BIOLOGY: unit 2

Chromosome

Thread-like structure located inside the nucleus of animal + plant cells, made of protein + deoxyribonucleic acid (DNA) - which contains the specific instructions that make each type of living creature unique. They are passed from parents to offspring.

does DNA replicate or duplicate?

replicate

DNA REPLICATION

→ semi-conservative process because when a new double-stranded molecule is formed, one strand will be from the original template molecule, one strand will be newly synthesised

→ bc each nitrogenous base can only pair with its complimentary base (A+T/C+G)

→ each new strand formed will be identical to the original strand separated from the template

HAPLOID

(n) A haploid cell contains 1 set of chromosomes/1 copy of each chromosome.

* female/male sex cells
* gametic
* humans have 23

DIPLOID

(2n) A diploid cell contains 2 sets of chromosomes/2 copies of each chromosome.

* blood/skin/muscle cells
* somatic
* humans have 46

What type of cell are sex cells/what do they become

male sex cell ( 23 haploid) + female sex cell ( 23 haploid) fertilise and fuse = zygote (diploid)

info on chromosomes

• XX = female (afab)

• XY = male (amab)

• typically 1 are the shortest and 22 longest

• double of 21 or 13 is usually Down syndrome

GENOME

Refers to all the genetic material in one haploid set of chromosomes, contains all the necessary information for an organism to function + reproduce throughout its lifespan. Every cell in an individual has a complete set of the genome.

nucleus containing DNA → cell w/ copies of inherited DNA → organism w/ traits inherited from both parents.

GENE

Short sections of DNA that code for proteins.

Proteins provide many essential functions in the body (‘building blocks of life’), ie antibodies that support immune functions + other essential molecules of the body, support muscle contraction + movements etc.

GENE LOCUS

The position of a gene in the chromosome.

HOMOLOGOUS PAIR

In sexually reproducing organisms, most cells have H.Ps. 1 from sperm and 1 from egg. (46, 23 pairs). Genes that control the same phenotypic characteristic are found at the same locus.

* basically shares same gene, are same size

ALLELE

Variation of a gene that encodes traits (variations of phenotypic characteristics).

* Chromosome 2: both carry the same genes, ie. how your chin forms. each parent has different version of this- dad may have narrow chin and mother may have square chin, these are the variations of gene- the alleles.
* one allele will be dominant. locus for eye colour gene, mums blue dads brown, one dominant.

DNA

the molecule of heredity (the genetic transmission of characteristics from one generation to another)

ouble-stranded helix, the 2 strands are held together by hydrogen bonds.

Comes together, combines + twists up (SA)

There are no same pairings (i.e. 2 gs)

• Prokaryotic cells (simple cell lacks a nucleus and membrane-bound organelles) = DNA is located in the chromosome within the nucleoid and plasmids.

• Eukaryotic cells  = DNA is located in the nucleus packaged into chromosomes.

Describe the structure of DNA ladder

• 2 antiparallel DNA strands

• held together by hydrogen bonds

• 2 sugar phosphate backbones

• double helix

Hydrogen bond

Helps form the double stranded helix, draw dotted

Why is there an order to the bases

The order of the bases within a DNA molecule provides information about the entire genome/particular genes.

What are the building blocks of DNA?

Nucleotides

The 3 things a Nucleotide contains

* Phosphate group
* Pentose sugar
* Nitrogenus base

Nitrogenus base

Makes up the ‘steps’ of the ‘DNA ladder’. There are 4 types:

* GUANINE
* CYTOSINE
* ADENINE
* THYMINE

Describe the bases’ pairing

Complimentary base pairing - the bases pair in a specific way.

PURINES | PYRIMIDINES

Guanine + Cytosine (3 BONDS)

Adenine + Thymine (2 BONDS)

Complimentary base pairing

They join with weak hydrogen bonds. This leads to the formation of a double-stranded molecule of two opposing chains of nucleotides.

What allows the bases to pair up?

Hydrogen bonds

Complimentary DNA strand to

GCTAAGTTCACTTG

CGATTCAAGTGAAC

Number of sets in a species: does it vary?

