6.1.2 Patterns Of Inheritance Flashcards
How do prokaryotes reproduce?
Asexual reproduction by Binary fission
What is the point of sexual reproduction?
To achieve genetic variation
What type of reduction is Meiosis and why?
A reduction divison
Daughter cells have 1/2 the original no. chromosomes
Haploid cells produced and used for sexual reproduction
Compare mitosis vs Meiosis
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
What is a diploid cell?
‘Body’ or ‘Somatic’ cell that contains the normal number of chromosomes
- 2n
- Chromosomes form homologous pairs
What is a haploid cell?
‘Sex’ cells/gametes
- n
- contain 1/2 normal no. chromosomes
How does fertilisation occur in humans?
Fusion of a sperm (n) + egg (n) to form a zygote
Zygote is a fertilised egg (2n)
What are homologous chromosomes
A pair of chromosomes that have the same genes
How many homologous chromosomes are in somatic cells?
23
What is a centromere?
The place two chromatids are held together
What is a chromatid?
Each side of a pair when two chromosomes join together
What is locus/loci
The position of a gene on a chromosome
What is a gene?
A section of DNA that codes for a polypeptide
What is an allele?
A different form of the same gene
What is a histone?
A protein involved in coiling DNA
What is polymerase
An enzyme that forms covalent bonds between phosphates + deoxyribose sugars in semi-conservative replication od DNA
What is a nucleotide?
Monomer of DNA - has a deoxyribose sugar, phosphate group + base
What occurs in prophase 1 of meiosis?
- Chromatin condenses and chromosomes supercoil
- Chromosomes shorten + thicken (will now stain) + centrioles move to opposite poles
- Homologous chromosomes form bivalents (same genes in different loci)
- Non-sister chromatids wrap around each other at chiasmata (Sections of chromosomes swap crossing over)
- Nuclear envelope disintegrates, nucleolus not visible, spindle fibres form + attach to chromosome’s centromeres
How does crossing over/allele mixing occur?
- Homologous chromosomes pair up to form bivalents
- Non-sister chromatids wrap + attach (Chiasmata)
- DNA is swapped (allele mixing)
- Chiasmata broken at anaphase 1
What occurs in metaphase 1
- Spindle fibres still attached to centromere of chromosomes
- Bivalents line up on equator of the cell
- Chiasmata remain
- Independant assortment of bivalents at equator
- Random segregation of bivalent at anaphase 1
What happens at anaphase 1
- Homologous chromosomes from each bivalent pulled to opposite poles
- Spindle fibres shorten
- Random segregation of bivalents
- Centromeres don’t divide
- Chiasmata separate
- Swapped chromatid lengths remain with new chromatid - now have new allele combinations
What happens at telophase 1?
- Spindle fibres broken down
- 2x nuclear envelopes reform around chromosomes (in animal cells)
- Cell membrane cleaves + cytoplasm divides by cytokinesis
What occurs in prophase 2?
- Nuclear envelope disintegrates again
- Chromosomes supercoil + condense
- Spindle fibres form from centrioles
- Chromosomes attach to spindle fibres at centromere
What occurs in metaphase 2?
- Chromatids arrange themselves on equator by spindle fibres attached to centromeres
- Independant assortment of chromatids
What occurs in anaphase 2?
- Centromeres divide + chromatids pulled to opposite poles by spindle fibres
- Chromatids randomly segregate
What occurs in telophase 2?
- Nuclear envelope reforms around haploid daughter cell nuclei
- In animal cells: The two cells divide to give 4 haploid cells
- In plant cells: a tetrad for four haploid cells is formed
Similarities between structures in cell of mitosis + meiosis
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
Differences between structures in cell of mitosis + meiosis
- 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
How does sexual reproduction produce genetic variation:
- 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
What is non-disjunction and why is it an issue?
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
How does genetic variation occur in meiosis?
- 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
Define a gene
A section of DNA that contains the complete sequence of bases (codons) to code for a polypeptide
Define an allele
Different versions of the same gene found on a particular locus of a chromosome
Define genotype
The alleles that the organism contains for certain genes
Define homozygous
A genotype that has two copies of the same allele at a particular gene locus
Define heterozygous
A genotype that has two different alleles for a specific gene
Define phenotype
The characteristics that are expressed in an individual determine by the genotype + environment
What is a dominant allele?
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)
What is a recessive allele?
An allele that is only expressed when homozygous/in absence of the dominant allele
