Genes and chromosomes Flashcards
meiosis and mitosis
what is heredity
the transmission of traits from one generation to the next
what is genetics
the scientific study of heredity and inherited variation
what are reproductive cells called
gametes
what do gametes do
they are reproductive cells that transmit genes from one generation to the next
what are the male and female gametes
sperm and eggs
where is DNA found in eukaryotes
in the nucleus, mitochondria and chloroplast
humans have ….. chromosomes in their somatic cells
46
what are somatic cells
all the cells of the body except from gametes and their precursors
what does each chromosome consist of
a single DNA molecule coiled in association with various proteins
what is a DNA molecule divided into
genes
what is the locus of a gene
the gene’s location along the length of the chromosome
what kind of reproduction produces offspring that are an exact copy of the parent
asexual reproduction
what happens in asexual reproduction
a single organism passes all of its genes to offspring without the fusion of gametes (gives rise to a clone)
how may differences arise in asexually reproducing organisms
either by mutation or error in cell division
what happens in sexual reproduction
two parents give rise to offspring that have unique combinations of genes inherited form the two parents
offspring vary genetically from siblings and parents
what is a life cycle
the generation to generation sequence of stages in the reproductive history of an organism from conception to production of its own offspring
when chromosomes become condensed for mitosis what characteristics can we analyse
- size
- position of centromeres
- patterns of coloured bands produced by certain chromatin binding stains
what is a karyotype
a display of chromosomes in their 23 pairs
what is the same in the 2 chromosomes of a pair
- length
- centromere position
- staining pattern
what are the pairs of chromosomes called
homologous chromosomes
both chromosomes of each homologous pair carry genes controlling the same ………………………
inherited characteristics
are the X and Y chromosomes homologous
not fully - only small parts are homologous
what kind of chromosomes are the X and Y chromosomes
sex chromosomes
what are all the other chromosomes that are not sex chromosomes called
autosomes
how many chromosomes of a pair do we inherit from each parent
one
what is the number of chromosomes in a single set represented by
n
what is a diploid cell
any cell with two chromosomes
what is the abbreviation for a diploid number of chromosomes
2n
what is the diploid number of chromosomes for humans
2n = 46
once DNA synthesis has occurred each chromosome contains 2 what
sister chromatids
is a cell still diploid once DNA has been synthesised
yes - even though the chromosomes are duplicated the cell is still diploid because it only has 2 sets of information regardless of the number of chromatids (the chromatids are merely copies of the information in one set)
which cells are haploid
gamete cells
what is the haploid chromosome number for humans
n = 23
what does the set of 23 chromosomes consist of
22 autosomes and a single sex chromosome
an unfertilised egg contains which sex chromosome
X
a sperm contains which sex chromosome
either and X or a Y
when does the human life cycle begin
when the haploid sperm of the father fuses with the haploid egg of the mother
the fusion of the nuclei of gametes is called what
fertilisation
what is the result of fertilisation and is it haploid or diploid
zygote - diploid - because it contains 2 haploid sets of chromosomes
how are the somatic cells of the body generated from the zygote
mitosis of the zygote and its descendant cells generates all the somatic cells
what are the only cells of the human body not produced by mitosis
the gametes
what cells do gametes develop from
the germ cells in the gonads (ovaries or testes)
gamete division involves which type of cell division
meiosis
meiosis reduces the number of sets of chromosomes from 2 to 1 combating the doubling that occurs in which process
fertilization
describe the alternation of generations life cycle that plants and algae exhibit
includes both diploid and haploid stages that are multicellular
- the multicellular diploid stage is called sporophyte
- meiosis in the diploid stage produces haploid cells called spores
- haploid cell divides mitotically, generating a multicellular haploid stage called the gametophyte (an organism)
- these gametophytes give rise to gametes by mitosis
- fusion of 2 haploid gametes results in diploid zygote
- the zygote develops into the next sporophyte generation
describe the animal life cycle
gametes are the only haploid cells
- meiosis occurs in germ cells during the production of gametes