Yes

n = number of sets, haploid = n, diploid = 2n

N stands for

Number of sets, eg. Haploid = n, Diploid = 2n.

Monoploidy

Common in insects

→  (Different to haploid as) all cells having 1n, not just the sex cells

Polyploidy

• when an organism acquires 1 or more extra sets of chromosomes.

triploid (3N), tetraploid (4N)

→ common in flowering plants

If the haploid # of chromosome sets in Red Kangaroos is 8, then the diploid # is

16

If the haploid # of chromosome sets in Catfish is 52, then the diploid # is

104

Where do cells make copies of DNA?

the nucleus

\#’ and 5’

Are at opposite ends of the DNA strand, used for directionality of DNA strand, Synthesis always happens form 5’ to 3’.

If a nucleic acid is found to contain 36% Adenine and 14% Guatine, what percentage of Thymine and Cytosine bases would it have?

36% Thymine

14% Cytosine

Fertilisation

When the nucleus of a sperm fuses with that an egg nucleus, the diploid number of chromosomes is restored.

•Male and female haploid (1n) sex cells fuse to form a diploid (2n) zygote.

•The new individual will have a different combination of DNA from that of either parent.

•This ensures variation among offspring and between offspring and parents.

MEIOSIS

Process that produces gametes that have a haploid number of chromosomes.

Germ cells: what, n/2n, where

* any biological cell that gives rise to the gametes of an organism that reproduces sexually
* they are 2n (diploid), they undergo meiosis 1 + 2 to create gametes (n) (haploid) (not mitosis)
* there are 2 cell divisions in meiosis, produces 4 daughter cells.
* Germ cells are found in the gonads
* Cell cycle still the same - G1, S, G2

M1: PROPHASE 1

• **by the end of interphase, the chromosomes have sister chromatids (appearing double stranded)

• Chromosomes condense, becoming visible

• Chromosomes align with homolungus pair + synapse

• Synapse = 1/more exchanges/crossovers occur between a matching segment of 1 chromosome with its pair, produces new combinations of genetic instructions, the sister chromatids are no longer identical.

• nuclear membrane disappears

M1: METAPHASE 1

* chromosomes line up across the equator of the cell
* chromosomes are NOT in single file (as mitosis), in pairs down the middle (metaphase plate)
* pairs are not mum/dad either side.

M1: ANAPHASE 1

• chromosomes are pulled by spindles to opposite poles of the cell (not sister chromotids)

M1: TELOPHASE 1 + CYTOKENISIS

• nuclear membrane forms around each group of separated single stranded chromosomes

• chromosomes decondense

• cell divides into 2 daughter cells

• each of these 2 cells now undergo meiosis 2

M2: PROPHASE 2

* chromosomes condense, becoming visible
* nuclear membrane disappears
* no crossing over/synapses occurs/no pairing of homologous pairs

M2: METAPHASE 2

* chromosomes line up across cell equator, different arrangements possible

M2: ANAPHASE 2

* chromatids are pulled by spindles to opposite poles of the cell

M2: TELOPHASE 2 + CYTOKENSIS

* nuclear membrane forms around each group of separated single stranded chromosomes
* chromosomes decondense
* cell divides into 2 daughter cells

Non-coding regions of genes

* junk DNA x (?)
* Some non coding regions have specialised functions such as starting DNA replication or packaging the DNA into a chromosome or helping chromosomes align during mitosis. 

AUTOSOMES

Matched pairs of chromosomes

* maternal chromosome, paternal chromosome
* alleles

human autosomes - how many?

The 46 chromosomes of a typical human cell consist of 22 pairs of autosomes (numbered 1 to 22) and one pair of sex chromosomes (XX or XY)

If exam multi-choice says all/always

typically not true bc it is likely there are exceptions

Distinct differences between the X and Y chromosomes

* X is very long compared to Y
* X contains about 900 genes
* Y contains about 55 genes

Karyotype

Visual display of an individual’s chromosomes set out in homologous pairs- which are arranged in order of length, usually from longest to shorter. Matched chromosomes by size, position of centromere and banding pattern to form pairs.

