pentose sugar is deoxyribose
- organic bases are A, T, C, G
- found mostly in the nucleus
- quantity is instant for all cells of a species (except gametes)
- chemically very stable
MRNA & tRNA:
- single polynucleotide chain
- mRNA = smaller than DNA but larger than tRNA, tRNA = smallest molecule of the 3
- mRA = single helix (except for a few viruses), tRNA = clover chapped molecule
- pentose sugar is ribose
- organic bases are A, U, C, G
- made in the nucleus bout found throughout the cell
- quantity varies from cell to cell & with level of metabolic activity
- mRNA = least chemically stable, tRNA = less chemically stable than DNA but more stable than mRNA
briefly outline the process of protein synthesis (4) (genetic info)
- DNA provides the instructions in the from of long sequences of bases
- a complimentary section of this sequence is made in the form of pre-mRNA (transcription)
- the pre-mRNA is spliced to from mRNA
- the mRNA is used as a template to which complimentary tRNA molecules attach & the animo acids they carry are linked to from a polypeptide (translation)
describe the process of transcription (4) (genetic info)
- 1) DNA helical breaks the hydrogen bonds between the 2 strands, causing them to separate & expose the nucleotides bases
- 2) complimentary RNA nucleotides move into place & from hydrogen bonds with the bases of the exposed nucleotides (C —> G, G —> C, A —> U, T —> A)
- 3) RNA polymerase joins the RNA nucleotides to the strand by forming phosphodiester bonds
- 4) when the RNA polymerase reaches ‘stop’ triplet code, it detaches & the pre-mRNA strand is complete
which type of cell does splicing take place in & why? (genetic info)
- eukaryotes
- introns are uncommon/don’t exist in prokaryotes
outline the process of slicing (2) (genetic info)
- intervening introns would prevent the synthesis of a polypeptide
- the base sequences corresponding to the introns are removed & the functional exons are joined together
outline the process of translation (6) (genetic info)
- once a molecule of mRNA has been transcribed, it moves out of the nucleus via a nuclear pore (in the nuclear envelope)
- in the cytoplasm, the mRNA combines with a ribosome
- amino acids become attached to tRNA molecules in the cytoplasm. Each tRNA molecule is specific to one amino acid & has an anticodon (sequence of 3 bases) that is complimentary to the codons on the mRNA molecule
- tRNA molecules attach to the ribosome & their anticodon pairs up with the appropriate codons on the mRNA strand
- the AAs transported by the tRNA molecules link together via peptide bonds & the tRNA molecules return to the cytoplasm to pick up more amino acids
- the ribosome moves along the mRNA strand & the AAs continue to join together until all the codons have been translated & the polypeptide is complete (ends when the ribosome reaches & recognised a ‘stop codon’
what happens to a completed polypeptide chain after protein synthesis? (3) (genetic info)
- the finished polypeptide is coiled or folded to produce its secondary structure
- the secondary structure is folded to produce the tertiary structure
- different polypeptide chains (along with any non-protein groups) are linked to form the quaternary structure
what is a gene mutation? (genetics)
- any change to one or more nucleotide bases, or a change in the sequence of bases, in DNA
outline & explain the 3 base substitution mutations (genetics)
- nonsense mutation: a base substitution that leads to a new codon that codes for a ‘stop’
- it leads to a truncated protein that is often unfunctional
- silent mutation: a base substitution that leads to a new codon that codes for the same amino acid as the original codon
- it has no effect on the protein
- Miss ensue mutation: a base substitution that leads to a new new codon that codes for a different amino acid to the original codon
- it has variable effects on the tertiary structure & function of a protein
which type of gene mutation is an example of a fame shift mutation? (genetics)
- base deletion
- base substitution is not a frame shift mutation
outline & explain a base deletion mutation (genetics)
- deletion mutation: a base deletion that causes each codon after to be read in a different reading frame
- a base is entirely removed
- has a big change to the primary sequence of the protein
what are diploid & haploid cells? (genetics)
- haploid = cells that only contain a single copy of each chromosome (gametes)
- diploid = cells in which the nucleus contains two sets of chromosomes
what is meiosis? (genetics)
- a process of cell division that produces 4 unique, haploid gametes (daughter cells)
- a diploid cell undergoes 2 divisions
why is meiosis important for sexual reproduction? (2) (genetics)
- in fertilisation, a haploid sperm nucleus fuses with a haploid egg nucleus, creating a zygote (which has the normal number of diploid cells)
- fertilisation is random - the zygote produced can have a random combination of chromosomes from both parents
how many sets of chromosomes does every diploid cell have? (genetics)
- 2 complete sets
- one comes from each parent
how many divisions occur in meiosis? (genetics)
- 2 nuclear divisions occur
what happens during meiosis 1 & 2? (genetics)
meiosis 1:
- introduces genetic diversity by randomly giving a cell’s genes in 2
- homologous chromosomes pair up & their chromatids wrap around each other
- equivalent proportions may be exchanged by crossing over
- by the end, the homologous pairs have separated, with one chromosome from each pair going into one of the daughter cells
meiosis 2:
- similar to mitosis
- each chromosome is split into its two chromatids
- chromatids move apart
- by the end, four unique daughter cells have been formed (in humans they contain 23 chromosomes)
state the 8 stage ‘meiosis chain’ (genetics)
- prophase 1
- metaphase 1
- anaphase 1
- telophase 1
- prophase 2
- metaphase 2
- anaphase 2
- telophase 2
state 2 ways in which meiosis brings about genetic variation (genetics)
- independent segregation of homologous chromosomes
- new combinations of maternal & paternal alleles by crossing over
outline the process of independent segregation during meiosis (5) (genetics)
- during meiosis 1 each chromosome lines up alongside its homologous partner
- when the arrange themselves in this line they do so at random
- one of each pair will pass to each daughter cells meiosis
- which one goes to the daughter cell (& which of any other pairs) depends of how the pairs are lined up in the parent cell
- since they are lined up at random, the combination of chromosomes of maternal & paternal origin that go into the daughter cell at meiosis 1 is also a matter of chance
how is variety produced from genetic combinations? (3) (genetics)
- each member of a homologous pair has exactly the same genes & therefore determines the same characteristics
- however, the alleles of the genes may differ
- the independent assortment of these chromosomes therefore produces new genetic combinations
what is the equation for number of possible chromosome combination? (genetics)
- 2 to the power of n
- n = number of pairs of homologous chromosomes
what is the equation for number of possible gamete combinations? (genetics)
- (2 to the power of n) squared
- n = number of pairs of homologous chromosomes
what are crossing over & recombination during meiosis? (genetics)
- crossing over = chromatids cross over one another many times
- recombination = broken off portions of chromatic recombine with another chromatid
outline the process of crossing over (5) (genetics)
- the chromatids of each pair become twisted around one another
- during this twisting process, tensions are created & portions of the chromatids break off
- these broken portions might then rejoin with the chromatids of its homologous partner
- usually it is the equivalent portions of homologous chromosomes that are exchanged
- in this way new genetic combinations of maternal & paternal alleles are produced
