WEEK 11 (Meiosis) Flashcards
What is Reproduction?
The process by which organisms give rise to offspring
What are the two basic types of reproduction?
Asexual reproduction & Sexual reproduction
What is the advantage of sexual reproduction?
Produces genetic variation in the offspring
What does Sexual reproduction involve?
The production of haploid gametes by MEIOSIS followed by FERTILISATION and the formation of a diploid zygote
Do sperm and egg contribute equally to the genetic character of the offspring? (YES/NO)
YES
Explanation: Despite their huge difference in size, sperm and egg contribute equally to the genetic character of the offspring
What is the difference between Germ-line cells and Somatic cells?
GERM-LINE CELLS = Give rise to haploid gametes by meiosis and the gametes spread genetic information into the next generation
SOMATIC CELLS = Form the body of the organism and are necessary to support sexual reproduction but themselves leave no progeny
What is the history of Meiosis?
In 1888 Theodor Boveri noted that the fertilised egg of a PARASITIC ROUNDWORM contains four chromosomes, whereas the worm’s gametes contain only two
What does meiosis involve?
One round of DNA replication followed by two rounds of cell division
Explanation: Mitosis and Meiosis both begin with a round of chromosome duplication and the reduction in chromosome number after meiosis occurs because a single round of duplication is followed by two successive cell divisions without further DNA replication
How long does Mitosis and Meiosis II take compared to Meiosis I?
- MITOSIS & MEIOSIS II = within hours
- MEIOSIS I = Last, months or even years
[Because of the long time spent in Prophase]
What is pairing?
Each duplicated paternal chromosome first locates and then attaches itself to the corresponding duplicated maternal homolog
What do duplicated homologous chromosomes do before lining up on the meiotic spindle?
Duplicated homologous chromosomes pair before lining up on the meiotic spindle to form BIVALENTS
Describe what happens in Crossing over
Non-sister chromatids in each bivalent exchange segments of DNA which is facilitated by the formation of a SYNAPTONEMAL COMPLEX
Describe how the Synaptomenal complex helps in Crossing over
As the duplicated homologs pair, the Synaptomenal complex
- Helps to hold the bivalent together
- Aligns the homologs so that strand exchange can readily occur between the non-sister chromatids
Multiple crossovers can occur between the duplicated homologous chromosomes in a bivalent (TRUE/FALSE)
TRUE
What is the importance of Chiasmata?
- CROSSOVER EVENTS create CHIASMATA between non-sister chromatids in each bivalent
- Chiasmata hold homologous chromosomes together during prophase I -> Chiasmata help ensure that the maternal and paternal homologs will segregate from one another correctly at the FIRST MEIOTIC DIVISION -> Allow for separation in Meiotic Anaphase I
What is a Kinetochore?
A disc-shaped protein structure associated with duplicated chromatids in eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart
What happens in Meiosis I?
- Pairing
- Crossing over
What is the difference in Metaphase of Meiosis I and Meiosis II?
- MEIOSIS I = Kinetochore microtubules of sister chromatids point in the SAME DIRECTION -> Arms of sister chromatids become unglued, allowing the duplicated homologs to separate
- MEIOSIS II = Kinetochore microtubules on each sister chromatid function INDEPENDENTLY -> allowing two sister chromatids to be pulled to opposite poles
Describe Meiosis II
- METAPHASE = The kinetochores of the sister chromatids point in opposite directions
- ANAPHASE = Cohesins holding the sister chromatids together at the centromere are degraded -> Allows kinetochore microtubules to pull the two sister chromatids to opposite poles
What can errors in chromosome segregation during meiosis result in?
Gametes with incorrect numbers of chromosomes
What is Non-disjunction?
The failure of homologous chromosomes or sister chromatids to separate properly during cell division
What is Down syndrome?