Provides info - sex, any extra/less chromosomes and therefore a genetic condition/disorder.

Humans - each H.P of autosomes given number from 1 to 22 and the sex chromosomes follow.

**write chromosomes as 46, 46 + x or 46 - x.

XY

male, mother contributed x and dad contributed y

XX

female, one x from mother one from father

PLOIDY

Complete sets of chromosomes in a cell

DIPLOID

Having 2 complete sets of chromosomes in a cell

HAPLOID

1 set of chromosomes in cell

MONOPLOID

Having a single set of chromosomes

POLYPLOID

More than 2 complete sets of chromosomes in a cell

ANEUPLOID

Having an abnormal number of chromosomes in a cell, like 1 less/extra

monosomy

having a missing chromosome, e.g. one instead of 2

INSERTION and DELETETION

Missing information is called a deletion, and duplicated information is called an insertion. 

Ideogram

Diagrammatic representation of the karyotype of an individual, useful in identifying the regions within the chromosome.

Chromosomal abnormality of Down syndrome -

Trisomy (3 copies of chromosome 21)

Chromosomal abnormality of Klinefelter syndrome -

More than 2 sex chromosomes (e.g. XXY, XXXY)

causes infertility

due to fusion of a Y sperm with a XX egg or XY sperm with an X egg

Chromosomal abnormality of Turner syndrome

Only one sex chromosome (X)

Chromosomal abnormality of Patau syndrome

Trisomy (3 copies of chromosome 13)

Chromosomal abnormality of Trisomy 8 syndrome

Trisomy 8 (3 copies of chromosome 8)

TRISOMY

having an extra copy of one of one’s chromosomes

How to match chromosomes for a Karyotype:

* size
* banding pattern
* centromere location

are x or y chromosomes longer?

The X chromosome is about three times larger than the Y chromosome

Metacentric

The centromere lies near the center of the chromosome

Submetacentric

Chromosomes have a centromere that is off-center, so that one chromosome arm is longer than the other. The short arm is designated "p" (for petite), and the long arm is designated "q" (because it follows the letter "p").

Acrocentric

The centromere is very near one end.

Dark bands on chromosomes

A karyotype analysis usually involves blocking cells in mitosis (METAPHASE/PROPHASE so chromosomes are condensed visible) and staining the condensed chromosomes with Giemsa dye. The dye stains regions of chromosomes that are rich in the base pairs Adenine (A) and Thymine (T) producing a dark band.

Centromere

Areas where the two chromatids are joined

Non disjunction

The chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)

How might non disjunction occur

• Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells) MEISOS 1

• n+ 1, n + 1, n - 1, n - 1

• Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected) MEIOSIS 2

• n + 1, n - 1, n, n

uniparental disomy 

A person receives two copies of a chromosome, or of part of a chromosome, from one parent and no copies from the other parent

lethal genes

female can survive with just X

male can’t survive with just Y

X0/Y0

chapter 6.3

does a dominant trait mean it is common/the most common

no

genotype

Genetic makeup of an organism, Genotypes make up the traits that an organism inherits from its parents (i.e. alleles)

has letter key (A or a)

**Alleles are dominant or recessive,** you inherit 2 alleles for each gene.

Particular combination of alleles (eg. gene controlling for hairline) -

•__Hetero__zygous – Ww (1 dominant, 1 recessive allele)

•__Homo__zygous **dominant** – WW (2 dominant alleles)

•__Homo__zygous **recessive** – ww (2 recessive alleles)

phenotype

The visible/measurable expression of the genetic make-up of an organism

i.e. the traits or characteristics that result from the genotype

\*\*if one/both are capitale then the trait will be shown -> capital 1st, e.g. Tt or TT then trait will be there, only not at tt. Allele from each parent, kid tt then parents both Tt (they both have trait)

e.g. hairline shape

Heterozygous

2 different alleles

(1 dominant, 1 recessive allele)

Ww

Homozygous dominant

2 of the same allele, both dominant

WW

Homozygous recessive

2 of the same allele, both recessive

ww

Why identical twins have an identical genotype but may show variations in the characteristics of their phenotype 

because epigenetic factors and environmental conditions may also affect gene expression.  Eg. One twin may play more sport, eat less carbohydrates, be more active.

