Cellular control and variation Flashcards
what is a gene?
a section of DNA that codes for a polypeptide (e.g. proteins, enzymes)
what is meant by the term ‘genetic code’
The base sequence down the length of DNA codes for the sequence of amino acids in a protein.
what are the characteristics of the genetic code?
- The code is a triplet code (3 bases code of one amino acid also known as a codon),
- the code is degenerate (several different codons can code for the same amino acid) reducing chances of a nonfunctioning protein being produced because of a point mutation
- some codons are STOP codons, these indicate the end of a polypeptide chain.
- The code is non-overlapping- each codon read individually
- almost universal: the same codons code for the same amino acids in almost all organisms
describe how a nucleotide sequence within a gene is used to construct a polypeptide?
.Transcription:
DNC helix ‘unzips’ and ‘unwinds’. RNA polymerase matches free RNA nucleotides to form a molecule that is complementary to the exposed template strand of the DNA molecules (base pairing). This molecule is known as mRNA.
mRNA leaves the nucleus via nuclear pores to ribosomes.
enzymes attach amino acids to their specific tRNA molecule which has an anticodon (triplet of bases) as part of the molecule. This is called amino acid activation and requires ATP.
Translation:
The mRNa molecule binds ro a ribosome. The anticodons on tRNA molecules attach to codons (3 bases) on mRNA molecule (complementary base pairing) and the ribosome moves one codon along to attach a second tRNA molecule, the corresponding amino acid of which forms a peptide bond with the adjacent amino acid via a condensation reaction. As the ribosome moves along the mRNA molecule more amino acids are added to the end of the polypeptide until the ribosome reaches a stop codon which causes the polypeptide to break loose from the ribosome.
What changes can gene mutations cause to the protein formed?
they cause changes to the sequence of nucleotides in DNA molecules. This means that the triplet code for the primary structure (the amino acid sequence) of the polypeptide coded by the gene is changed.
A change in 1 DNA codon (e.g. by substitution) may result in a different amino acid being put into the polypeptide chain thus changing the H bonding in the secondary structure or the ionic/hydrophobic/disulphide bonding in the tertiary structure. The shape of the protein can be changed, resulting in changes to its solubility or function
what are 3 ways in which a nucleotide sequence can be affected by mutation? which have the most significant effects?
insertion&deletion - one or more nucleotides may be deleted from or added to the sequence. This alters the reading frame of the triplet code, altering all subsequent triplets so having the greatest affect.
substitution - one nucleotide may be substituted for another, producing a different triplet code that may code for a different amino acid/be a STOP triplet
How are proteins provided with the energy to form their tertiary structure and why is this important?
cyclic AMP activates proteins by altering their three-dimensional structure
This is important for proteins to have the complementary shape to the molecules they bind to, e.g. enzymes and their substrates, antibodies and their antigen, hormones and their membrane receptor
Explain the neutral effects mutations can have on the way a protein functions
Mutation can occur in non-coding part of the DNA - not all DNA codes for proteins. The changed codon could still code for the same amino acid or a different version with the same function thus having no effect on the protein shape or function.
Explain the harmful effects mutations can have on the way a protein functions
a mutated gene produces a new allele if the gene.
Many mutated alleles give rise to genetic diseases e.g. sickle cell anaemia which occurs die to base substitution changing 1 amino acid in the beta polypeptides of haemoglobin, changing it to a hydrophobic amino acid which makes the haemoglobin less soluble and less able to carry oxygen (red blood cells become distorted - sickle shaped)
Explain the beneficial effects mutations can have on the way a protein functions
Some mutations produce beneficial alleles e.g. decrease in melanin production in the skin of early humans migrating to temperate climates: paler skin allows more vitamin D to be made in the skin in climates with less sunshine. (vitamin D deficiency = rickets)
What is the lac operon?
A group of genes that control the production of enzymes needed for lactose metabolism in the bacterium E.coli
Describe the structure of the lac operon
Part of the circular DNA found in bacterium.