Down syndrome is a condition when an individual has an extra chromosome on chromosome 21
DIAGNOSIS:
- The first trimester ultrasound shows increased nuchal translucency and hypo plastic nasal bone
- Markers for Down syndrome are increased hCG and increased inhibin
CAUSES:
- Nondisjunction
- Unbalanced Robertsonian translocation
- Post Fertilisation mitotic error
SYMPTOMS:
- Intellectual disability
- Flat face
- Small ears
- Slanting eyes
- Small mouth
- Low muscle tone and loose joints
- Short neck, arms and legs
What is the most important risk factor for Trisomy?
Maternal age
What are the three most common types of autosomal trisomy?
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
[Trisomy = The gain of a chromosome]
What is Edwards Syndrome and what are the symptoms?
Babies with Edwards’ syndrome have more of chromosome 18 in some or all of the cells in their body. It is also known as Trisomy 18.
SYMPTOMS:
- Abnormally shaped head
- Rocker-bottom feet
- Intellectual disability
- Nondisjunction
- Clenched fists with overlapping fingers
- Low-set ears
- Micrognathia (small jaw)
- Congenital heart disease
- Death usually occurs by age 1
What is Patau syndrome and what are the symptoms?
Patau’s syndrome is a serious rare genetic disorder caused by having an additional copy of chromosome 13 in one or all of the body’s cells. It’s also called trisomy 13.
SIGNS AND SYMPTOMS:
- Severe intellectual disability
- Small head
- Heart defects
- Cleft palate
- Extra fingers
- Small or missing eyes
- Death usually occurs by 1
What is Klinefelter syndrome and what are the symptoms?
Klinefelter syndrome is when boys are born with an extra X chromosome (XXY). To compensate for the extra X chromosome, one X chromosome will be inactivated (Barr body).
SYMPTOMS:
- Taller than average structure
- Longer legs due to delayed epiphyseal closure which leads to increased bone length
- Absent, delayed or incomplete puberty
- After puberty, less muscle and less facial and body hair compared with other teens
- Small, firm testicles (TESTICULAR ATROPHY)
- Small penis
- Enlarged breast tissue (GYNECOMASTIA)
- Abnormal Lydia cell function - decrease testosterone and increase LH and oestrogen
What is Triple X syndrome?
Also known as Trisomy X is characterised by the presence of an extra X chromosome in each cell of a female
SYMPTOMS:
- Taller than average
- Learning difficulties
- Decreased muscle tone
- Seizures
- Kidney problems
What is Turner syndrome (Monosomy X)
Turner syndrome, a condition that affects only females, results when one of the X chromosomes (sex chromosomes) is missing or partially missing (XO)
Sex chromosome (X, or rarely Y) is loss often due to NONDISJUNCTION during meiosis or mitosis. Meiosis errors usually occur in paternal gametes (sperm missing the sex chromosome) and Mitosis errors occur after zygote formation (loss of sex chromosome in some but not all cells)
SYMPTOMS:
- Short stature
- Lymphatic defects resulting in WEBBED NECK
- 1st degree amenorrhea (late period)
- menopause before even starting period (pregnancy possible using IVF & Progesterone)
- decrease in oestrogen leads to increase in LH & FSH
- No Barr body
- Shield chest
What is Mosaicism?
A chromosomal change in only some of the cells
[Women with Turner syndrome caused by X chromosome mosaicism are said to have MOSAIC TURNER SYNDROME]
What is the difference between Meiosis I and Meiosis II?
- MEIOSIS I = separates homologous chromosomes
- MEIOSIS II - separates sister chromatids
Where does Pairing of homologous chromosomes occur, Meiosis or Mitosis?
Meiosis
Explanation: In Mitosis there is no pairing of homologous chromosomes
Where does Crossing over occur, Meiosis or Mitosis?
Meiosis
Explanation: In Mitosis crossing over does not occur
What is Asexual Reproduction?
Involves one parent and produces offspring that are genetically identical to each other and to the parent
What is Sexual reproduction?
Involves two parents and produces offspring that are genetically unique
What is Meiosis?
The type of cell division that produces gametes
