MEIOSIS

Question 1

Diploid organisms have two versions of each ……………………., also known as …………………

Answer 1

Chromosome

Homologues

Question 2

Homologues are either …………………. or ………………… and only one is packaged into a gamete.

Answer 2

Maternal

Paternal

Question 3

Pairing of homologues before segregration (the extra step of meiosis) allows for ……………………………….

Answer 3

Crossing over/ homologous recombination

Question 4

At which phase do homologues join up

Answer 4

Meiotic prophase 1

Question 5

What is the purpose(s) of crossing over

Answer 5

1. ALIGNS CHROMOSOMES READY FOR ANAPHASE

2. ALLOWS FOR GENETIC RECOMBINATION OF MATERNAL AND PATERNAL DNA

Question 6

WHAT HELPS PAIRING OF HOMOLOGUES

Answer 6

1. DNA BASE PAIRING

2. SYNAPTONEMAL COMPLEX

Question 7

HOW MANY TIMES CAN CROSSING OVER OCCUR

Answer 7

AT LEAST ONCE BUT NO MORE THAN FOUR TIMES

Question 8

HOW CLOSE ARE PAIRED HOMOLOGUES

Answer 8

400NM APART

Question 9

WHAT MAKES UP THE SYNPATONEMAL COMPLEX

Answer 9

THE AXIAL CORE - PROTEINS THAT BIND TO CHROMATIN VIA COHESIN ARE CROSS LINKED BY TRANSVERSE FILAMENTS TO FORM THE SYNAPTONEMAL COMPLEX

Question 10

WHAT ARE THE PHASES OF CROSSING OVER

Answer 10

INTERPHASE
LEPOTENE
ZYGOTENE
PACHYTENE
DIPLOTENE

Question 11

WHAT IS THE DIFFERENCE BETWEEN MITOSIS AND MEIOSIS

Answer 11

IN MITOSIS: SISTER CHROMATIDS SEPARATE TO FORM A DIPLOID CELL

IN MEIOSIS 1: HOMOLOGUES SEPARATE (BOTH KINETOCHORE ON ONE CHROMOSOME ARE ATTACHED TO THE SAME POLE
END RESULT IS A HAPLOID CELL

Question 12

WHAT CAN GO WRONG IN MEIOSIS

Answer 12

ABNORMALITIES IN CHROMOSOME NUMBER

CHROMOSOME STRUCTURAL REARRANGEMENTS

Question 13

WHAT IS MONOSOMY

Answer 13

ONLY HAVING 1 COPY OF A CHROMOSME

Question 14

EXAMPLE OF MONOSOMY

Answer 14

USUALLY EMBRYONIC LETHAL
EG MONOSOMY 45 X - TURNERS SYNDROME
COMPLETE OR PARTIAL ABSENCE OF SECOND SEX CHROMOSOME IN PHENOTYPIC FEMALES
POOR GROWTH
SHORT STATURE
DELAYED PUBERTY
CONGENITAL HEART DEFECTS

Question 15

WHAT IS TRISOMY

Answer 15

3 COPIES OF A CHROMOSOME

Question 16

EXAMPLE OF TRISOMY

Answer 16

USUALLY EMBRYONIC LETHAL
TRISOMY 21 - DOWNS SYNDROME
TRISOMY 22
TRISOMY 18 - EDWARD SYNDROME

USUALLY DIE BEFORE OR SHORTLY AFTER BIRTH

Question 17

WHAT IS POLYPLOIDY

Answer 17

WHOLE SETS OF CHROMOSOMES DUPLICATED

Question 18

HOW DOES ANEUPLOIDY OCCUR

Answer 18

THIS OCCURS BY CHROMOSOME NON DYSJUNCTION EITHER IN HOMOLOGOUS CHROMOSOMES OR SISTER CHROMATIDS UPON SEPARATION IN MITOSIS OR MEIOSIS 1 OR MEIOSIS 2
IF OCCURS IN MEIOSIS ONE YOU WILL HAVE NO NORMAL GAMETES PRODUCED

