T5 - Mitosis + Meiosis Flashcards
describe the relationship between DNA molecules, chromosomes and chromatids
- DNA double helix wound around histone proteins
- DNA wound around histone = nucleosome
- many nucleosomes, like beads on a string
- wound around each other, into solenoid structure
- wound futher to form chromatin
- wound further to form chromosome
- one chromosome = one DNA molecule
chromatin structure changes during cell cycle, between euchromatin and heterochromatin
outline and describe chromosome and chromatid structure
- chromosome is very long DNA structure
- single unreplicated chromosome is condensed during cell cycle
- when replicated, X shaped - two sister chromatids held together by centromere (and now one chromosome = two DNA molecules)
- each chromatid consists of one DNA molecule, and has a p + q arm
- p = short
- q = long
- telomeres are found at the end of chromosomes (more info next card)
what are telomeres
- found at the end of chromosomes
- detects the end of the chromosome
- repeat sequences at the end of each chromosome / chromatid
- each time during cell replication, chromosomes are shortened
- because the ends are protected by telomeres, only that part of the chromosome is lost
- therefore don’t lose any precious DNA
- ie prevents loss of genes
describe how chromosomes can be categorized
using the position of centromere
- metacentric middle
- submetacentric a bit further up
- acrocentric near the top
- telocentric top
this differs between different types of chromosomes, allowing this to categorize chromosomes.
using size and shape
- seven groups based on size, position of centromere and local differences
- groups A-G
- X is ‘C’ group member
- Y is ‘G’ group member
how are metaphase spreads can be used in chromosome analysis
- need to rupture the cell membrane
- during metaphase
- this is when chromosomes are condensed for replication
- see the whole karotype
- can then arrange in numerical order + differentiate between different chromosomes using size, shape and chromatid position
- karotype may be used to look for abnormalities in an individual’s chromosome number or structure
karotype = individual’s complete set of chromosomes
appreciate how chromosome banding patterns can be used in karotypes and ideograms
- G-banding involves enzymatic digestion followed by a Giesma stain, creating characteristic banding pattern
- chromosome painting is where flourescent markers label the different parts of chromosomes, and processed by computer, leads to characteristic colours
- can help identify translocations, duplications and loss of Y chromosomes
- can help to identify different chromosomes (may be difficult to distinguish based on shape and size etc)
- can help generate ideograms, where genetic content and locus of different genes are mapped onto diagram of chromosome
- can help to screen for genetic disorders
ideogram = diagrammatic representation of karotype
explain the difference between a gene and allele
gene is a sequence of nucleotides that codes for a particular protein / polypeptide
an allele is an alternative form of a gene. Alleles have the same locus on the chromosome.
Alleles determine the organism’s genotype, and therefore their phenotype.
describe the general outline of each phase within the process of mitosis
cell division that occurs in somatic cells, producing two identical diploid daughter cells with the same chromosome content as the parental cell
- PROPHASE: breakdown of nuclear membrane, spindle fibres appear, chromatin condenses into chromosomes, nucleolus disappears
- PROMETAPHASE: spindle fibres attach to chromosomes by the kinetochore, chromosomes condense further
- METAPHASE: chromosomes align, checkpoint at this stage
- ANAPHASE: kinetochore microtubules shorten, kinetochores walk along fibres of tubulin, pulling chromosomes into each half of new cell, centromeres divide, sister chromatids move to opposite poles
- TELOPHASE: nuclear membrane reforms, chromosomes decondense, spindle fibres disappear, cytoplasm begins to cleave
- …cytokinesis where cytoplasm divides
kinetochore is found on the centromere
what are the main phases of the cell cycle
- G1 metabolic changes prepare the cell for division
- S phase DNA synthesis replicates the genetic material, and each chromosome now consists of two sister chromatids
- G2 phase metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis
- M phase a nuclear division (mitosis), followed by a cell division (cytokinesis)
- G0 phase a resting or quiescent phase when the cell is not growing or dividing (some cells enter this phase for varying time periods)
