m&m common qn Flashcards
qn: Describe 2 ways in which meiosis leads to variation [4m]
TAKE NOTE OF KEYWORDS
- independent assortment of homologous chromosomes** at the METAPHASE PLATE** during METAPHASE I**
- and separation of HOMOLOGOUS CHROMOSOMES** at ANAPHASE I** ultimately results in different combinations of parental chromosomes in the gametes*;
- Crossing over** of non-sister chromatids of homologous chromosome** occurs at PROPHASE I* at points called chiasmata ;
- where equivalent portions of these chromatids break and rejoin*, resulting in the exchange of genetic material, hence new combination of alleles on the chromatid**
describe anaphase (KEYWORDS LOOKOUT)
- the CENTROMERES DIVIDE** and SISTER CHROMATIDS** separate and each sister chromatid IS NOW A DAUGHTER CHROMOSOME*
- KINETECHORE MICROTUBULES** shorten and pull the chromosomes to OPPOSITE POLES(!!!!!) *** (not end!!!)
- always mention separation to OPPOSITE POLES when talking about ANAPHASE
outline the roles of centromeres
STRUCTURE:
- they are non-coding tandem repeat sequences at one location along the length of a chromosome
FUNCTION
- where SISTER CHROMATIDS** adhere to each other
- and allow proteins called KINETOCHORES* and subsequently KINETOCHORE MICROTUBULES** to attach
- thus allowing the alignment of chromosomes at THE EQUATOR* during METAPHASE*
- and the subsequent separation of sister chromatids* to OPPOSITE POLES***
qn: anaphase is the least frequently observed stage in mitosis. Suggest 2 reasons for this [2m]
- anaphase is a very short-lived stage* and quickly proceeds to telophase;
- and separation of SISTER CHROMATIDS** occurs very fast
qn: explain how cancer is a result of uncontrolled cell division? [3m]
cancer results when
- proto-oncogenes mutated to ONCOGENES* via GAIN-IN -FUNCTION** mutation which cause cell to divide excessively
- tumour suppressor genes** go through LOSS-OF-FUNCTION** mutation, cells lose their ability to stop dividing
- in uncontrolled cell division, the LACK OF APOPTOSIS* and CONTACT INHIBITION** result in tumour formation
- METASTASIS occurs and secondary tumours* are formed
qn: why does the chromosome appear as a double arm structure? [2m]
- in the s phase of interphase*;
- each chromosome has undergone semiconservative DNA replication* to form 2 IDENTICAL SISTER CHROMATIDS**
- that are held together at the centromere
Explain what is meant by homologous chromosomes. [2m]
- a pair of chromosomes, one of paternal and one of maternal origin,which have the same genes* but different alleles *at CORRESPONDING LOCI**
- they are similar in SIZE, SHAPE, CENTROMERE POSITION AND STAINING PATTERN**
Outline the role of centrioles in mitosis [2m]
- Pair of centrioles at each pole* and their position determines the polarity of the cells*;
- centrioles form part of the microtubule-organising centre, and are involved in organising the synthesis of spindle fibres
- which lead to the separation of chromosomes* after centromeres divide
qn: describe and explain what happens to the nuclear envelope during mitosis [4m]
- during prophase*, nuclear envelope disintegrate and form vesicles
- (explain) to allow the spindle fibres to attach to the centromere*
- during telophase, vesicles fuse and nuclear envelope reforms
- to enclose chromosome inside the nucleus
Explain what happened during meiosis I that could have led to the chromosomes with an extra X chromosome [3]
- Non disjunction of sister chromatids* of X chromosomes to separate properly to opposite poles during anaphase I**
- Leading to formation of ovum with 2X chromosomes
- Which is fertilised by another gamete with normal number of X chromosome
Suggest possible outcome of a disorganised metaphase plate
- Homologous chromosomes** may not separate equally during anaphase 1
- Meiosis cannot continue and no germ cells are formed
What is the significance of the mitosis cell cycle
- production of genetically identical** daughter nuclei with same number * and type* of chromosomes* and the same alleles* so that genetically identical daughter cells** can be produced for :
1) cell growth
- INCREASE NUMBER OF CELLS by producing more cells genetically identical to the existing ones
2) Regeneration and cell replacement
- DAMAGED CELLS REPLACED by cells genetically identical to original ones, retaining the same function *
3) Asexual reproduction
- produce genetically identical OFFSPRING
: offspring will be suitably adapted to the same conditions that have allowed their parents to thrive
: maintains genetic stability
Why is there a need to regulate the mitotic cell cycle tightly?
- Cell cycle is TIGHTLY REGULATED as it is important for normal growth and development. Regulation is at certain control points known as checkpoints* which are G1, G2, M phase. These checkpoints are stop and go-ahead signals which determine whether or not the cell cycle can proceed
- Cancer occurs when the dysregulation of checkpoints of cell division** or the cells escape the cell cycle control mechanism** that normally regulates the growth —> uncontrolled cell division of cells (i.e. tumour formation ) and possibly cancer
What is the significance of meiotic cell cycle?
- FOR MAINTENANCE OF CHROMOSOME NUMBER in every generation**
Reduction division: production of 4 haploid gametes** from 1 diploid parent cell**
Chromosome number halved so that chromosome number** can be restored* upon fertilisation*
—> chromosome number of species remains the same* AFTER MANY GENERATIONS
- FOR GENETIC VARIATION IN OFFSPRING IN EVERY GENERATION
- CROSSING OVER between non-sister chromatids of homologous chromosomes results in new combination of alleles* on chromatids*
- independent assortment** of homologous chromosomes at METAPHASE PLATE and their subsequent separation** during metaphase I and anaphase I respectively
and the random orientation* of non-identical sister chromatids** of each chromosome* at the metaphase plate** and their subsequent separation during metaphase II and anaphase II respectively
—> results in gametes with different combination of maternal and paternal chromosomes***
- RANDOM FUSION OF GAMETES
—> during sexual reproduction/ fertilisation results in offspring with variety of genotype and possibly phenotypes
Benefits of variation :
Allows population to adapt to environmental changes as individuals selected for favourable characteristics which allow them to survive in the environment , while those selected against will die off
