chromosomes and meiosis
events in metaphase 1
- homologous pairs of chromosomes line up at the metaphase plat, moved by microtubules anchored to the poles of the cell
events in telophase 1
- in some cell types, the nuclear membrane reforms around the now haploid chromosomes and the chromosomes decondense - cytokinesis occurs simultaneously, forming two cells
events of prophase 1
- replicated chromatin condenses into chromosome - homologous chromosomes pair up, aligning genes by gene - crossing over between homologous, non-sister chromatids
drawing of crossing over
- synapsis: pouring of homologous chromosomes, forming a bivalent/tetrad - crossing over occurs that the chiasmata - pieces of chromosomes (and genes there on) are exchanged - produced genetic recombinants in the gamete chromosomes
prophase 2
- the nuclear membrane breaks down and chromosomes condense, if necessary. the spindle fibres form
note
-replicated chromosomes are composed of two identical sister chromatids -unreplicated chromosomes = no sister chromatid
three events that occur during prophase 1 of meiosis
1. pairing of homologous chromosomes 2. crossing over 3. condensation
Meiosis 1 vs Meiosis 2
1: homologous chromosomes separate 2: sister chromatids separate
describe the process of creating a karyogram
a cell is "frozen" in metaphase of mitosis by the application of chemicals that disrupt the mitotic spindle, a hypotonic solution is added; water enters the cell causing it to swell and burst, separating the chromosomes from each other. the chromosomes are stained and viewed with a microscope. a photograph of the chromosomes is taken for organization
distinguish between a karyogram and karyotype
a karyogram is a graphical/photograph representation of the karyotype. a karyotype is the characteristics of the chromosomes of an individual (number, type, shape)
tetrad
a pair of homologous chromosomes
bivalent
a pair of joined homologous chromosomes
describe two procedures for obtaining fatal cells for production of a karyotype
amniocentesis - a needle is inserted through the abdomen into the uterus. amniotic fluid is withdrawn, which contains cells from the foetus that can be used in karyotyping. a risk is uterine contractions chlorinic villas sampling - a thin tube is inserted into the uterus via the vagina and cervix. a small sample of placenta is removed, which contains fatal cells that can be used in karyotyping. a risk is bleeding
outline the advancement in knowledge gained from the development of autoradiography techniques
autoradiography is used to produce an image of a radioactive substance. the technique is used in cellular and molecular biology to visualise structures. for example, autoradiography can be used to visualise chromosomes, bands of DNA in electrophoresis gels, tissue samples, and single cells
state an advantage of being diploid
being diploid means you have two copies of each chromosomes, and therefore have two copies of each gene that the chromosome contains. so, if one of the chromosomes carries a detrimental allele of a gene, there is a second copy of the gene whose allele may be able to counter the affects of the negative version
outline sex determination by sex chromosomes
biological sex is determined by which sex chromosomes are present XX= female XY= male the male parent determines the sex of the offspring by either passing on an X chromosome (to make a female) or a Y chromosome (to make a male offspring)
state the difference between independent assortment of genes and segregation of allies
both independent assortment and segregation occur during meiosis. segregation refers to alleles of the same gene separating into different gametes. independent assortment is when different genes located on different chromosomes move independently from each other
Describe Cairn's technique for producing images of DNA molecules from E. coli
cairn's radioactivity labled DNA to produce images of the molecule, this allowed humans to visualise and measure the length of DNA - DNA was replicated many times in the presence of radioactive thymine nucleotides - the radioactive DNA was left for a period of time, covered with a sheet of photographic film. the radioactive thymine exposed the film - the radioactivity of the DNA exposed the film, leading to an image of DNA. in Cairn's case, he observed the circular DNA of a bacterial cell
Outline conclusions drawn from the images produced using Cairn's autoradiography technique
cairn's was able to see that prokaryotic chromosomes are circular. the also observed DNA replication occurring at the replication fork