G1, S and G2 is collectively known as interphase
what are the main stages of meiosis
special type of cell division for germline cells, and produces 4 non-identical cells with half the chromosome complement of the parental cell → divided into meiosis I and meiosis II
- PROPHASE I: nuclear membrane dissolves, chromosomes codense, homologous chromosomes form bivalents, crossing over occurs
- METAPHASE I: spindle fibres form and connect to bivalents at centromeres and align them along the middle of the cell
- ANAPHASE I: spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
- TELOPHASE I: chromosomes decondense, forms two haploid daughter cells, nuclear membrane may reform
- PROPHASE II: chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles
- METAPHASE II: spindle fibres attach to chromosomes at centromere and align them along the cell equator
- ANAPHASE II: spindle fibres contract and seperate the sister chromatids, and these (now chromosomes) move to opposite poles ✷
- TELOPHASE II: chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
germline cell = gamete
✷ sister chromatids may not be identical at this stage, as crossing over has already occured
what is recombination
meiosis
- during prophase I homologous chromosomes pair up, forming bivalents
- crossing over occurs
- formation of chiasmata
- genetic exchange takes place between a pair of homologous chromosomes
- breaking off of part of chromosome and being joined to a chromatid on the homologous chromosome
- introduces variety of alleles
how is genetic diversity introduced during meiosis
- Crossing-over: The exchange of regions of DNA between 2 homologous chromosomes caused by twisting as they line up along the equator of the cell.
- Independent assortment: the random orientation of each pair of chromosomes along the midline of the cell (ie sometimes the maternal one is on the left, sometimes on the right).
- Random segregation: the random distribution of alleles among the four gametes.
name the three main cycle checkpoints, and describe in simple terms what happens at each
G1
- mian decision point for cell
- once it passes this checkpoint, it is comitted to cell division
- checks for cell size, nutrients, molecular signals, DNA integrity and growth factors
- contact inhibition: point at which cell decides it’s grown enough and growth stops
- if not ok, cell arrests at this point
G2
- checks for DNA integrity and whether DNA is completely copied during S phase
- ensures identical copy of DNA for daughter cells
M aka spindle checkpoint
- cell examines whether all sister chromatids are correctly attached to spindle microtubules
- checks for chromosome attachment to spindle at metaphase plate
nondisjunction leads to…
aneuploidy
The umbrella term for an incorrect number of chromosomes in a cell- i.e. not 46 - is known as aneuploidy
- can be caused by non-disjunction during mitosis or meiosis
- polyploidy: where all chromatids fail to seperate equally, leading to some 2n cells
- aneuploidy: where one chromosome doesn’t seperate equally, leading to some n+1 or n-1 cells
- caused when chromosomes do not properly attach to spindle fibres / metaphase plate
- produces daughter cells with abnormal chromosome numbers
Aneuploidy can cause a number of genetic conditions depending on whether chromosomes are, missing or in excess, and which specific chromosomes are affected
mitotic non-disjunction leads to…
trisomy
- one of the daughter cells will have 3 copies of one chromosome (triosmy) and one will have only 1 copy (monosomy)
- if non-disjunction occurs in the first cell division in a developing embryo (first post-zygotic division), a trisomy and a monosomy cell will be produced
- the monosomy cell will be destroyed, leaving only trisomy cell to divide
- all cells in body will now be trisomy
- may lead to conditions such as Down’s Syndrome
- non-mosaic karotype
mosaicism
- if non-disjunction happens at a later cell division
- by this point, multiple sucessful divisions have occured, meaning there are also surviving diploid cells
- in long term, some cells will be derived from trisomy and some will be derived from healthy normal diploid cell line
- more than one cell line in body = mosaicism
- this can be spread throughout the tissues of the body, or might be limited to a specific tissue
non-disjunction is the faliure of one or more pairs of sister chromatids to seperate normally during nuclear division… results in an abnormal distribution of chromosomes in the daughter cells
