6. Meiosis Flashcards
What is Genetics?
Genetics is the scientific study of heredity and variation
What is Heredity?
Heredity = transmission of traits (characteristics) from one generation to the next
What is Variation?
Variation = differences in appearance that offspring show from their parents and siblings
What are Genes?
Genes are segments of DNA that contain instructions for building and functioning of living organisms.
What are Gametes?
- Reproductive Cells
- Sperm and Egg Cells
What are Alleles?
Alleles
- different versions of specific genes
What is a LOCUS?
specific location on a chromosome holding a gene
What is FERTILIZATION?
Fertilization is the union of
gametes (sperm cell and egg cell)
DIPLOID ZYGOTE
The fertilized egg is called a diploid zygote
- has one set of chromosomes from each parent
(so a total of two sets of chromosomes)
What are Somatic Cells?
all other cells in body excluding egg and sperm cells
What produces eggs and sperm?
At sexual maturity, the ovaries produce egg cells while testes produce sperm cells
Chromosomes in Somatic vs Gametic Cells?
Gametic Cells (sperm and egg)
- haploid
- one set of chromosomes
- 23 chromosomes
Somatic Cells are
- diploid
- two sets of chromosomes
- 46 chromosomes
How are Gametes produced?
Gametes are the only types of human cells produced by meiosis, rather than mitosis
What are Homologous Chromosomes?
(4)
The two chromosomes in each pair are called
homologous chromosomes,
one chromosome is from the mother and one is from the father
can exist as unreplicated or replicated
- Homologous chromosomes are the same size (i.e. length), and they have genes which are arranged in the same order
What are CONDENSINS?
*Condensins are protein rings along the length of chromosomes for the purpose of compacting chromosomes just before cell division
What is a Karyotype?
A karyotype is an ordered display of chromosomes of an individual cell
What is Colchicine Drug?
After growing cells, a drug called colchicine is added before the karyotype processing
– Colchicine arrests the cell at metaphase during cell division
– At metaphase, the chromosomes are most condensed
Male vs Female Sex Chromosomes?
The sex chromosomes are called X and Y
o Human females have a homologous pair of X chromosomes (XX)
o Human males have one X and one Y chromosome (XY)
What are Autosomes?
The 22 pairs of chromosomes that do not determine sex are called autosomes
What is a Diploid Cell?
A diploid cell (2n) has two sets of chromosomes
What is Down Syndrome?
- Down syndrome due to three copies of chromosome 21
– Most common chromosome abnormality in humans
– Typically associated with a delay in cognitive ability and physical growth
What is Klinefelter syndrome?
Klinefelter syndrome, an XXY male
– Individual has male sex organs, but are sterile
– May display feminine characteristics
– Intelligence is normal
What is Turner syndrome?
Turner syndrome, an XO female
– Do not mature sexually during puberty (sterile)
– Short stature and normal intelligence
Result of Meiosis?
The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis
* Each daughter cell has only half as many chromosomes as the parent cell
What is Reductional Division?
Meiosis I starts with diploid parent cell and results in two haploid daughter cells with replicated chromosomes; it is called reductional division
What is Equational Divison?
Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called equational division
What is a Centrosome?
Centrosome
– Organelle that serves as a microtubule organizing center
– Replication leads to two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out of them
What is an Aster?
Aster
– A radial array of short microtubules that extends from each centrosome, connecting to proteins on the inner surface of cell membrane
What are the 2 types of Spindle Microtubules?
– Kinetochore microtubules =
capture chromosomes by binding to kinetochore proteins
– Non-kinetochore microtubules =
(also known as polar microtubules) = do not
capture chromosomes
Division in meiosis I occurs in what four phases?