Consists of 4 parts:
promoter: site where RNA polymerase attaches and then moves along the DNA to transcribe the genes
operator: where the lac repressor protein attaches via a complementary shaped binding site
structural genes - 2 genes coding for enzymes, beta-galactosidase which digests lactose into galactose and glucose, and lactose permease which makes the bacterial cell more permeable to lactose.
how is the lac repressor protein coded for?
by a regulatory gene that is not part of the lac operon.
Describe the sequence of events in the lac operon when lactose is absent, or enough glucose is present.
What are the advantages of this control?
The structural genes are ‘switched off’
Regulatory gene transcibes and translates lac repressor protein which binds to the operator. RNA polymerase is unable to bind to the promoter and move along the DNA so genes are not transcribed into mRNA and beta-galactose and lactose permease enzymes are not synthesised.
advantages:
- energy saved
-amino acids saved for vital proteins
- no waste of space from unnecessary enzymes
- glucose respired (easier, more energy released)
Describe the sequence of events in the lac operon when lactose is present
structural genes are ‘switched on’
The small amount of lactose able to enter passively (without help of lactose permease) binds to the repressor protein, changing its shape so it can no longer bind to the operator. RNA polymerase is able to bind to the promoter and move along the DNA to genes for beta-galactosidase and lactose permease are transcribed to mRNA and enzymes are synthesised.
What is the body plan of an organism?
The body plan of an organisms is the orientation of structures (e.g. head and tail) and the positions of specialised tissues and organs.
Describe the genes that control body plans - the homeotic genes.
homeotic / regulatory, (gene) ;
contains, 180 base pairs / homeobox sequence, that codes for homeodomain on protein. Gene product binds to DNA and initiates transcription / switches genes on / off - control of development / body plan.
In animals, the homeobox is common in genes concerned with the control of developmental events such as segmentation, the establishment of an anterior-posterior axis, and the activation of genes coding for body parts such as limbs.
Homeotic genes and thus the homeobox sequence is similar in plants, animals and fungi.
What is apoptosis?
Apoptosis is programmed cell death can act as a mechanism to change the body plan. Excess cells (e.g. those of a tail or webs between digits during the development of humans) are made to shrink, fragment and the remains are engulfed by phagocytic cells. Controlled by cell signalling.
Describe the events in meoisis of Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Prophase - chromosomes condense and supercoil, nuclear membrane breaks down, centrioles duplicate and move to opposite poles, form spindle fibres. Chromosomes form pairs called bivalents in synapsis. Crossing over occurs.
Metaphase - bivalents line up on equator, spindle fibres attach to centromeres.
Anaphase - Homologous paris are separated to opposite poles. (centromeres do not split)
Telophase - Cytokinesis occurs. Chromosomes disappear, nuclear envelope reforms.
What are homologous chromosomes?
Chromosomes that:
- are the same size and shape
- carry the same types of genes
- may have different alleles of these genes
- originate from each parent
They pair up to for bivalents during prophase
Describe the events of Meoisis 2
Start with 2 haploid cells, each with 1 chromosome from each pair.
Prophase 2 - (in both cells) chromosomes condense and supercoil, nuclear membrane breaks down, centrioles duplicate and move to opposite poles, form spindle fibres.
Metaphase 2 - single chromosomes line up at equator. Spindle fibre attaches to centromere.
Anaphase 2 - sister chromatids pulled to opposite poles (pulled apart at centromere)
Telophase 2 - cells divide, nuclear membranes reform so 4 genetically different haploid cells are produced (e.g. half as many chromosomes)
What is an allele?
A variation of a gene.
Formed by a gene mutation.
Controls a variation in the phenotype e.g. blue as opposed to brown eyes.
Explain the term locus
The specific location of a particular gene on a chromosome. Alleles of the same gene occupy the same gene locus, but on the opposite homologous chromosome.
What is the genotype?
The types of genes and alleles an individual has for a characteristic.
What is the phenotype?
The physical (external), physiological (internal) and behavioural characteristics an organisms as a result of their genotype and/or the environment. Some characteristics are due solely to the genotype: eye colour Some are due to both genotype and environment: weight Some are due solely to the environment: language(s)
What is meant by the term dominant?
the allele that determines the phenotype in a heterozygote - eg B in a Bb individual results in brown eye colour.
A dominant allele is the one expressed during protein synthesis.
