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What is enzyme induction?
A phenomenon whereby bacteria vary the synthesis rates of specific enzymes in response to environmental changes, such as the type of food available.
___ enzymes are synthesised at ___ rates, according to the cell's _____.
Give an example of enzyme induction.
- E.coli normally respires glucose but when placed in a lactose (disaccharide sugar) culture medium with no glucose, it soon increases the rate of synthesis of the two enzymes needed to respire lactose (lactose permease to transport lactose into cell and beta-galactosidase to hydrolyse it into glucose and galactose).
- The lactose here is called the inducer.
What is an operon?
A length of DNA, made up of structural genes and control sites. The structural genes code for proteins, such as enzymes. The control sites are the operator region and the promoter region.
Describe the different parts of the lac operon.
- Structural genes: Z codes for enzyme beta-galactosidase, and Y codes for lactose permease.
- Operator region: O, is a length of DNA next to the structural genes. It can switch them on or off.
- Promoter region: P, is length of DNA to which RNA polymerase binds to begin the transcription of the structural genes, Z and Y. (Operator and Promoter do not code for polypeptides)
- (Also, don't forget the regulator gene, I, which is not part of the operon and is some distance from it.)
- [I -----POZY]
Describe how the lac operon works when lactose is absent.
- 1. Regulator gene (I) expressed (transcriped & translated), and the repressor protein is synthesised. This repressor protein has two binding sites - one that binds to lactose and one that binds to operator region.
- 2. Repressor protein binds to operator region, and in doing so covers part of promoter region next to it, where RNA polymerase normally attaches.
- 3. RNA polymerase cannot bind to promoter region, so structural genes cannot be transcribed into mRNA, and so beta-galactosidase and lactose permease cannot be synthesised.
Describe how lac operon works when lactose is present. (and glucose isn't I think)
- 1. Lactose (inducer) molecules bind to the other site in repressor protein. This causes it to change shape so that its binding site cannot now bind to the operator region.
- 2. This leaves promoter region unblocked. RNA polymerase can now bind to it and initiate transcription of mRNA for genes Z and Y.
- 3. Because this operator-repressor-inducer system acts as a molecular switch, it allows synthesis of the two lac proteins required to respire lactose. As a result, lactose permease can take up lactose into cells, and can convert lactose into glucose and galactose using beta-galactosidase (the sugars can them be used for respiration).
What is the advantage of E.coli having this lac operon system to induce enzyme formation?
So that resources are not wasted in making unnecessary enzymes when lactose is not present.
What is involved in controlling the development of body plans of organisms? Give its definition.
Homeobox genes: a set of genes that control the development of the body plan of an organism, including the polarity (head and tail ends) and positioning of the organs.
Homeobox genes in different organisms are very ___. All animals have homeobox genes that are ___ ___, they are ____ with each other.
- recognisably similar
The fact that homeobox genes are very similar in all animals tell us what? How can we describe these genes?
- This implies that the activity of the homeobox genes are absolutely fundamental to the development of an animal body that works, and that a mutation in a homeobox gene is so disastrous that the organism is usually not able to survive.
- As these genes have not changed much for millions of years, we say they are highly conserved.
Homeobox genes code for the production of proteins called ___ ___. What is the role of these proteins?
- transcription factors (I think 60 amino acids)
- The proteins bind to particular region of DNA and cause it to be transcribed. In this way, a single homeobox gene can switch on a whole collection of other genes.
- Eg. The normal Antp gene (I think esp in relation to fruit flies) switches on all genes involved in production of a leg.
The homeobox genes are arranged in clusters known as... Also, what are substances that govern pattern of tissue development by activating the homeobox genes called?
- Hox clusters
- (Vertabrates have 4 Hox clusters)
What is programmed cell death that occurs in multicellular organisms called?
Apoptosis - it is an essential part of development of all animals.
Outline the process of apoptosis as a mechanism to change body plans.
- 1. Enzymes break down the cytoskeleton
- 2. Cytoplasm becomes dense, with organelles tightly packed.
- 3. Cell surface membrane changes and small bits called blebs form.
- 4.Chromatin condenses, nucleus starts to disintegrate and nuclear envelope breaks. DNA breaks into fragments.
- 5. Cell breaks into vesicles that are ingested and digested by phagocytes by phagocytosis. Cellular debris disposed of and does not damage any other cells or tissues.
How is apoptosis controlled?
By diverse range of cell signals - some from inside the cells, some from outside. (eg. Nitric oxides, cytokines by cells of immune system, hormones - they induce apoptosis).
Explain why apoptosis does not damage nearby cells and tissues.
Because the hydrolytic enzymes are enclosed in vesicles and these are ingested by phagocytes.
What do you call an untidy cell death that often is damaging?
Each parent produces special reproductive cells, called ___, which fuse together at ___ to produce a ___.
