6.2-6.3 Mitosis and Meiosis

Fill in the blanks: mitosis is the process of ___ division by which ___ (number) ___ ___ (describes genetic material) ___ nuclei are produced from a ___ cell

nuclear, 2, genetically identical, daughter, parent

Why do all living organisms need to produce cells by mitosis?

growth, repair and for some organisms asexual reproduction

Fill in the blanks: during interphase, DNA is in the form of ___, which appears as a dark mass in the nucleus, before it is replicated into __ ____ ____, held together by a ____

chromatin, 2 sister chromatids, centromere

Describe in detail what happens in prophase?

the chromatin fibres (made of proteins, RNA and DNA) begin to coil and condense into chromosomes. The centrioles move to opposite poles of the cell and begin to organise the formation of microtubules that will from the spindle. The nuclear envelope begins to break down into vesicles and the nucleolus disappears

Fill in the blanks: spindle fibres develop from the ____ and are made from ___ threads that are formed in the ___

centrioles, tublin, cytoplasm

Why does the nuclear envelope break down in prophase and what does it break down into?

it breaks down into vesicles so that it is easier to move the chromosomes

Fill in the blanks: in metaphase, ____ of the spindle fibres attach to the ____ from either side and pull in _____ direction. This causes the chromatids to line up at the equator of the cell, often called the ____ ___.

microtubules, centromeres, opposite, metaphase plate

What happens during anaphase?

the centromere of each pair of chromatids splits as the spindle fibres contract and pull the individual chromatids apart and towards opposite poles, which create a V-shape

How do the spindle fibres contract during anaphase?

motor proteins ‘walk’ along the tubulin threads

How do the spindle fibres get their energy to contract?

the energy comes from ATP (from aerobic respiration) in mitochondria which gather around the fibres

What happens in telophase?

The separated chromosomes reach opposite poles and new nuclear membranes from around them, as well as new nucleolus. The chromosomes begin to decondense and uncoil, and the spindle fibres break down.

What stage of mitosis does this image show?

anaphase

What stage of mitosis does this image show?

metaphase

What stage of mitosis does this image show?

prophase

What stage of mitosis does this image show?

telophase

What is cytokinesis?

the division of the cytoplasm to create two daughter cells

Describe the process of cytokinesis in animal cells

Fill in the blanks: a ____ ____ forms in the middle of the cell as the ___ ___ is pulled inwards by the cytoplasm. When the plasma membranes from each side of the cell are in close proximity they fuse. Contractile proteins called ____ are found along the equator of the cell which cause the membranes to separate.

cleavage furrow, cell-surface/plasma membrane, actins

Why can’t cytokinesis in plant cells happen the same way it does in animal cells?

as plants have cell walls, a cleavage furrow cannot be formed

Describe the process of cytokinesis in animal cells

Fill in the blanks: ___ assemble along the ___ and fuse with each other, forming connecting channels - ____ - which fuse with the plasma membrane, dividing the cells into two. New section of cell __ form along the new membrane.

vesicles, equator, plasmodesmata, wall

Give 3 advantages of asexual reproduction

• only 1 parent required (saves time + energy)

• genetically identical offspring (successful adaptations passed on)

• quick dispersal and spread/colonisation

• rapid multiplication

What is the main disadvantage of asexual reproduction?

the lack of genetic variation in offspring (unless mutations occur) so evolution by natural selection cannot happen and organisms cannot adapt to changing environments

Fill in the blanks: meiosis is a form of ___ cell division in which _ (number) genetically ___ daughter cells are produced through _ (number) divisions

nuclear, 4, different, 2

What is mean by a pair of homologous chromosomes?

A pair of chromosomes, one maternal and one paternal, that contain that same genes at the same loci, but may contain different alleles

What is a karyotype and what are they used for?

a visual map of the complete set of chromosomes in an organism, which can be used to detect genetic conditions by spotting abnormalities in chromosome count or arrangement

Describe what happens in prophase I of meiosis

• chromosomes condense and coil

• nuclear envelope breaks down

• spindle fibres begin forming from the centriole

• chromosomes pair up in their homologous pairs, forming bivalents

• crossing over of non-sister chromatids may occur - this point is called the chiasma

Describe what happens in metaphase I of meiosis

• the bivalents randomly line up along the equator/metaphase plate (independent assortment)

• spindle fibres attach to the centromeres

Describe what happens in anaphase I of meiosis

• each pair of homologous chromosomes is pulled apart

• the crossed over areas of DNA separate forming recombinant chromatids

Describe what happens in telophase I of meiosis

• chromosomes assemble at each pole and uncoil, and the nuclear envelope reforms

• cell undergoes cytokinesis

After meiosis I, are the daughter cells produced haploid or diploid?

haploid

True or false? Telophase I of meiosis doesn't occur in plant cells

true

Describe what happens in prophase II of meiosis

• chromosomes coil and condense

• nuclear envelope breaks down

• spindle formation begins but at a right angle to the previous formation

Describe what happens in metaphase II of meiosis

• chromosomes arrange randomly on the metaphase plate (independent assortment)

• chromosomes may cross over again

Describe what happens in anaphase II of meiosis

• centromeres divide and the individual chromatids are pulled to opposite poles

Describe what happens in telophase II of meiosis

• nuclear envelopes from around each of the 4 haploid nuclei as the chromosomes uncoil

• the two cells divide by cytokinesis to form 4 haploid cells

State 4 ways that genetic diversity is created during meiosis

crossing over, independent assortment, random fertilisation, mutations

Describe the process of crossing over

• in prophase I non-sister chromatids of homologous chromosomes can become entangled so that portions break off and region the other chromatid, exchanging alleles

• the point of contact is the chiasma (plural: chiasmata)

• this forms recombinant chromosomes

What is independent assortment and how does it happen?

the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I and II

How do you calculate the number of possible chromosome combinations?

2ⁿ (where n is the number of chromosomes in a haploid cell)

How does random fertilisation increase genetic diversity?

each gamete carries genetically different alleles and any random male gamete can fuse with a random female gamete