Genetics - 3.3 Meiosis

Question 1

Understandings:

Answer 1

• One diploid nucleus divides by meiosis to produce four haploid nuclei
• Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number
• DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids
• The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation
• Orientation of pairs of homologous chromosomes prior to separation is random
• The halving of the chromosome number allows a sexual life cycle with fusion of gametes
• Crossing over and random orientation promotes genetic variation
• Fusion of gametes from different parents promotes genetic variation

Question 2

Meiosis

Answer 2

= the process by which sex cells (gametes) are made in the reproductive organs - involves the reduction division of a diploid germline cell into 4 genetically distinct haploid nuclei

Process;
1st - reduction division (meiosis I):
1) interphase
2) Prophase 1
3) Metaphase 1
4) anaphase1
5) Telophase 1

2nd - mitotic division (meiosis II):
6) prophase 2
7) metaphase 2
8) anaphase 2
9) telophase 2

https://ib.bioninja.com.au/standard-level/topic-3-genetics/33-meiosis/meiotic-division.html

Question 3

Meiosis 1 - reduction division

Answer 3

The first division separates the homologous chromosomes into two intermediate cells

= separates pairs of homologous chromosomes to 1/2 the chromosome # (diploid -> haploid)

Question 4

meiosis 2 - Mitotic division

Answer 4

The second division is merely a mitotic one in nature, where the chromatids are pulled apart but the number of chromosomes remains the same. This allows large numbers of gametes to be produced

=separates sister chromatids (created by the replication of DNA during interphase)

Question 5

1) Interphase - meiosis 1

Answer 5

Before division - DNA is replicated (s phase) to produce 2 genetically identical copies

In a non-dividing cell, the chromosomes are not visible as discrete structures because they are uncalled to make the DNA information available for protein synthesis - one diploid parent cell 0 homologous pair of chromosomes - meiosis is preceded by DNA replication during which each of the chromosome replicates. For each chromosome there are now two genetically identical sister chromatid - it is at this stage that gene mutations may occur - ie they may create new versions of genes (alleles)

Question 6

sister chromatids

Answer 6

held together by a single centromere - they are later separated during meiosis 2 (following separation of homologous chromosomes in meiosis 1)

Question 7

2) prophase 1 - meiosis 1

Answer 7

The chromosomes condense. the homologues, each consisting of 2 sister chromatids, pair up in a process = synapsis to form bivalents. At this stage, the arms of the chromatids can become entangled and segments of chromosomes can be exchanged ina. process = crossing over

Question 8

3) metaphase 1 - meiosis 1

Answer 8

The bivalents line up at the ‘equator’ (the metaphase plate) of the cell in a way that is random = resulting in independent assortment of maternal and paternal chromosome

Question 9

4) anaphase 1 - meiosis 1

Answer 9

= homologues separate

• it is a this stage that mistakes may occour in the separation process, resulting in abnormal numbers of chromosomes being passed on to the gametes. If single chromosomes fail to separate the event = ANEUPLOIDY, if complete sets of chromosomes fail to separate = POLYPLOIDY (not viable in humans but possible in plants)

Question 10

5) Telophase 1 - intermediate cell

Answer 10

is that phase when the chromosomes have finished moving to opposite ends of the cell

Question 11

6) prophase 2 - meiosis 2

Answer 11

spindle apparatus forms, chromosomes migrate towards the metaphase plate

Question 12

7) metaphase 2 - meiosis 2

Answer 12

chromosomes line up on the metaphase plate

Question 13

8) anaphase 2 - meiosis 2

Answer 13

sister chromatids (now individual chromosomes) seperate

Question 14

9) telophase 2

Answer 14

= gametes (egg or sperm)

Question 15

key meiosis points:

Answer 15

= the cell divides 2x but the DNA only replicates ONCE
= at the end of meiosis, cells have 1/2 the chromosome numbers. There are 4 cells which have different combinations of chromosomes and alleles due to INDEPENDENT ASSORTMENT and CROSSING OVER
= meiosis is called a REDUCTION DIVISION where homologous chromosomes separate into gametes (ie each gamete has 1/2 the number of chromosomes
= the chromosome number is resorted (2N) when two gametes fuse at FERTILIZATION

Question 16

Meiosis and VARIATION

Answer 16

Variation:
1) crossing over
2) random assortment of chromosomes
3) random fusion of gametes from different parents

(A human can produce more than 8 million gametes with different combinations of chromosomes and alleles - hence a couple could (theoretically) have more than 64 million children before two of them are identical)

Question 17

Meiosis and variation - 1) meiosis and allele segregation

Answer 17

each pair of alleles is sorted into different gametes = and subsequently into different offspring:

due to:
- each allele being carried on separate homologous chromosomes that are separated during meiosis (ie an individual with 2 different alleles for a trait (Ss) will produce gametes in which 1/2 will contain the allele S and 1/2 will contain the s alleles)

Question 18

Meiosis and variation - 2) (independent) ASSORTMENT

Answer 18

chromosomes are sorted independently of each other -> during meiosis, alleles on one pair of homologous chromosomes separate independently from allele pairs on other chromosomes = ie the pairs separate randomly ===> *

(these alleles will be inherited in the offspring in predictable ratios determined by the genotype of parents)

Gamete combination = 2n (n = haploid #) - # of possible gamete combinations are dependent on # of homologous pairs

==> * as in the subsequent assortment of chromosomes into gametes -> the final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome following anaphase 1

• during metaphase 1 - homologous chromosomes line up at the equator as bivalents in one of 2 arrangements - maternal copy left/paternal copy right OR paternal copy left/maternal copy right
  
  • orientation of pairs of homologous chromosomes = random as is the subsequent assortment of chromosomes into gametes - final gametes will differ depending on whether they got… maternal copy left/….