list mechanisms by which a species chromosome number can change
chromosome splitting will increase the number of chromosomes - one chromosome with genes a, b, c, d, and e - chromosome with duplicated centromere sequence - chromosome splits if centromeres are pulled to opposite poles during meiosis - RESULT: 2 chromosomes when there used to be 1 chromosome fusing will decrease the number of chromosomes - RESULT: one chromosome when there used to be 2
list the characteristics by which chromosomes are arranged on the karyogram
chromosomes are photographed and arranged by size (largest to smallest), banding pattern and centromere position
list three ways in which the types of chromosomes within a single cell are different
chromosomes within a cell are different in: - size; number of base pairs - which genes they carry - sequence of nitrogenous bases - location of the centromere - banding pattern when stained
state that crossing-over occurs during prophase 1
crossing over, the exchange of genetic material between sister chromatids of homologous chromosomes, occurs during prophase 1 of meiosis
define diploid
diploid means the cell or organism contains two compete sets of chromosomes, one from each parent
describe the use of a karyogram to diagnose down syndrome
down syndrome is caused by a nondisjunction of chromosomes #21, resulting in three chromosomes #21, which can be observed in a karyogram
state that DNA is replicated in interphase before meiosis
during "S phase" of interphase before meiosis, the DNA molecule(s) in the cell replicated, creating two identical copies held together at the centromere
describe random orientation and independent assortment
during metaphase 1 of meiosis, the way the pairs of chromosomes align at the cell equator is random; this means that anaphase 1 either chromosomes (maternal or paternal) can end up at either pole (and eventually either daughter cell). also, which way a pair is facing is independent (does not affect) of the way other homologous pairs are facing random orientation = within a homologous pair independent assortment = between homologous pairs
outline the process and result of crossing over
during prophase 1 of meiosis, homologous chromosomes exchange pieces of non-sister chromatids. this results in new combinations of genes, creating "recombinant chromosomes." crossing over occurs at a location on the chromosomes called the chiasmata, the point at which the pairs homologous chromosomes are connected/in contact
list example haploid cells
eggs and sperm are haploid cells
outline the formation of a diploid cell from two haploid gametes
gametes (egg and sperm) are haploid, which means they have on complete set of chromosomes. when the gametes fuse during. fertilisation, the two sets of chromosomes (one from the egg and one from the sperm) combine to create a diploid zygote
define haploid
haploid means that the cell or organism contains only 1 set of chromosomes; there are not chromosome pairs
describe the discovery of meiosis
hertwig (1876) - observed one cell dividing to create four cells, making sea urchin eggs van bender (1883) - discovered how chromosomes move during meiosis by studying roundworms Weidmann (1890) - theorised how the meiotic divisions could lead to daughter cells with only half of the genetic information ("germ plasm")
events in anaphase 1
homologous chromosomes are pulled to opposite poles of the cell
define homologous chromosomes
homologous chromosomes are two chromosomes, one of paternal original and one of maternal origin
explain the relationship between the number of human and chimpanzee chromosomes
humans have 46 chromosomes and chimpanzees have 48 chromosomes. there is evidence that two chimp chromosomes fused to create 1 human chromosomes (chromosome 2 in humans)
sister chromatids
identical strands of DNA formed during DNA replication in interphase, joined at the centromere region
discuss difficulties in microscopic examination of dividing cells
in early microscopic examination of cells, the cells had to be preserved before viewing. this killed the cells and prevented processes like meiosis from being observed in action. additionally, it wasn't until DNA stains were discovered that the behaviour of chromosomes during meiosis could be observed
State the number of nuclear chromosome types in a human cell
in the nucleus of a human cell there are 23 types of chromosomes. males would have an additional type, the Y chromosome. there are 22 autosomes and 2 types of sex chromosomes.
