U1L6 Flashcards
Meiosis reduces
the number of
chromosome sets from diploid to haploid
What produced in sexual life
cycles contributes to evolution
Genetic Variation
Hereditary
Is the transmission of traits from
one generation to the next
Variation
Is demonstrated by differences in appearance that offspring show from parents and siblings
locus
Each gene has a specific chromosomal location
Genes are passed to the next generation via reproductive
cells called
Gametes
Asexual reproduction
a single individual passes genes to its offspring without the fusion of gametes
Clones
Is a group of genetically identical
individuals from the same parent
Sexual reproduction
Two parents give rise to offspring having unique combinations of genes inherited from both
Each replicated chromosome
consists of 2 identical sister chromatids
Gametes (sperm or egg) contain a single set of chromosomes, and are
Haploid
what exhibit an
alternation of generations
Plants and some algae
only diploid cells
can undergo meiosis
Meiosis takes place in two sets of cell
divisions called
Meiosis I and II
The two cell divisions result in
4 daughter cells
Each daughter cell in meiosis has only
half as many chromosomes as the parent cell
After interphase the sister chromatids are held together by
Cohesions
Describe the process of crossing over
The non sister chromatids are broken at precisely corresponding positions
A zipper-like structure called the synaptonemal complex holds the homologs together
DNA breaks are repaired, joining DNA from one non sister chromatid to the corresponding segment of another
Comparison between mitosis and meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to parent cell
Meiosis reduces the number of chromosome sets from two
(diploid) to one (haploid), producing cells that differ genetically from each other and from parent cell
Mutations (changes in an organism’s DNA) are the
original source of genetic diversity
Mutations
create different versions of
genes called alleles
Three mechanisms contribute to genetic variation
Independent assortment of chromosomes
Crossing over
Random fertilization
How many different combinations of
maternal and paternal chromosomes
can be packaged in gametes in an
organism whose diploid number is
16?
2n = 16
n = 6
2^6 = 256
The number of combinations possible when chromosomes assort independently into gametes is
2^n n = haploid number
Crossing over produces
Recombinant
chromosomes, which combine DNA from each parent
Crossing over begins very early in
prophase I, as homologous chromosomes pair up gene by gene
In crossing over
homologous portions of two
non sister chromatids trade places
Crossing over contributes to genetic variation by
combining DNA from two
parents into a single
chromosome
Random fertilization adds to genetic variation
Because any sperm can fuse with any ovum(unfertilized egg)
In nondisjunction
pairs of homologous chromosomes do not separate normally during meiosis
What happens in prophase I
Nuclear envelope breakdown, spindle formation, crossing over: DNA molecules of nonsister chromatids are broken and rejoined
What happens in metaphase I
Pairs of homologuous chromosmes are arranged at the metaphase plate
What happens at Anaphase I
Breakdown of proteins responsible for sister chromatid cohesion
Move toward opposite poles
What happens in Telophase I and cytokinesis
Each half of cell has become a haploid set of chromosomes
Two haploid cells form consisting of two sister chromatids
Cleavage furrow formation
What happens in prophase 2
Spindle apparatus forms, move toward metaphase II plate
Metaphase II
Chromosomes are positioned at the metaphase plate
Kinetochores of sister chromatids are attached to microtubules extending from opposite poles
Anaphase II
Breakdown of proteins holding sister chromatids together
Move toward opposite poles as individual chromosomes
Telophase II and cytokinesis
Chromosomes begin de-condensing
Meitoic division of one parent cell produces 4 daughter cells each with a haploid set of chromosomes
Daughter cells are genetically distinct from one another and from the parent cell
Independent assortment
refers to the random distribution of different chromosome pairs into gametes. In other words, the inheritance of one gene is independent of the inheritance of another gene located on a different chromosome.
Segregation
alleles of a gene pair separate or “segregate” during the formation of gametes (eggs and sperm) in a sexually reproducing organism
