Meiosis Flashcards
Hereditary
the transmission of treats form one generation to the next
variation
differences in appearances the offspring show from parents and siblings
genetics
the scientific study of hereditary and variation
genes
hereditary units
- offsprings acquire genes form parents by inheriting chromosomes
- made up of DNA
-passed via gametes
gametes
reproductive cells (seems and eggs)
fertilization
sperm and eggs unite, passing on genes of both parents to their offspring
somatic cells
46 chromosomes, all cells of body except games and their precursors
- 23 pairs, 1 from mother and 1 from father
locus
each gene’s specific position on a certain chromosome
asexual reproduction
single individual passes Gennes to its offspring without fusion of gametes
sexual reproduction
two parents give rise to offspring that have unique combinations of genes inherited form the two parents
clone
a group of genetically identical individuals form the same parent (asexual)
life cycle
the generation to generation sequence of stages in reproductive history of an organism
karyotype
an ordered display of the pairs of chromosomes from a cell
homologous chromosomes/homologs
two chromosomes in each pair
- same length and shape and carry genes controlling same inherited characters
- each pair includes one chromosome form each parent
sex chromosomes
X and Y
- females: XX, males: XY
autosomes
remaining 22 pairs of chromosomes
a diploid cell (2n)
has two sets of chromosomes
-for humans the diploid number is 46 (2n=46)
in DNA synthesis each chromosome is…
replicated
- each chromosome consists of two identical sister chromatids
haploid (n)
a single set of chromosomes (a gamete)
- human haploid number is 23
- 23 consists of 22 autosomes and a single sex chromosome (egg is X, sperm is X or Y)
zygote
fertilized egg, has one set of chromosomes from each parent
- produces somatic cells by mitosis and develops into an adult
Ovaries and testes produce…
haploid games
gametes are the only types of human cells produced by…
meiosis rather than mitosis
meiosis results in
one set of chromosomes in each gamete
fertilization and meiosis alternate in…
sexual life cycles to maintain the chromosome number
gametes fuse to form a
diploid zygote that divides by mitosis and develop into a multicellular organism
mitosis and meiosis are preceded by…
the duplication of chromosomes
meiosis has two sets of cell division:
meiosis I and meiosis II
- result in 4 daughter cells; each has half as many chromosomes as the parent cell
stages of meiosis
homologs chromosomes duplicated = sister chromatids, may have different versions of genes (alleles)
Meiosis 1
homologous pairs separate
Meiosis II
sister chromatids separate
- 4 new haploid cells
prophase I
homologous chromosomes pair up and align, crossing over happens, forms chiasmata
- forms recombinant chromatids
crossing over
non-sister chromatids exchange DNA segments
chiasmata
each homologs pair has 1 or more X-shaped regions; exist where crossing over happens
synopsis during prophase I
- synaptonomal complex
- DNA of mom and dad chromatid are broken at matching points
- then crossing over happens
synaptonomal complex
zipper-like structure that forms when homologous pair up
Mitosis summary
conserves number of chromosome sets, producing genetically identical
Meiosis summary
chromosomes sets from diploid to haploid; cells differ from each other and parent
- has 2 divisions after replication
- meiosis I: synapsis and crossing over in prophase I, alignment of homologous at metaphase plate, separation of homologs during anaphase I
mutations are the source…
of genetic diversity, create different version of genes called alleles - reshuffling of alleles during sexual reproduction produces genetic diversity
three mechanisms contribute to genetic variation:
- independent assortment of chromosomes
- crossing over
- random fertilizations
independent assortment of chromosomes
homologous pairs orient randomly at metaphase I -> each pair sorts mom and dad homologs into daughter cells independently of the other pairs
- possible combos = 2n = 8.4 million
random fertilization
any sperm can confuse with any egg; 70 trillion possible combos
blending hypothesis
idea that genetic material from the two parents blend together (WRONG)
particulate hypothesis
idea that parents pass on discrete heritable units (genes)
- mendel confirmed with garden pears
Mendel discovered the basic principles of hereditary by…
Breeding garden peas in carefully planned experiments (stamens + eggs)
- worked with peas because because of characters (distinct heritable features) and traits (character variants)
- could control mating
two distinct alternative forms of mating
- true breeding
- hybridization
true-breeding
plants that produce offspring of the same variety when they self-pollinate
hybridization
mated two contrasting, true-breeding varieties
generation names
P generation (true breeding parents), F1 generation (the hybrid off spring of P generation) and the F2 generation (when F1 self pollinate or cross pollinate)
Results of Mendels experiment
all F1 were purple, F2: 3/4 purple + 1/4 white (3:1 ratio)
- In F1 plants, the heritable factor for white flowers were hidden/masked by purple flower factor
- Mendel saw same thing in 6 other pea plant characters
purple flower and white flower
purple: dominant trait
white: recessive trait
Mendel’s Model explains 3:1 ratio in F2 offspring: 1st statement
1st: alternative versions of genes account for variations in inherited characters -alleles at specific locus
Mendel’s Model explains 3:1 ratio in F2 offspring: 2nd statement
for each character, an organism inherits two alleles, one from each parent
Mendel’s Model explains 3:1 ratio in F2 offspring: 3rd statement
if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive trait) has no noticeable effects in appearance
Mendel’s Model explains 3:1 ratio in F2 offspring: 4th statement
law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Punnett Square
predict results of genetic cross
- a capital letter represents a dominant allele, lowercase represents a recessive allele
homozygous
two identical alleles (PP or pp) - true breeding
heterozygous
two different alleles (Pp)
phenotype and genotype
phenotype: physical appearance
genotype: genetic makeup
How can we tell the genotype of an individual with the dominant phenotype?
testcross: breeding mystery individual with a homozygous recessive individual (pp)
any gamete produced by an individual heterozygous for a trait has a…
50% probability of carrying the dominant and 50% of carrying the recessive
