6.1.2 Patterns Of Inheritance

Question 1

How do prokaryotes reproduce?

Answer 1

Asexual reproduction by Binary fission

Question 2

What is the point of sexual reproduction?

Answer 2

To achieve genetic variation

Question 3

What type of reduction is Meiosis and why?

Answer 3

A reduction divison
Daughter cells have 1/2 the original no. chromosomes
Haploid cells produced and used for sexual reproduction

Question 4

Compare mitosis vs Meiosis

Answer 4

Mitosis
- Diploid (2N) cells made
- Cells are clones (genetically identical)
- Same genetic information in parent cell + daughter cell
- 2 cells made
- 1 division of the nucleus + 1 division of the cell by cytokinesis

Meiosis
- Haploid (n) cells made
- Cells are genetically different (variation)
- Daughter cells show genetic variation from parent cell
- Daughter cells have 1/2 no. chromosomes as parent cell
- 4 cells made
- 2 divisions of nucleus, 2 divisions of cell by cytokinesis

Question 5

What is a diploid cell?

Answer 5

‘Body’ or ‘Somatic’ cell that contains the normal number of chromosomes
- 2n
- Chromosomes form homologous pairs

Question 6

What is a haploid cell?

Answer 6

‘Sex’ cells/gametes
- n
- contain 1/2 normal no. chromosomes

Question 7

How does fertilisation occur in humans?

Answer 7

Fusion of a sperm (n) + egg (n) to form a zygote
Zygote is a fertilised egg (2n)

Question 8

What are homologous chromosomes

Answer 8

A pair of chromosomes that have the same genes

Question 9

How many homologous chromosomes are in somatic cells?

Answer 9

23

Question 10

What is a centromere?

Answer 10

The place two chromatids are held together

Question 11

What is a chromatid?

Answer 11

Each side of a pair when two chromosomes join together

Question 12

What is locus/loci

Answer 12

The position of a gene on a chromosome

Question 13

What is a gene?

Answer 13

A section of DNA that codes for a polypeptide

Question 14

What is an allele?

Answer 14

A different form of the same gene

Question 15

What is a histone?

Answer 15

A protein involved in coiling DNA

Question 16

What is polymerase

Answer 16

An enzyme that forms covalent bonds between phosphates + deoxyribose sugars in semi-conservative replication od DNA

Question 17

What is a nucleotide?

Answer 17

Monomer of DNA - has a deoxyribose sugar, phosphate group + base

Question 18

What occurs in prophase 1 of meiosis?

Answer 18

1. Chromatin condenses and chromosomes supercoil
2. Chromosomes shorten + thicken (will now stain) + centrioles move to opposite poles
3. Homologous chromosomes form bivalents (same genes in different loci)
4. Non-sister chromatids wrap around each other at chiasmata (Sections of chromosomes swap crossing over)
5. Nuclear envelope disintegrates, nucleolus not visible, spindle fibres form + attach to chromosome’s centromeres

Question 19

How does crossing over/allele mixing occur?

Answer 19

1. Homologous chromosomes pair up to form bivalents
2. Non-sister chromatids wrap + attach (Chiasmata)
3. DNA is swapped (allele mixing)
4. Chiasmata broken at anaphase 1

Question 20

What occurs in metaphase 1

Answer 20

1. Spindle fibres still attached to centromere of chromosomes
2. Bivalents line up on equator of the cell
3. Chiasmata remain
4. Independant assortment of bivalents at equator
5. Random segregation of bivalent at anaphase 1

Question 21

What happens at anaphase 1

Answer 21

1. Homologous chromosomes from each bivalent pulled to opposite poles
2. Spindle fibres shorten
3. Random segregation of bivalents
4. Centromeres don’t divide
5. Chiasmata separate
6. Swapped chromatid lengths remain with new chromatid - now have new allele combinations

Question 22

What happens at telophase 1?

Answer 22

1. Spindle fibres broken down
2. 2x nuclear envelopes reform around chromosomes (in animal cells)
3. Cell membrane cleaves + cytoplasm divides by cytokinesis

Question 23

What occurs in prophase 2?

Answer 23

1. Nuclear envelope disintegrates again
2. Chromosomes supercoil + condense
3. Spindle fibres form from centrioles
4. Chromosomes attach to spindle fibres at centromere

Question 24

What occurs in metaphase 2?

Answer 24

1. Chromatids arrange themselves on equator by spindle fibres attached to centromeres
2. Independant assortment of chromatids

Question 25

What occurs in anaphase 2?

Answer 25

1. Centromeres divide + chromatids pulled to opposite poles by spindle fibres 2. Chromatids randomly segregate

Question 26

What occurs in telophase 2?

Answer 26

1. Nuclear envelope reforms around haploid daughter cell nuclei 2. In animal cells: The two cells divide to give 4 haploid cells 3. In plant cells: a tetrad for four haploid cells is formed

Question 27

Similarities between structures in cell of mitosis + meiosis

Answer 27

**Both Involve** - Chromatin condensing, chromosomes supercoiling + shortening + thickening - Centrioles moving to opposite poles of the cell - Centrioles produce spindle fibres - Nuclear envelope disintegrates - Spindle fibres attach to centromere of chromosomes - Spindle fibres shorten - Nuclear envelope reform - Spindle fibres break down

Question 28

Differences between structures in cell of mitosis + meiosis

Answer 28

- Mitosis: Homologous chromosomes don't form bivalents - Meiosis: Homologous chromosomes form bivalents - Mitosis: No chiasmata + crossing over - Meiosis: Non sister chromatids wrap around at chiasmata (leading to crossing over) - Mitosis: Chromosomes line up along equator - Meiosis: Bivalents line up along equator (Metaphase 1) - Mitosis: Centromeres divide at anaphase - Meiosis: Centromeres don't divide at anaphase 1 - Mitosis: Spindle fibres form once - Meiosis: Spindle fibres form twice - Mitosis: Nuclear envelope disintegrates once - Meiosis: Nuclear envelope disintegrates twice

Question 29

How does sexual reproduction produce genetic variation:

Answer 29

- Offspring produced from two individuals + random fusion of gametes occur at fertilisation - Random mutations occur at any time, changing DNA base sequence - Non-disjunction can occur

Question 30

What is non-disjunction and why is it an issue?

Answer 30

When chiasmata do not break at anaphase the chromosomes don't seperate Gametes will not be haploid so there's an incorrect chromosome number, leading to chromosome mutation

Question 31

How does genetic variation occur in meiosis?

Answer 31

- Independent assortment followed by random segregation in Meiosis 1 - Crossing over of non sister chromatids (chiasmata) - Independant assortment followed by random segregation of chromatids in Meiosis 2 - Random mutations - Random fertilisation

Question 32

Define a gene

Answer 32

A section of DNA that contains the complete sequence of bases (codons) to code for a polypeptide

Question 33

Define an allele

Answer 33

Different versions of the same gene found on a particular locus of a chromosome

Question 34

Define genotype

Answer 34

The alleles that the organism contains for certain genes

Question 35

Define homozygous

Answer 35

A genotype that has two copies of the same allele at a particular gene locus

Question 36

Define heterozygous

Answer 36

A genotype that has two different alleles for a specific gene

Question 37

Define phenotype

Answer 37

The characteristics that are expressed in an individual determine by the genotype + environment

Question 38

What is a dominant allele?

Answer 38

An allele that is always expressed in a phenotype (even if a different allele for the same gene is present in the genotype e.g. heterozygous)

Question 39

What is a recessive allele?

Answer 39

An allele that is only expressed when homozygous/in absence of the dominant allele