Meiosis is a ___ division. Resulting ___ cells have ___ the original number of chromosomes. They are ___ and can be used for sexual reproduction. What are the original cells called?
- haploid (half of chromosomes of original cell - only one set of chromosomes).
- Orignal cells (with 46 chromosomes in humans) are called diploid cells.
1) Meiosis involves two seperate divisions, what? 2)Also name the stages in each division. 3) What comes before meiosis though, and what happens here?
- 1) Meiosis I and Meiosis II
- 2) Each division has 4 stages - prophase, metaphase, anaphase and telophase (PMAT).
- 3) Before meiosis I, interphase happens - where DNA replicates - so each chromosome now consists of 2 identical sister chromatids, joined at centromere. Cell now contains 4, rather than the original 2 copies of each chromosome.
Describe the first stage of Meiosis I. And behaviours of chromosomes and associated behaviour of nulcear envelope and centriole.
- Prophase I
- Chromosomes: Chromatin condense and undergo supercoiling - shortening and thickening (now chromosomes can take up stains and be seen with light microscope)
- Chromosomes come together in their homologous pairs, to form a bivalent. Each member of pair consists of same genes at same loci, and each pair consists of one maternal and one paternal chromosome.
- Non-sister chromatids wrap around each other and attach at points called chiasmata (sigular - chiasma).
- They swap sections of chromatids with one another in process called crossing over.
- Nuclear envelope: Nucleolus disappears and nuclear envelope disintegrates
- Centrioles: move to opposite ends of cell, and form spindles (protein microtubules)
Describe the second stage of Meiosis I. (And like last time, behaviour of chromosome)
- Metaphase I
- Bivalents line up across equator of spindle, attached to spindle microtubules at centromeres. Chiasmata still present.
- Bivalents arranged randomly (random assortment) with each member of homologous pair facing opposite poles.
- This allows chromosomes to segregate independently when they are pulled apart in anaphase I.
Describe the third stage of Meiosis I.
- Anaphase I
- Homologous chromosomes in each bivalent pulled by spindle to opposite poles
- Remember - centromeres do not divide.
- The chiasmata separate and lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached.
Describe the fourth stage of Meiosis I.
- Two new nuclear envelopes form around each set of chromosomes at each pole (in most animal cells).
- Plasma membrane folds inwards and cell divides by cytokinesis (might even be called Cytokinesis I).
- Chromosomes uncoil
- [In most plant cells, the cell goes straight from anaphase I to meiosis II. ]
Outline the four stages in Meiosis II.
- [Remember, this division is in a plane at right angles to meiosis I.]
- Prophase II: If nuclear envelope has reformed, it breaks down again.
- Chromosomes condense
- Centrioles replicate and move to poles. Spindles form (at right angles to previous spindle axis).
- Metaphase II: Chromosomes arranged (by random assortment) on equator of spindle. Attached to spindle at centromeres.
- Anaphase II: This time centromeres divide and chromatids pulled to opposite poles by spindle fibres. Chromatids randomly segragate.
- Telophase II: Nuclear envelope reform around haploid daughter nuclei.
- In animals, two cells now completely divide, to give 4 haploid cells, whereas in plants, a tetrad of four haploid cells formed.
What is a bivalent?
A homologous pair of chromosomes, each consisting of two sister chromatids, paired up for meiosis.
List the ways in which meiosis leads to genetic variation. (4)
- Crossing over: during prophase I, to shuffle alleles.
- Genetic reassortment (independent assortment): due to random assortment and susequent segregation of maternal and paternal chromosomes in the homologous pairs, during Meiosis I. (metaphase I, then anaphase I)
- Genetic reassortment (independent assortment): due to random assortment and segregation of sister chromatids during Meiosis II. (metaphase II, then anaphase II)
- Random mutation
Why does fertilisation increase genetic variation?
- By randomly combining two sets of chromosomes, one from each of the two genetically unrelated individual gametes.
- (Usually only one ovum released from ovary at a time, and about 300million sperm (all genetically different), fertilises the ovum to make the zygote).
About chiasmata. What is another role of chiasmata during meiosis? Also, how many cross-over events occur on average in a pair of human chromosomes?
- Chiasmata also hold the maternal and paternal homologous chromosomes together on the spindle.
- About 2-3 cross over events occur on each pair of human chromosomes.
What is an allele?
A version of a gene. An allele of a gene has a difference in the DNA base sequence that is expressed as (translates into) slightly different polypeptides.
From the individual reassortment of chromosomes alone, how many genetically different games can be theoretically be made?
- 2n where n=the haploid number of chromosomes.
- Eg. humans - 223
What is the locus? (plural loci)
Specific position on a chromosome, occupied by a specific gene.
What is crossing-over?
- Where non-sister chromatids exchange alleles during prophase I of meiosis.
- (Where a chromatid of one chromosome in a bivalent can cross a chromatid of the other, forming a chiasma).