Example of environmental impacts on phenotype 

Lower temperature activates dark pigment production and growth of black hair. (at birth, rabbit has a white fluffy body but soon grows black hairs on its ears, nose, feet and tail, but if a cold pad is fixed to a shaved area on its back for a few weeks, black hair starts to grow beneath the pad).

also: siamese cat owners have found that dark areas enlarge in winter and reduce in summer.

environmental factor: chlorophyll

• chlorophyll will only develop where light is available.

• Plants may only develop flower buds when night length and temperature is a suitable duration.

• Hours of darkness triggers flower production 

Chemical modification of DNA

Chemical modification of DNA can also influence the phenotypic expression of genes.

• Scientists found a pattern shown by altered DNA, where a methyl group or “chemical cap” was attached to part of the molecule.

• This DNA methylation switched off the expression of the genes containing the modified DNA, so the trait wasn’t expressed.

• Another type of structural DNA modification is caused by histone modification in the DNA molecule.

These changes normally occur during the individual’s lifetime but may also be transmitted to offspring.

EPIGENETICS: what is it

* Epigenetics is the study of heritable changes in the phenotype of an organism caused by modification of gene expression due to physical changes in the structure of DNA, rather than an alteration of the genetic code.


* Several environmental lifestyle factors including diet, obesity, tobacco smoke, alcohol consumption and enviro pollutants can modify epigenetic patterns.


* Current research investigates the role of epigenetics in various cancers, mental retardation and syndromes which involve chromosomal abnormalities.
* \

cancer and epigenetics

Cancer has been linked to epigenetics, diseased tissue from colorectal cancer patients was found to have less DNA methylation compared to normal tissue.

Genes which are methylated are generally switched off and not expressed, therefore loss of DNA methylation can cause higher gene activation.

Too much methylation could undo the work of tumour suppressor genes, allowing cancer to occur.

Fragile X syndrome is an inherited intellectual disability, usually in males – promoter region of FMR1 gene is methylated, turning off the gene which produces an important protein.

Coffin-Lowry and Rett syndromes.

Next generations of GG x gg

1. Gg Gg Gg Gg
2. GG Gg Gg gg - genotype 1:2:1/25%-50%-25%, phenotype 3:1

codominance

there are two things with both traits showing through, neither specifically more dominant

e.g. blood types - AB, AO, BO, OO, BB, AA (6 types)

hybrid genotype

Hh, means heterozygous

Punnett squares

Used to find the genotypes and phenotypes of potential/ offspring of to parents using their genotypes

how do we know if a trait is dominant or recessive?

•If it is heterozygous, whichever looks like = dominant. Cross 2 homozygous (dom x rec) , A x Yy, heterozygous, whichever phenotype expressed is the dom.

Incomplete dominance

•Sometimes the dominant gene doesn’t always mask the recessive gene

•This results in a mixed form

•This is called incomplete dominance

You can see it when you cross two pure-bredparents

As neither allele is dominant, the letter C represents the gene (colour) and the subscript (R and W) represent the alleles (red and white).

SW 177

Codominance

Different to incomplete dominance as it **expresses both alleles** in the phenotype 

SW 178

similarities and differences between incomplete dominance and codominance

Incomplete dominance

-Mix of both alleles

-Red x white = pink

-Codominance

-Expresses both alleles

-Red x white = red/white spots/stripes

-Both use superscript  (extra letter)

-gene: C

-Red colour: C^r white colour = C^w

-Heterozygous = C^rC^w

-C^rC^r= pure red, C^wC^w= pure white

dominant trait (typically)

is always expressed in the phenotype if it is present, be it a heterozygous or homozygous dominant

recessive trait (typically)

is expressed in phenotype only when the genotype is homozygous recessive

is masked by the dominant trait in heterozygous