Question 19

WHAT IS THE IMPACT OF ADDITIONAL SEX CHROMOSOMES

Answer 19

MINOR PROBLEMS , NORMAL LIFE SPAN

Question 20

WHAT IS THE IMPACT OF LACKING SEX CHROMOSOMES

Answer 20

45Y - NOT VIABLE
45X - TURNERS SYNDROME
ONLY 1% SURVIVE TO TERM

Question 21

WHAT ARE THE TWO MODELS OF POSSIBILITY WHEN WE SEE LETHALITY

Answer 21

1. HAPLOINSUFFICIENCY - IN TERMS OF SEX CHROMOSOMES THE DOSE IS CRITICAL
2. IMPRINTED GENES ON X - MONOALLELIC EXPRESSION IS LOST

THIS ASSUMES X INACTIVATION

Question 22

WHAT IS X INACTIVATION

Answer 22

IN FEMALES ONE OF THE X CHROMOSOMES IS RANDOMLY INACTIVATED (MOST OF THE GENES ARE NOT CONTRIBUTING)

Question 23

WHY ARE SOME GENES ON X INACTIVATED CHROMOSOMES STILL EXPRESSED

Answer 23

SOME REGIONS OF DNA ON THE SEX CHROMOSMES (PAR) BEHAVE ON A PSEUDOAUTOSOMAL BASIS IE THEY ACT LIKE AUTOSOMAL CHROMOSOMES AND THEREBY REQUIRE EXPRESSION FROM BOTH COPIES OF THE GENE - THEREFORE THE DOSAGE MATTERS

Question 24

GIVE AN EXAMPLE OF X INACTIVATION AND LETHALITY

Answer 24

USUALLY IN FEMALES AN X IS INACTIVATED - THIS IS RANDOM. IN TURNER SYNDROME, ON ONE FULL X IS PRESENT. IF THE FULL X IS DEEMED TO BE THE X THAT IS INACTIVATED THEN THERE IS NO TRANSCRIPTION.
IF THERE FULL X IS DEEMED ACTIVE THEN IT IS STILL HAPLOINSUFFICIENT SINCE THE PAR GENES REQUIRE TWO COPIES AND SO NOT ENOUGH CAN BE TRANSCRIBED.
BOTH RESULT IN DEATH.
IN MOSAIC TUNERS SYNDROME, SOME OF THE CELLS HAVE BOTH COPIES AND SOME OF THE CELLS DONT THEREFORE 1% CAN SURVIVE.

Question 25

WHY DO WE SEE LETHALITY WITH IMPRINTED GENES ON X IN ANEUPLOIDY THEORETICALLY

Answer 25

IF A GENE IS IMPRINTED ON X THEN THE GENE ON X IS SILENCED AND THEREFORE MONOALLELIC WITH 100% CONTRIBUTION FROM THE OTHER COPY. IF THERE IS A FAULT WITH THIS COPY/OR THE CHROMOSOME IS LOST THEN THERE IS NO GENE EXPRESSION

Question 26

WHICH OF THE TWO LETHALITY MODELS IS CORRECT

Answer 26

HAPLOINSUFFICIENCY

Question 27

HOW CAN WE TEST THAT THE HAPLOINSUFFIENCIENCY MODEL IS CORRECT

Answer 27

1. HESC IN CULTURE - SOME CAN RANDOMLY LOSE A SEX CHROMOSOME
2. IF WE SCREEN BOTH TYPES AND LOOK FOR GENES THAT ARE PSEUDOAUTOSOMAL/ESCAPED X INACTIVATION THEN 37 GENES MEET THIS CRITERIA
3. ONLY 3 OF THE 37 ARE EXPRESSED AND ESCAPE X INACTIVATION (ARSE, STS, TBL1X)
4. NONE SHOWED IMPRINTING (LOOKED FOR SNP IN THE GENOME THAT ARE HETERO BUT HOMO IN THE GENE YOU ARE LOOKING AT.)