- gametes undergo no further cell division prior to fertilization
- after fertilization the diploid zygote divides by mitosis producing a multicellular organism that is diploid
describe the life cycle seen in most fungi and some protists
the only diploid stage is the single celled zygote
- gametes fuse and form a diploid zygote
- meiosis occurs with a multicellular diploid offspring developing
- meiosis produces not gametes but haploid cells
- these cells then divide by mitosis and give rise to either unicellular descendants or haploid multicellular adult organism
the haploid organism carries out further mitosis producing cells that develop into gametes
can haploid and diploid cells undergo mitosis
yes
can haploid and diploid cells undergo meiosis
no - only diploid cells can undergo mitosis because haploid cells only have a single set of chromosomes that cannot be further reduced
what happens before both meiosis and mitosis
chromosomes are duplicated
how many cells does meiosis result in
4 haploid cells
how many cells does mitosis result in
2 diploid cells
what are sister chromatids
2 copies of one chromosome closely associated - sister chromatid cohesion
what do sister chromatids make up
one duplicated chromosome
what do homologous chromosomes contain
individual chromosomes that were inherited from each parent - they look alike but have different versions of genes at corresponding loci with each version being called an allele
what are the steps of prophase I
prophase I (duplicated chromosomes pair up and exchange segments)
- spindle formation and nuclear membrane breakdown
- chromosomes start to condense
- each duplicated chromosome pairs with its homolog and crossing over occurs (DNA form non-sister chromatids are broken and re-joined to each other
- duplicated homologous chromosomes pair up and exchange segments
- each homologous pair has chiasmata where crossing over has occurred
- microtubules from the poles attach to the kinetochores (found at the centromere of each homolog)
- microtubules move homologous pairs towards the metaphase plate
what are the steps of metaphase I
metaphase I (chromosomes line up by homologous pairs)
- pairs of homologous chromosomes are now arranged at the metaphase plate with one chromosome of each pair facing each pole
- both chromatids of one homolog are attached to kinetochore microtubules from the pole they are facing
what are the steps in anaphase I
anaphase I (two homologous chromosomes of each pair separate)
- breakdown of sister chromatid cohesion proteins allows homologs to separate
- homologs move towards separate poles as directed by spindle apparatus
- sister chromatid cohesion persists at the centromere causing chromatids to move as a unit towards the same pole
what are the steps in telophase I and cytokinesis
telophase I and cytokinesis (2 haploid cells form, each chromosome still consists of 2 sister chromatids)
- each half of the cell has a complete haploid set of duplicated chromosomes
- each chromosome is composed of 2 sister chromatids and one or both chromatids contain non-sister DNA
- cytokinesis (division of the cytoplasm) usually occurs simultaneously with telophase I forming 2 haploid daughter cells
does chromosome duplication occur between meiosis I and meiosis II
NOOOO
what are the steps in prophase II
prophase II
- spindle apparatus forms
- chromosomes each still composed of 2 chromatids associated at the centromere are moved by microtubules towards the metaphase plate
what are the steps of metaphase II
metaphase II
- chromosomes are positioned at the metaphase plate
- because of crossing over in meiosis I the 2 sister chromatids of each chromosome are not genetically identical
- the kinetochores of sister chromatids are attached to the microtubules extending from the poles
what are the steps of anaphase II
anaphase II
- breakdown of proteins holding sister chromatids together at centromere allows chromatids to separate
- the chromatids move towards opposite poles as individual chromosomes
what are the steps in telophase II and cytokinesis
telophase II and cytokinesis
- nuclei form and chromosomes begin decondensing
- cytokinesis occurs
- the result is 4 daughter cells each with a haploid set of unduplicated chromosomes
- the four cells are genetically distinct from each other and the parent cell
what are the 4 steps in the cell cycle
- G1 - growth
- S - DNA synthesis
- G2 - growth
- M - mitosis
after interphase (g1, S, g2) the chromosomes have been duplicated and the sister chromatids are held together by what proteins
cohesins
in prophase I when the DNA molecules of 2 non-sister chromatids breaks at corresponding points what happens to the chromatin
it starts to condense
what is the synaptonemal complex
