Meiosis and Genetic Variation Flashcards by Gitanjali Sharma

What are the gametes foud in males?

Sperm cells

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What are gametes found in females?

Egg cells

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Why do gametes join together at fertilisation?

To form a zygote- this rapidly divides and develops into a new organism

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What are diploid ?

Normal body cells having (2n) - cell contains two of each chromosome one from mother and father

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What does it mean that gametes have a hapolid number?

(n) number of chromosomes

Only one copy of chromosome

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What specifically happens at fertilisation?

Haploid sperm fuses with haploid egg

making a cell with normal diploid number of chromosomes

Half of chromosomes are from father (sperm) and half are from mother (egg)

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How does gamete produce genetic diversity?

Fertilisation is random - any sperm can fertilise an egg

Random fertilisation produce a zygote with a different combination of chromosomes to both parents

Mixing of genetic material in sexual reproduction

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Diagram of fertilisation

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Where does meiosis take place?

Reproductive organs

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What happens before Meiosis?

(Interphase I)

The DNA unravels and replicates

Two copies of chromosomes called chromatids - two centromeres

Nuclear membrane is present

Nucleolus is present

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What happens in Meiosis 1- Early Prophase?

DNA condenses and becomes visible - double armed chromosomes each from two sister chromatids

Sister chromatids are joined together at the middle by a centromere

Centrioles begin to move

Nucleolus disappears

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What happens in Meiosis 1 - Mid-prophase

Homologous chromosomes pair up and lie along side each other (SYNAPSIS)
Each pair claled a BIVALENT
Centrioles move further towards poles

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What happens in Meiosis 1 - Late Prophase 1

Chromatids tangle at a point called CHASMATA)

Chromosomes may break and rejoin

Crossing OVER occurs

Reformed chromatids called RECOMBINANTS

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What happens in Meiosis 1 - Metaphase?

Nuclear membrane breaks down

Spindle forms

Bivalent moves to the equator and attach to the spindle by the centromere

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What happens in Meosis 1 - Anaphase 1?

Contraction of spindle fibres pulls the whole chromosome towards opposite poles

Homologous chromosomes separate

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What happens in meosis 1- telophase?

Nuclear membrane reforms around the set of chromosomes

Spindle fibres disappear

Centromere divides into two

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What happens in cytokinesis in meiosis 1?

Division of cytoplasm

short phase

What happens in meiosis 2 -Prophase 2?

Centrioles move towards opposite poles

What happens to meiosis 2-Metaphase2?

Nuclear membranes disappear

Spindle forms

Chromosomes move to the equator and attach to the spindle fibres by the centromere

What happens to the meiosis 2- Anaphase 2?

Centromeres divide

Sister chromatids move towards opposite poles

What happens in meiosis 2-telophase2?

Each chromatid now called chromosomes

Nuclear membrane reforms around the group of chromosomes

Spindle fibres disappear and centrioles may divide

What happens to the cytokinesis in meiosis 2?

Division of cytoplasm

Four haploid daughter cells

Each cell is genetically differeny

What are homologous pairs?

Each pair is same size

Have different alleles

What are the two types of events that cause genetic variation in meiosis?

Crossing over of chromatids
Independent segregation of chromosomes

What does crossing of chromatids mean?

Crossing over of chromatids in meiosis 1 means that each of the four daughter cells formed from meiosis contains chromatids with different alleles

What happens in the independent segregation of chromosomes?

1. Each homologous pair of chromosomes in cells is made up of one chromosome of mum and one from dad 2. When homologous pairs are separated in meiosis 1 , completely random which chromosome from each pair ends up in daughter cell 3. Four daughter cells produced by meiosis have completely different combination of maternal and paternal chromosomes 4. Shuffling of chromosome lead to genetic variation in potential offspring

What is outcome of mitosis?

Produce cells with the same number of chromosomes as parent cells

Outcome of meiosis

Produces cell with half the number of chromosomes as parent cell

What is bivalent?

Pair of homologous chromosomes

What is chiasmata?

paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.

Give two differences between the nuclei removed from the embryo cells and the nuclei discarded from the unfertilised egg cells.

embryo cell diploid, egg cell haploid; contain different alleles/forms of the colour gene;

Explain why meiosis is important in sexual reproduction , apart from producing gametes that are genetically different

produces haploid cells / chromosome number halved; fertilisation; maintains the diploid / chromosome number (in next generation

Most plants produce seeds after fertilisation in sexual reproduction. However, dandelions produce small, windblown seeds without fertilisation taking place. Suggest two advantages to the dandelion of being able to reproduce from these seeds, as well as from pieces of root.

dispersal / prevent overcrowding / competition / colonise ; increased number of (proven) offspring; *(not quicker*

If embryos with more than eight cells are split up, the separated cells fail to develop into new embryos. Suggest why.

cells lost ability to _control_ development / no longer totipotent / cells have differentiated/become specialised;

An egg cell from a cow was fertilised in a laboratory and allowed to develop into an eight-celled embryo. This was split into four parts, each of which developed into a new embryo. This is shown in the diagram. The new embryos were later transferred into different surrogate cows. Give two advantages to a farmer of using embryos produced by this procedure.

offspring with favourable characteristics / high meat/milk yield; pedigree embryos into non-pedigree mothers / not risking pedigree mothers / rare breeds conserved; sex/gender selection;

What is the number that all daughter cells end up with meiosis?

23 chromosomes

What happens with mutations with meiosis?

Produce contain variations in the number of whole chromosomes or part of chromosomes - chromosome mutations

What do chromosome mutations lead to?

Errors caused by meiosis and inherited conditions - error in gamete (hereditary cells)

What chromosome mutation lead to Down Syndrone?

Non-disjunction - failure of chromosome to separate properly chromosome 21 can lead to syndrome

What leads to Down's syndrone?

* Person having an extra copy of chromosome 21 /extra part * Non-disjunction mean that chromosome 21 fail to separate properly during meiosis , so gets cell an extra copy * When gamete with extra copy fuses with another gamete at fertilisation , resulting zygote will have three copies of chromosome 21