meiosis 6.3 Flashcards
what is meant by diploid
- presence of two complete sets of chromosomes in an organism’s cells
- ^each parent contributing a chromosome to each pair
what are gametes
sex cells (sperm and eggs)
what is a zygote
The fertilised egg of an animal
what produces gametes
Gametes are formed by a form of cell division known as meiosis
What is the difference between meiosis and mitosis
- mitosis: produces 2 idential daughter cells, used for growth and repair
- meiosis: nucleus divides twice, produce 4 non-identical haploid gametes
What is meant by reduction division
Each gamete contains half of the chromosome number of the parent cell - it is haploid. Meiosis is therefore known as reduction division
what is meant by homologous chromosomes
- essentially identical but can contain different alleles
- have same gene sequence, loci, centromere location and chromosomal length
explain what is meant by an allele
Different versions of the same gene
what are the characteristics of genes on homologous chromosomes
- have same genes at same loci
^be same length and size when visible in prophase
what are the 2 main divisions of meiosis
meiosis I and meiosis II
what is another name for the first division that occurs in meiosis I and why has it got this name
- reduction division, pairs of homologous chromosomes are separated into 2 cells
- Each cell is haploid
explain what is produced at the end of meiosis II
The second division is similar to mitosis, and the pairs of chromatids present in each daughter cell are separated, forming 2 more cells.
4 haploid daughter cells are produced in total
what are the stages of Meiosis I
prophase I
Metaphase I
Anaphase I
Telophase I
what occurs in Prophase I
Chromosomes condense, the nuclear envelope disintegrates, the nucleolus disappears and spindle formation begins (as in prophase of mitosis)
what are bivalents
- set of two homologous chromosomes lying adjacent to each other held together at chiasmata
what is the difference between prophase I in meiosis and prophase in mitosis
The difference in prophase 1 is that the homologous chromosomes pair up, forming bivalents.
what is crossing over and when does it occur (what stage and when in the stage)
the exchange of DNA between paired homologous chromosomes (one from each parent) that occurs when chromosomes (large molecules of DNA) are moved through the cytoplasm as they are brought together and the chromatids entangle
crossing over occurs in prophase I
what is the difference between metaphase I in meiosis and metaphase in mitosis
Metaphase I is the same as metaphase in mitosis except that the homologous pairs of chromosomes assemble along the metaphase plate instead of the individual chromosomes
what is independent assortment
The orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair. It can result in many different combinations of alleles facing the poles
why would you want independent assortment
it results in genetic variation
when does independent assortment occur
it occurs in Metaphase I and II
what is the difference between anaphase I in meiosis and anaphase in mitosis
Anaphase I is different from anaphase of mitosis as the homologous chromosomes are pulled to the opposite poles and the chromatids stay joined to each other
What are chiasmata
Sections of DNA on ‘sister’ chromatids, which became entangled during crossing over, now break off and rejoin - sometimes resulting in exchanging of DNA. The points at which the chromatids break and rejoin are called chiasmata
why would you want crossing over
To bring about genetic variation
how does genetic variation occur from the crossing over during meiosis
- crossing over forms recombinant chromatids
- genes exchanged may contain different alleles
- genetic variation arises from new combination of alleles
- sister chromatids are no longer identical
- explain the process of telophase I in meiosis
- how does this differ from telophase of mitosis
- chromosomes assemble at poles and nuclear membrane reforms
- chromosomes uncoil
- cytokinesis then occurs dividing cell into 2.
- here is where cells go from diploid to haploid
- the same apart from product (one is dipolid other is haploid)
what are the stages of meiosis II
prophase II
metaphase II
anaphase II
telophase II
what occurs in prophase II
In prophase II the chromosomes, which consist of 2 chromatids, condense and become visible again. The nuclear envelope breaks down and spindle formation begins
How do metaphase II and metaphase I differ in meiosis
- homologous chromosomes independently assorted along metaphase plate in metaphase I
- individual chromosomes in metaphase II
What occurs in metaphase II to produce genetic variation
there is independent assortment again and more genetic variation is produced
How does anaphase I and anaphase II differ
- anaphase I: homologous chromosomes pulled to opposite poles
- anaphase II: sister chromatids of chromosome pulled to opposite poles after division of centromere
what occurs in telophase II
The chromatids assemble at the poles at telophase II like telophase of mitosis. The chromosomes uncoil and form chromatin again. The nucleus envelope reforms and the nucleolus becomes visible
what is produced at the end of telophase II
Cytokinesis results in division of the cells forming 4 daughter cells in total. The cells will be haploid due to the reduction division. They will also be genetically different from each other, and from the parent cell , due to the processes of crossing over and independent assortment