it holds one homolog tightly to the other in prophase I
what happens during synapsis in prophase I
the DNA breaks are closed up so that each broken end is joined to the corresponding segment of the non-sister chromatid (a paternal chromatid is joined to a piece of the maternal chromatid beyond the crossover point and vice versa)
what do the points of crossing over become visible as
chiasmata
what happens in prophase of mitosis
- chromatin fibres become tightly coiled
- nucleoli disappear
- each duplicated chromosome appears as 2 identical sister chromatids joined at their centromeres
- the spindle apparatus begins to form (spindle fibres extend from the centrosomes and shorter microtubules extend from asters)
- centrosomes move to opposite poles
what happens in the prometaphase of mitosis
- nuclear envelope fragments
- the chromosomes have become more condensed
- a kinetochore has formed at the centromere of each chromatid (2 per chromosome)
- microtubules attach to kinetochores
- non-kinetochore microtubules interact with the opposite pole lengthening the cell
what happens in the metaphase of mitosis
- chromosomes have all arrived at the metaphase plate (their centromeres lie at the metaphase plate)
- the kinetochores of the 2 sister chromatids are attached too opposite poles
what happens in the anaphase of mitosis
- shortest stage
- cohesion proteins are cleaved, parting sister chromatids and each chromatid becomes an independent chromosome
- chromosomes move to poles as kinetochore microtubules shorten
- the cell elongates as the non-kinetochore microtubules lengthen
what happens in telophase of mitosis
- 2 daughter nuclei form in the cell and nuclear envelopes arise
- nucleoli reappear
- chromosomes become less condensed
- any remaining spindle microtubules depolymerise
what happens during cytokinesis
- happens in late telophase
- the cytoplasm divides
- involves the formation of a cleavage furrow that pinches the cell in 2
what is the cleavage furrow
a shallow groove in the cell surface near the old metaphase plate
on the cytoplasmic side of this is a contractile ring of actin filaments associated with myosin
interaction between actin and myosin cleaves the cell in 2
in interphase what happens to the centrosome
it duplicates
what is the main difference between meiosis and mitosis
meiosis reduces the number of chromosome sets from 2 to 1 whereas mitosis conserves the number of chromosome sets whether this be 2 or 1
which 3 events are unique to meiosis
- synapsis and crossing over (prophase I)
- alignment of homologous pairs at the metaphase plate (metaphase I) (rather than individual chromosomes in mitosis)
- separation of homologs (in mitosis sister chromatids separate) (anaphase I)
(these all occur in meiosis I)
in what steps of meiosis and mitosis are cohesions cleaved
mitosis - metaphase (cohesion at centromere - allows sister chromatids too separate)
meiosis - anaphase I (cohesion at arms - allows homologs to separate) and anaphase II (cohesion at a centromere - allows sister chromatids to separate)
mitosis is pretty much identical to meiosis I/II
meiosis II
shuffling of alleles during sexual reproduction produces …………..
variation
each member of a sexually/asexually reproducing population have a unique combination of traits
sexually
what 3 mechanisms contribute to genetic variation from sexual reproduction
independent assortment
crossing over
random fertilization
what is independent assortment
the random orientation of pairs of homologous chromosomes at metaphase of meiosis I
- at metaphase I homologous chromosomes each consisting of one maternal and one paternal chromosome are situated in the metaphase plate
- each pair may orient with either its paternal or maternal homolog closer to a given pole
- 50% chance that daughter cells of meiosis I will get paternal/maternal chromosome of a certain homologous pair
what is crossing over and how does it contribute to variation
crossing over produces recombinant chromosomes - individual chromosomes that carry genes from 2 different parents. it produces new combinations of maternal and paternal alleles
how does random fertilisation contribute to increased variation
it adds to the genetic variation arising from meiosis
what is the original source of different alleles
mutation
what are the differences between sexual and asexual reproduction
sexual - requires a lot of energy investment - fusion of gametes - allows variation asexual - requires less energy - no fusion of gametes - no variation - clones
are germ cells haploid or diploid
diploid
what is epigenetics
the study of the chemical modification of specific genes or gene-associated proteins of an organism
give an examples of an epigenetic tag
methylation - influences the level of transcription