compare divisions of meiosis 1 and meiosis 2
meiosis 1 - reductive division, diploid -> haploid - chromosomes remain replicated - crossing over occurs - proceed by interphase with an S phase (DNA replication) - results in two haploid cells meiosis 2 - non-reductive division, haploid -> haploid - chromatids of a chromosome separate - proceed by interkinesis, no S phase - results in four haploid cells
why is meiosis 1 a reductive division
meiosis 1 is referred to as a reductive division, because the chromosome number begins as diploid (2 of each chromosomes type) and at the end of meiosis 1 the chromosomes number is haploid). the chromosome number is reduced
outcome of allele segregation
mender's law of segregation described the consequence of the behaviour of chromosomes during meiosis; the law states that a pair of alleles (variations of the same gene) separate into different gamete cells
telophase 2
nuclear membrane forms around each set of chromosomes and the chromosomes decondense - cytokinesis occurs simultaneously to create four haploid cells
state that chromosome number and type is a distinguishing characteristic of a species
organism with differing numbers of chromosomes are not likely able to interbreed; all the members of a species will have the same number of chromosomes
state a similarity and difference found between pairs of homologous chromosomes
similarities - same genes at the same locus - majority of the same dan base sequence - same length (in bp) - same centromere position - same staining pattern differences - different origin (one from mom and one from dad) - different alleles of the genes (slight different in base sequences in the gene
anaphase 2
sister chromatids of the replicated chromosomes are pulled towards opposite poles of the cell
State the human cell diploid number
somatic human cells have a diploid number of 2n=46 - there are two of each chromosomes for a total of 46 chromosomes
describe the attachment of spindle microtubules to chromosomes during meiosis 1
spindle microtubules are long protein fibres that attacher chromosomes to the poled of the cell. in meiosis 1, the homologous chromosomes of a pair are each attached to different poles. the way the chromosome pairs align, and therefore the pols to which each chromosomes is attached, is random
describe the mechanism by which the SRY gene regulated embryonic gonad development
the "sex determining region y" (SRY) gene codes for a protein (also called SRY). the Sry protein is a transcription factor, which activates other genes that cause the development of seminiferous tubules in the embryonic gonads. this turns the gonad into a testes. cells in the testes (celled Leydig cells) then start secreting testosterone. the SRY gene is activated in weeks 6-8 of embryonic development. since the gene is located only on the Y chromosome, the presence of the Y chromosome (the SRY gene) leads to development of male characteristics. without the Y chromosomes (and SRY gene) the embryo will develop as female.
non-sister chromatids
the chromatids of two homologous chromosomes
State the human cell haploid number
the eggs and sperm of humans are haploid n=23 - there is one of each chromosomes for a total of 23 crhomosomes
state two consequences of chiasmata formation between non-sister chromatids
the formulation of chiasmata formation between non-sister homologous chromosomes has two consequences: 1. stabilising bivalents during alignment on the metaphase plate 2. increasing genetic variation by facilitating crossing over
compare sexual and asexual lifecyles
the lifecycle is the period of time that an organism passes through until producing offspring of its own. the lifecycle can involve sexual or asexual reproduction sexual - two parents - meiosis - results in genetic variation asexual - one parent - fission, mitosis or budding - requires less energy
define chiasmata
the location where two homologous non-sister chromatids exchange genetic material during crossing over
state the minimum chromosome number in eukaryotes
the minimum chromosome number in eukaryotes is 2n=2. the "jack jumper ant" and Australian daisy both only have two chromosomes per cell
explain why the chromosome number of a species does not indicate the number of genes in the species
the number of chromosomes does not indicate the number of genes. it's possible to have one large chromosome with many genes, or many smaller chromosomes with fewer genes. likewise, it's possible to have larger chromosomes with relatively few genes or smaller chromosomes that are packed full of genes
synapsis
the process of the pairing of homologous chromosomes during prophase 1
explain why the typical number of chromosomes in a eukaryotic species is an even number
the reason why most eukaryotic organisms have an even number of chromosomes is sexual reproduction, in which each parent gives one set of chromosomes, resulting in an even number in the offspring. for example: parent 1 = n = 13 parent 2 = n = 13 - offspring 2n=26
metaphase 2
the replicated chromosomes are moved by microtubules to the metaphase plate. since the cell is haploid it is impossible for chromosomes to be paired
explain why meiosis must occur as part of a sexual lifecycle
the sexual lifecycle combines genetic information from two parents. in order to maintain the correct number of chromosomes in the offspring, the parents must undergo meiosis to create gametes with half of the genetic information
define bivalent
the structure of a pair of homologous chromosomes physically attached at a chiasmata
describe random orientation chromosomes during meiosis 1
the way the chromosomes of a homologous pair orients at the metaphase plate during meiosis 1 is random; this means that the maternal or paternal chromosome are equally likely to move to either pole of the cell during anaphase 1. this also means that the orientation of one homologous pair does not affect the orientation of any other homologous pairs
outline the structure and function of the two human sex chromosomes
the x-chromosome is the larger of the two sex chromosomes; a length of 15 million bp and containing 1805 genes. the y-chromosomes is much smaller than the x-chromosome; a length of 57 million bp and ~460 genes
deduce the sex of a individual given a karyogram
to determine sex from a karyogram, examine the last pair of chromosomes; XX= famles XY = males