Topic 3: Meiosis Flashcards
The ovum and sperm are two specialised animal cells called _________.
_________ are made by the process of_________.
The sperm and ovum fuse during ____________, to form a ________.
gametes
gametes
meiosis
fertilisation
zygote
Most human body cells, including the ones in the ovaries and testes that make gametes, contain __ chromosomes
Half of these are from the ______ and half are from the ______.
46
father
mother
A chromosome from the father and a chromosome from the mother, which contain the same _____ at the same ____ ____ (in the same position), with their _________ in the same position form a _____________ _____ ____ __________.
genes
gene loci
centromeres
homologous pair of chromosomes
Key definition: define haploid (1 point)
- HALF the number of chromosomes found in {body/SOMATIC CELLS}.
Key definition: define diploid (1 point)
- The FULL number of chromosomes found in {body/SOMATIC CELLS}.
Consider a diploid cell in the ovary with 46 chromosomes, which will produce eggs by _______.
When it divides by meiosis, _ haploid cells, each with __ chromosomes are formed.
__ chromosomes will be needed to make these _ cells.
So the cell needs to carry out ____ ________ BEFORE meiosis.
If a copy of every one of the 46 {chromosomes/DNA molecules} is made, this gives __ {chromatids/DNA molecules}.
meiosis
4
23
92
4
DNA replication
92
How does a cell prepare for cell meiosis?
3 steps
(It prepares by doubling the quantity of {chromosomes/DNA})
- Chromosomes undergo DNA replication – each chromosome makes a copy of itself
- The two chromosomes are attached to each other at a region called the CENTROMERE.
- Each chromosome (now an X shape) is now made of 2 SISTER CHROMATIDS WHICH ARE IDENTICAL.
What does meiosis form? (1 thing)
- Gametes e.g. human sperm and egg
Where does meiosis take place? (1 point)
- In the mammalian ovaries and testes
Meiosis has one type of ________ division.
nuclear (nucleus)
Each diploid gamete producing cell (in ovary/testis) makes __ _______ _______, each with _____ the number of chromosomes as parent cell.
4 haploid gametes
half
In meiosis, the gametes produced are NOT ___________ _________.
This leads to _________ _________ in offspring.
genetically identical
genetic variation
What is the {role/importance} of {meiosis/making gametes}?
2 points
- HALVES THE CHROMOSOME NUMBER, PRODUCING HAPLOID {GAMETES/cells} so that the diploid number is restored at fertilisation.
- PROMOTES GENETIC VARIATION in gametes through INDEPENDENT ASSORTMENT OF CHROMOSOMES and CROSSING OVER OF ALLELES BETWEEN CHROMATIDS.
Describe the process of meiosis
4 steps
- Homologous chromosomes line up in pairs
- (MEIOSIS I - first division) Homologous chromosomes then separate
- (MEIOSIS II - second division) Sister chromatids separate
- 4 haploid gametes are formed
In both meiosis I and meiosis II (both divisions), chromosomes attach to _______ _____.
_______ _____ contract and shorten to pull the __________ (meiosis I) or _________(meiosis II) to opposite ends of the cell.
This ensures each gamete only receives one _____ of each _____.
spindle fibres
spindle fibres
chromosomes
chromatids
allele
gene
Key Summary: State the two important features of meiosis which lead to genetic variation in the 4 gametes.
(2 points)
- Crossing over of chromatids
- Independent assortment OF CHROMOSOMES
NOTE: In exams, if meiosis is not mentioned in the question, for extra mark say: (point 1) + (point 2) + “in meiosis”
Independent assortment of chromosomes occurs during ________ _ when the homologous chromosome pairs line up ready to divide
The way that the chromosome pairs line up is _______ and with 23 pairs there are a huge number of different arrangements
The different arrangements result in different combinations of chromosomes which come together in the ________.
meiosis I
random
gametes
Key Summary: Explain Independent assortment of chromosomes
3 points
- RANDOM ALIGNING OF {HOMOLOGOUS/maternal and paternal} CHROMOSOMES on the equator of the cell
- Creates {new/DIFFERENT} COMBINATIONS OF CHROMOSOMES, SO ALSO NEW COMBINATIONS OF ALLELES, in the gametes
- OCCURS DURING METAPHASE OF MEIOSIS I when homologous chromosome pairs line up on the equator
During ________ __ ________ _, homologous chromosomes come together in pairs and chromatids come into contact.
____-________ __________ may cross over at a point called a _______.
prophase of meiosis I
Non-sister chromatids (chromatids on different chromosomes)
chiasma
At each cross over point (chiasma), chromatids _____ and _____, exchanging alleles (+ sections of DNA) between ____-_______ __________.
Several chiasmata may occur along a chromosome.
Crossing over ___________ ________, so produces {_________ _________ / _________ _________} – contain new combinations of alleles from both parents = genetic variation.
break
rejoin
non-sister chromatids
recombines alleles
{recombinant chromatids/recombinant gametes}
Key Summary: Explain crossing over
4 steps
- During MEIOSIS I, when homologous chromosomes pair up, chromatids come into contact and NON-SISTER CHROMATIDS may cross over at a point called a CHIASMA.
2 (this point is important). At a chiasma, non-sister chromatids in the same chromosome pair break and rejoin, exchanging alleles between them.
- This RECOMBINES ALLELES, so produces {recombinants/RECOMBINANT CHROMATIDS} and gametes which contain new combinations of alleles from both parents.
- This produces genetic variation in gametes.
NOTE: A non-sister chromatids are chromatids on different chromosomes.