Question 19

Meiosis and variation - RANDOM ORIENTATION

Answer 19

= when homologous chromosomes line up in metaphase 1
- their orientation towards the opposing poles in random
- the orientation of each bivalent occurs independently = different combinations of maternal/parental chromosomes can be inherited when bivalents separate in anaphase 1
—>

1) total # of combinations that can occur in gametes is 2n
2) humans have 46 chromosomes (n=23) = can produce 8,383,608 different gametes by random orientation
3) if crossing over also occurs, the # of different gamete combinations becomes immeasurable.

Question 20

Meiosis and CROSSING OVER:
(recombination, crossover, chiasmata)

Answer 20

When the chromatid finally separates out the independent chromosomes, the ones with the new combination of alleles = RECOMBINANTS - this process = CROSSING OVER (if the original chromosomes are AB and ab, now the recombinants are Ab and aB)

the RESULT of this process is an exchange of alleles (this makes the chromatids unique because the genes swapped around and so all gametes are different)

Crossover - a result of this exchange of genetic material = new gene combinations are formed on chromatids = RECOMBINATION

CHIASMATA = homologous chromosomes held together - once these are formed = homologous chromosomes condense as bi v/r(?)alents and then separated in meiosis
-> if crossing over occurs = all 4 haploid daughter cells will be genetically distinct (daughter chromatids - no longer identical)

Question 21

Sexual reproduction (information);

Answer 21

1) asexual reproduction
2) sexual reproduction

=> the 1/2ing of the chromosome # allows a sexual life cycle with fusion on gametes
- most sexually reproducing organisms = diploid (ie they have 2 copies of every chromosome (maternal parental copy)) -> in order to reproduce, these organisms need to make gametes that are haploid (one copy of each chromosome)

• fertilisation of 2 haploid gametes (egg + sperm) = formation of a diploid zygote that can grow via mitosis - if chromosome # was not 1/2ed in gametes, total chromosome #s would double each generation (POLYPLOIDY)

Question 22

sexual reproduction - ASEXUAL reproduction

Answer 22

= the daughter cells are exactly like the parent cells (ie they receive exact copies of the chromosomes so long as no mistakes have been made in the copying of the chromosomes (mistakes = mutations))

Question 23

sexual reproduction - SEXUAL reproduction

Answer 23

= the zygotes have a different set of chromosomes from their parents and different from their siblings (= due to the chromosomes being thoroughly mixed)

(more on pg 25 of work/note-book)

Question 24

sexual reproduction - variation

Answer 24

variation in inherited chromosomes =
- makes individuals look different from each other
- species has a better chance of survival
(ie feeding into the concept of natural selection)

(further broken down on page 25 of work/notes-book)

Question 25

Chromosome mutations (= ANEUPLOIDY) ( + NON-DISJUNCTION)

Answer 25

= a condition where one or more chromosomes are missing from or added to the normal somatic cell chromosome number => aneuploidy usually results from non-disjunction during meiosis (ND may occur in meiosis 1 and 2) 3 main types (of aneuploidy): 1. autosomal aneuploidy 2. sex chromosome aneuploidy 3. polyploidy

Question 26

Karyotyping (application - NEED TO KNOW HOW TO DO!!)

Answer 26

Karyotyping is the process by which chromosomes are organised and visualised for inspection -Karyotyping is typically used to determine the gender of an unborn child and test for chromosomal abnormalities Cells are harvested from the foetus before being chemically induced to undertake cell division (so chromosomes are visible) The stage during which mitosis is arrested will determine whether chromosomes appear with sister chromatids Finally, chromosomes are stained and photographed, before being organised according to structure The visual profile generated is called a karyogram further/more in-depth info = https://ib.bioninja.com.au/standard-level/topic-3-genetics/33-meiosis/karyotyping.html

Question 27

non-disjunction:

Answer 27

= refers to the chromosomes failing to separate correctly resulting in gametes with one extra/missing chromosome (= AANEUPLOIDY) May occur via: 1) failure of homologous to separate in anaphase 1 (= 4 affected daughter cells) 2) failure of sister chromatids to separate in anaphase 2 (= 2 affected daughter cells) can cause: 1. chromosomal abnormalities 2. down syndrome - more info on bioninja!

Question 28

after non-disjunction

Answer 28

= one cell may have 2 copies of a homologous chromosome and the other has none. if these 2 cells are fertilized by a normal gamete = 1) a zygote with 3 copies of the homologue = TRISOMY 2) a zygote with only 1 homologue = a condition called MONOSOMY (this can occur with the autosomes or the sex chromosomes and can be influenced by the age of parents)

Question 29

autosomal aneuploidy

Answer 29

= formed by non-disjunction of the autosomes 1. Down's syndrome (this is trisomy 21 because there are 3 copies of chromosome 21. It produces a child with characteristic facial features, mental deficiency, an enlarged tongue and various internal disorders. The incidence of Down's Syndrome increases in cases where the mother is over 35 years old) 2. trisomy 18 and trisomy 13 (causes serious abnormality in babies, many organs are affected and the babies die at a younger age)