Division in meiosis I occurs in four phases:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
What occurs in Prophase 1? (3)
(Prophase I typically occupies more than 90% of the time required for meiosis)
- Replicated chromosomes condense and spindle microtubules begin to form from centrosomes
- In synapsis, homologous replicated chromosomes loosely pair up, forming a bivalent
- In crossing over, non-sister chromatids exchange DNA segments to produce chromosomes with a combination of maternal and paternal alleles
- Cells in prophase I are diploid
What Occurs in Metaphase 1? (3)
- Paired homologs line up at the metaphase plate (cells in metaphase I are diploid)
- Microtubules from one pole are attached to the kinetochore of one replicated chromosome of each bivalent
- Microtubules from the other pole are attached to the kinetochore of the other replicated chromosome of each bivalent
What Occurs in Anaphase 1? (3)
- In anaphase I, paired homologs separate (cells in anaphase I are diploid)
- One chromosome moves toward each pole, guided by the spindle microtubules (kinetochore microtubules shorten while non-kinetochore microtubules lengthen)
- Sister chromatids remain attached at the centromere
What Occurs in Telophase 1 & Cytokinesis?
(3)
- In the beginning of telophase I, the homologs (as replicated chromosomes) finish migrating to the poles of the cell (cells in telophase I are diploid)
- Afterwards, cytokinesis occurs, ultimately forming two haploid daughter cells
- In animal cells, a cleavage furrow forms;
in plant cells, a cell plate forms
Division in meiosis II occurs in what four phases?
(all cells in the following phases are haploid)
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II and cytokinesis
What Occurs in Prophase II? (2)
- In prophase II, spindle microtubules form
- In late prophase II, replicated chromosomes (each still composed of two sister chromatids) move toward the metaphase plate
- NOTE – There is no synapsis, no bivalents, no chiasmata and no crossing over compared to prophase I
What Occurs in Metaphase II? (3)
- Replicated chromosomes are arranged at the metaphase plate
- Due to crossing over in meiosis I, the two sister chromatids of each replicated chromosome are no longer genetically identical
- Kinetochore microtubules attach to sister chromatids
What Occurs in Anaphase II? (2)
- In anaphase II, the sister chromatids separate
- The sister chromatids of each replicated chromosome now move as two newly individual chromosomes toward opposite poles of the cell
What Occurs in Telophase II and Cytokinesis? (5)
- In telophase II, the chromosomes
(which appear unreplicated) arrive at opposite poles - Nuclei form and the chromosomes begin decondensing
- Each cell then undergoes cytokinesis
- At the end of meiosis, there are four
daughter cells, each with a haploid set
of unreplicated chromosomes - Each daughter cell is genetically distinct
from the others and from the parent cell
What Mechanisms Contribute to Genetic Variation? (3)
Three mechanisms contribute to genetic variation:
– Independent assortment of chromosomes
– Crossing over
– Random fertilization
What is the Independent Assortment of Chromosomes?
- Homologous pairs of replicated chromosomes (as bivalents) orient randomly at metaphase I of meiosis
- Each bivalent sorts maternal and paternal homologs (as replicated chromosomes) into daughter cells independently of each other
- The number of combinations possible when replicated chromosomes assort independently into gametes is 2n, where n is the haploid number
- For humans (n = 23), there are ~ 8.4 million (223) possible haploid combinations of unreplicated chromosomes that gametes can receive
What is Crossing Over?
- Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
- Crossing over begins very early in prophase I, as homologous replicated chromosomes pair up
- 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
What is Random Fertilization?
- Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)
- The fusion of two gametes (each sperm or egg cell has one of ~ 8.4 million possible haploid chromosome combinations from independent assortment)
– 223 = ~ 8.4 million - Zygote can have one of ~ 70 trillion
possible diploid combinations
– 8.4 million x 8.4 million - ~70 trillion
HUMAN FEMALE SOMATIC CELL KARYOTYPE
HUMAN MALE SOMATIC CELL KARYOTYPE
HUMAN GAMETIC CELL KARYOTYPE (egg or sperm)
HUMAN GAMETIC CELL KARYOTYPE (sperm)
difference between Anaphase I and Anaphase II
